USB-C to 300V Troubleshooting

In my last post I described the challenges in using the LT3750 as a high voltage power supply. Namely, the power supply needs to be restarted after it reaches its target voltage, since it is a one-shot “charger”. An additional problem is that when inspecting the output voltage, it didn’t quite match the calculated output.

Output Discrepancy

It was surprising to get this plot of expected vs measured output, as reported by the power supply. I would expect some scaling to be wrong, or to have some DC offset due to reworking the voltage divider with 5% resistors instead of 1%, but the hockey stick on the left side of the plot is pretty egregious. Not only do the values take off there, but the concavity of the line actually seems to change! This needed further investigation.

I actually didn’t believe it at first, but after verifying the plot with a few other instruments, and after testing the ADC, rechecking the voltage divider, and a lot of head scratching I realized the problem was in the LT3750, not in the rest of the circuitry.

This was puzzling because on the surface the circuit seemed to do what I expected, aside from the range of 75-100 counts (RBg max = 970 ohms). After scouring the datasheet, I realized that there is a limit on the current into the RVout pin of 1mA. This is not really highlighted in the datasheet. I had calculated a 12k resistor for Rvout in order to get a wide Vout range. However, this violated the rule for for Rvout, and my suspicion is that the proportional current source cant keep up, meaning Vrbg has a hard time overcoming the voltage in the vout comparator, resulting in an overshoot in the output voltage.

With this hunch, I replaced Rvout with a 27k resistor. This narrowed the usable output range by bringing up the minimum output voltage- ouput voltage is proportional to Rvout/Rbg, so its easy to have a higher voltage. Rbg is limited, per the datasheet, to 2.5k or less. This is annoying, since it would be nice to have a range from about 60V-300V, because my bench supply tops out at 60V. however, at the expected output voltage, the 27k resistor nearly limits the input current to 1mA, so it would be hard to go much smaller.

At least the output is sane now! Ultimately, I could introduce another digipot on the RVout side, but its probably not worth the hassle and the cost of another set of parts for what I want to do with this. The right way to get this power supply might actually be to just use something with continuous feedback to avoid having to kick off another charge cycle.

Output Accuracy:

With the output related to the input, the next step was to relate the potentiometer codes to output voltage. Sadly, this needed to be compared to some instrument, and my multimeter does not have peak hold. So I hooked the supply up to my scope and measured the peak voltage (for a few cycles) per code. I know the expression for the output code is:

Where B is the terminal resistance/parasitic resistance of the digipot (Rbg), and A is a bunch of stuff (turns ratio, a constant, the Rvout resistor value). It doesn’t matter since I am just fitting this curve. The parameters A and B were extracted with scipy, and plugged back into the control code to control output voltage.

Improving ADC reads with rp2040 PIO:

The output stability was initially very bad. This is because the sampling was running very slowly- even running on its own core, reading/writing GPIOs and evaluating expressions takes a lot of time- here you can see the clock rate into the ADC was only 6.7kHz- much lower than the minimum 5 MHz. To put it in perspective, the raspberry pi is only running 6x faster than the max speed that that the adc can be sampled. The slowest sample speed would allow 25 instructions per- reading, including whatever is needed to generate the PWM.

Even at high speeds, it takes a lot of time to get a good sample. Since the output is a bit at a time, each bit only carries one bit of information- this sounds obvious but that means that to get a typical 8 bit resolution, we don’t need to sample 8 bits, we need to sample 256 bits. even running at 20MHz, that still means only one reading every 12.8 microseconds, and that would give worse than 1 volt resolution.

In theory, with proper filtering (which I’m not doing), the adc can provide 16 bit resolution, which is incredible. I decided to settle for taking 12 bits of samples and decimating them, running at about 18 MHz for about a 200 microsecond conversion time (plus some small overhead).

This required using the PIO of the RP2040. PIO is super cool- its like a teeny tiny extra microcontroller that gets run with a tiny(!!!) 9 instruction set. This instruction set is extremely limited- even more so than typical assembly. For example, you can do something like load a number into a 32 bit register, but you cant load a number that is bigger than 31. There is no automatic incrementing of registers, but you can decrement some registers using the jump command. Its a beautiful, but strangely functional (and very fast) mess.

Basically what my code does is load a counter into the y register of how many pwm cycles to do, and loads the maximum number into the x register – 0xFFFFFFFF. Then it starts clocking out pwm cycles, and uses the jump instruction to decrement the x register and y register. When the y register is empty, it reports the x register back to the main program and restarts.

With a little math, this results in how many “high” bits there were in the data. This duty cycle is related back to the read voltage through some simple math from the AMC3306 datasheet and the resistor divider information. This tended to read really high at low voltages (20V when the output was shorted), so I added a correction/adjustment factor to bring it back in line with reality.

Output Stability:

I wanted to get a very rough idea of the output stability. It’s not easy to to claim any kind of stability or even claim a power output without a variable load of some kind, but I wanted to at least be able to set an output voltage and look at how the supply can regulate against the internal bleeder resistors.

Overall its not too bad- the max is within a volt of the setpoint, and the ripple seems to be about 10V, although given the resolution when zoomed out this far on the scope I am not totally confident in that. Given the hackery and amount of new stuff that went into this, I am satisfied with the outcome.

What I would change:

In the scheme of things, this project went pretty well- I tried at least four or five new processes, so anything working at all is a small miracle. However, there are a couple things I would change if I did it again. Here is a very public bug/ wishlist:

Indicators for hazardous voltage presence, CHARGE and nDone would be neat and blinky. Its nice to have activity indicators. I could actually add most of these with some rework, but it would have been smart to put them on the board!

I do have provisions to add an I2C screen, but no real plans to use it at the moment. This is another nice to have, to get feedback instead of using the console or a dmm, which is limiting. Originally I didn’t expect to use the feedback delta sigma modulator, so this information would not have existed in the system. Now that I have it, it seems silly not to display output voltage.

Increasing the allowable output voltage would also be easy with a substitution of output diode and output caps. It would be wise to choose output caps well in excess of the possible output voltage- even at the expense of space. Currently, my power supply can be commanded (accidentally) to self destruct!

Obviously it would also be great to go back and add the right parts for all the r’s and c’s that are bodged in.

Fix the feedback divider for better range!

PIO errata:

side loading is awesome! it basically allows you a free instruction to set output pins while you do other stuff. The “other stuff” I am doing is nopping, but this could be super useful.

labels at the end of your code will get optimized out. even if they have a nop()[x] after them. however, a regular nop() will keep them in the code.

adc pins used in the pio need to be explicitly enabled in the way you want, e.g. pin = Pin(28, Pin.IN) or pin = Pin(28, Pin.OUT). The PIO state machine class will not do this for you.

Adjustable High Voltage USB PD Power Supply

A usb-pd that nobody wanted or asked for. However, I needed one.

When I say high voltage, I don’t mean 20V, I mean around 300V, for high-voltage DC needs like electrophoresis or electrowetting. This is a gratuitous and impractical project, since you can buy an electrophoresis power supply for about $100. However, I wanted to do a more compilated design, and I “needed” a test PCB for my hotplate, so I made this.

The goal of this project was to make something worthy of being a lab instrument, e.g. not completely unsafe, and not held together with string and gum. I want to “have it all” with this instrument and even more than that I wanted an excuse to use a lot of new parts and tools.

With respect to safety, I wanted to essentially live up to a standard of a double insulated tool- that anything conductive sticking out of the case (unless it is live) has double or reinforced insulation. To avoid USB ground loops or accidental ground-referencing due to an evil USB port (or a short of shield to ground), all USB ports are isolated from each other and from the high voltage.

In short, in addition to this project being impractical, its also somewhat risky in terms of project bringup- it was possible I missed something or that a circuit just won’t work and would be impossible to fix.

High Voltage Generation

Ignore the loose solder blobs…those got cleaned off.

I wanted to use a flyback converter to take advantage of the inherent isolation of the transformer. The 300V out is not ground referenced, which makes it a lot safer since you have to touch both wires to get zapped (this will hurt). To maintain isolation, the output voltage cannot be sensed directly (with a conductor). One option is to use an opto isolator, but that requires more board area, cost, and things that can go wrong.

Another option is to use a flyback controller with “PSR”. PSR is primary side regulation, which senses the voltage induced on the primary while the secondary coil is discharging. This cleverly uses the transformer itself to avoid extra parts or breaching the isolation. There are a few controllers with this feature.

I also wanted the flyback to be adjustable- this is not as easy since it seems like these controllers are most often used when “just one” voltage is needed over and over again (many are for charging photoflash capacitors). The trick was finding one where only one resistor value needed to be changed, which I could do using a digipot.

I landed on the LT3750, which checks all the boxes. An additional factor was that coilcraft sells a special inductor that plays nicely with this controller, and for my application which has very high primary side currents (low DCR), low leakage inductance, and needs a relatively high inductance to be slow enough for the controller.

For safety, I added a high value bleeder resistor to the output caps, so they will discharge below the hazardous voltage threshold within a minute or two of disabling the supply.

N.B.: I later figured out that choosing a controller without continuous feedback was a problem- I made it work but its not ideal, see what went wrong.

USB Isolation

This device has three ports- two USB and one BNC. It’s important to prevent any conduction from one port to another, to avoid ground loops or ground referencing. The power USB port is isolated from the high voltage by nature of the converter. The data usb port (also for programming the RP2040) also needs to be isolated from the power USB port.

To do this I used an ADUM3160 which is a full speed, reinforced isolator. this provides a great deal of protection from voltages on either side of the isolator, which is important to protect both myself and my computer!

This worked off the bat at full speed, with little care given to the trace lengths/impendence ,since I kept them short. Surprisingly, the main design issue in the whole project was using the wrong jumper footprint, which left the device speed undefined on the rp2040 side. a single blob of solder fixed this (shown above).


At the heart of this all is an RP2040. I’ve never used one of these (or micropython) but they have a pretty good hardware design guide and it looked simple enough to one-shot the design. Surprisingly, soldering the QFN went smoothly, and the usb bootloader worked flawlessly, which is an unusual and totally delightful experience.

Bringup with the RP2040 has been similarly delightful- using the REPL, its easy to query devices over I2C and to make sure that all my buttons etc. are hooked up correctly in only a few minutes. It was also easy to write scripts to quickly test the power supply. I can see a lot of reasons to use this in the future.

What Went Wrong + The RP2040 is a Hammer

The LT3750 has PSR, but no feedback about what the voltage is unless it is charging. This means that if it is charging, it is regulating the voltage, and if it is not charging, it has no idea what the output voltage is. Checking what the output voltage is can only be done by pumping a little energy into the output and seeing what happens, which increases the output voltage (if done frequently).

Days since magic smoke released: 0

To maintain the output voltage for different loads, the output needs to be checked frequently. This checking can easily drive the output voltage up (based on the minimum resistor value, to about 600V), which is enough to blow up the output caps, fry whatever thing you were running off the power supply etc. – in short, its nasty.

If not checked frequently enough, the output voltage will sag under different loads. What works for maintaining voltage across the bleeder resistors might not work for a real load.

Fortunately, I had planned to use the AMC3336-Q1 to measure the output across the isolation barrier. This part is super convenient to use because it is powered from the isolated side through some integral magnetics. This eliminates needing to do funny business to get a low voltage source referenced to the high voltage side.

Sadly the AMC3336 was out of stock, so I ended up with the AMC3306m25. This is the same part, but it only has a 250mV range, which is ok because that only required a small change in the resistor divider.

By using the AMC33X6, I can measure the output and kick on the converter if the output drops from within a few volts of my specified output.

One annoying thing about the AMC33X6 series is that the output is fairly high frequency digital output. The output is basically is a 5-20Mhz square wave that needs to be measured synchronously with a CLKIN signal that also needs to be generated (at least on my board) by the micro. The digital output actually encodes raw ADC information in terms of duty cycle. The proper way to process this is with a digital filter (and who knows- maybe I’ll do that) but currently I am just decimating (averaging) it.

If this sounds like a job for some low level micro peripherals, you are right. Or if you think this is the perfect application for the raspberry pi PIO, you would also be very right. However, the rp2040 also has an entire 125MHz core that I wasn’t using, so instead of doing it the hard way, I did it the easy way and used the second processor to generate the clock signal and to read/process the data in.

This works reasonably well, and I’m ready to start developing the rest of the application for which I need this high voltage supply.


If you are interested in any of the specifics of this project, you can get the source here.

Hot Plate Usability Upgrades

My hotplate, while very functional, was not user friendly. It was a small matter of software (SMOP) to solve these issues.

The buttons work!

The hotplate can be turned on and off and controlled in increments of 10c using just the buttons. This frees it from needing to be hooked up to the computer with another USB cable. I originally imagined that having computer control would be neat (and it would be), but its not really necessary for anything that I am doing right now, and it impedes my workflow to do so.

There is a screen!

This screen really would be better off rotated 90 degrees. the final hotplate controller will have that, but for now I will just have to use my imagination. This screen is at a funny angle because the original plan was to control this through the serial port and specifically not to have a screen.

To Do:

This thing needs some kind of case, both to protect the electronics and to make it easy to use and store. I will probably 3d print some kind of temporary mounting arrangement, and also spin up a finished version of the user interface. That UI will probably get assembled on the hotplate!

Tin Bismuth Solder + PCB hotplate

My first SnBi soldering project

With my pcb hotplate built, I needed a test for my new tool. My goal is to be able to easily do prototypes of pretty much anything in-house, to avoid the delay, cost, and general PITA of outsourcing board fab. I also want to switch to a lead-free process.

Motivation to go Lead-Free

There are a couple of advantages I see with moving to lead-free SnBiAg solders, especially at home. The first is that the liquidus temperature is much, much lower- around 140 C. Eutectic tin-lead solders come in around 185C, and SAC lead free solders melt around 220. This lower temperature has two pleasant benefits. First, you won’t end up scorching your parts/silkscreen/soldermask with hot air trying to get some chip off the board. Second, everything is easier/faster to heat up since less energy is required.

This lower temperature also seems like it should reduce the energy in the flux fumes, which I can only assume is better than the alternative (use a fume extractor, kids).

The other advantage is that there is no lead in lead free solder. It’s 2023- we know lead is bad. I have done a lot of soldering with lab/food separation, and while leaded solder is probably fine if you have a lab setting, its not something you want to get in your carpet/personal desk/place where you might sleep or eat. Realistically there’s always a little loose solder created when soldering (think of what is the bucket of a brass sponge).

That said, I’m keeping my tin-lead solder (for good reasons).

How Does It Compare?

A quick test- most of the paste printed well (look at C12 and C19)

One limitation is that SnBi solder can’t be used with leaded solder, since this can form a Sn-Pb-Bi alloy with a very low melting point (below 100C). And, for some reason, it does not seem to be available in small wire diameters. Hopefully this will change, since having a huge 1mm solder wire sort of prevents any kind of small SMD rework using wire.

Another limitation (as a result of not playing nice with lead) is that leaded pad finishes are inappropriate for SnBi alloys. This, combined with fine-pitch parts, lead me to use OSP. Both leaded and unleaded solder seemed to have less-good wetting on this surface than on ENIG/HASL type finishes. I think that SAC HASL is probably OK for a surface finish but I have not researched it. ENIG would have been preferable for the QFN.

It also seems to take a long time for the solder to get wicked up into solder wick, which can be frustrating. I think a larger tip would help here, but my best “normal size” tip is currently reserved for leaded soldering.

And that’s about it. Overall a good replacement, especially for prototype boards that don’t get hot!

A First Project

I wanted to really see what I could get away with, so I designed a board with a QFN, some fine pitch USB-C connectors, and a handful of of fine pitch leaded SMD components. There were also some larger parts like buttons, crystals, and a whole module, that represent “bulky” parts. To raise the stakes, I decided I would go crazy and use all kinds of $10 ICs.

To solder this, I used the chip-quick NC191LT50, which is a paste that does not need to be refrigerated. It printed nicely using a stainless stencil.

Test run showing bridged pins, some issues wetting

I then put it on the PCB hotplate and took it up to about 160C. Since this is a four layer board, it took a few seconds for the top to heat up. The soldering went surprisingly well, but the QFN had some bridged pins.

Since the solder is low temp, removing the IC was a breeze and the board stayed totally un-scorched. I quickly wiped up some of the solder, and hot-aired it back on. It is now working just fine!

Larger parts were soldered with the same alloy, but with solder wire. I only had a very small soldering iron tip to use (a non-leaded one), so it was hard to get good contact, but the solder did eventually wet everything nicely.

Overall this went shockingly smoothly, given the diversity of parts and using a while new process!

Conclusion: Tin Bismuth Silver solder is here to stay

I am pretty happy with this stuff, especially for projects where a stencil makes sense/is affordable. I’d hate to solder fine pitch components with the huge wire, but I think a paste (even from a syringe) and a hotplate will go a long way.

PCB Hotplate Continued

The hot plating continues! And with the boards actually in hand, I can compare my estimates to some real world measurements.

In the end, I made both FR4 and aluminum heaters. These heaters ended up having slightly different properties. Oddly, the aluminum PCB has a higher resistance (4 ohms) than the copper pcb, 2.9 ohms). Read on to figure out what this did to the design.

Designed Vs Measured Resistance:

The design resistance of the heater track was 3.3 ohms. Clearly, this was not “nailed” for either of the boards, but it was close. I was surprised that both boards didn’t overshoot the resistance, due to the addition of a few extra squares of conductor on the connector island- however the FR4 heater seems to have a more generous allotment of copper, and the aluminum PCB may have slightly less copper.

Unfortunately, this means that at higher temperatures, the power delivered to the aluminum board is much lower than desired. The calculated value for the resistance at 200C is about 6.7 ohms, and the measured value is 7-8 ohms. This means that the maximum power available at 20V is 50-60W, or less. This is pretty close to the theoretical minimum power needed to reach 200C, which is why this plate struggles to achieve 200C. As you can see, the heater is on for the entire time (until I pulled the plug). Time units are in 10ms intervals. Fortunately, this validates the power delivery assumptions I made in the past post.

To recap, Qdot is the amount of heating power needed to sustain a particular delta T, given some convection parameters. I assumed a delta T of 175k, with an area of about .01m2, and h being 25 W/m2k, resulting in a power delivery at the heater of 43W. while this is not totally spot on, its in the right ballpark. It suggests that lowering the resistance and actually delivering 100w would be very helpful to removing this 200c limit. I think 10-20 is a reasonable conservative estimate of convective cooling, but its conservative in the wrong direction if you are designing a heater.

On the other hand, the resistance of the FR4 heater is low. At low temperatures, running at 100% PWM (all on), will cause the power supply to brown out. This means that the power delivery actually needs to be cut back (PWM’ed) when the heater is cooler, so that the overall current draw is lower. Once the heater is in the vicinity of 100C, it can be run “full blast”, aka more or less shorted across the power supply.

Does It Blend?

Both the aluminum and FR4 heaters were able to reach 200c depending on the conditions. Both were capable of melting solder directly on the plate, as you can see. The aluminum heater would benefit from an improved (lower resistance) heater trace.

The remote probe feature allows for the sensing of the temperature of the PCB being soldered. This is good, since that is where the temperature actually matters. This can cause the heater to get much hotter (20c or so) than the top of the target board, which in this photo, resulted in some discoloration of the solder mask of the heater.

Temperature Gradients and Overheating:

The hot plate design needs to achieve two tasks: be cold at the connector (so as not to de-solder it), and it needs to be hot (and ideally, evenly hot) across the surface.

The worst case for de-soldering the connector is with the aluminum heater, since the aluminum substrate should conduct heat pretty well. Up at peak temperatures, the connector area only got to about 85C. Certainly warm, but not nearly hot enough to damage the connector.

In terms of heater gradients, the aluminum PCB had about a 10C difference from the center to the corner at 200c. The gradient to the “neck” of the connector island was much worse (13+ C), since the thick traces there (and the substrate) act like a heat sink. The corners are cooler since there is some radiation and extra convection out of the sides of the heater.

The FR4 board has a even higher gradient, in excess of 50C during heat up, and staying steady at about 50C after several minutes. This is annoying, since it makes the outer areas of the PCB too cold to solder on. The center can be overdriven to compensate, but that leads to overheating and discoloring the solder mask.

New Design Goals + Limitations:

Its clear that hot plate soldering is not the ideal way to solder- something like a toaster seems a lot better and faster, since it can more evenly heat the target PCB. The main issue is that in order to get the top of the PCB hot enough to reflow, the bottom has to be quite a bit hotter. This is worse with traditional lead-free solders, since they need to hit about 220C. Lead-Tin eutectic solders need to hit 185 or so, and bismuth lead-free solders only need to reach 140 C (there is a medium caveat here).

You can see the results of this heating on an adafruit board (above), that I gently reflowed. The left board is new from adafruit, and the right one is reflowed. It didn’t really damage the board, but it certainly would be an unacceptable outcome if a manufacturer overheated a board like this, and the lead free solder (I assume) on top barely reached the liquidus point, and would re-solidify as soon as I moved a component.

The medium sized caveat is that any lead in the process can poison your bismuth solders forming a bismuth-lead crystal that has a very low melting point (95C!). I think the jury is still out on using bismuth solder for stuff that gets hot, but for most prototyping it seems fine. For low temperature solders, the hotplate is fine because your whole board stays under typical soldering temperatures.

For leaded solders the hot plate will likely also work ok, since the heater can be at about 200c and the surface of the target PCB can be at 180. However, for lead-free SAC (Sn Ag Cu) alloys, I think the hot plate will likely damage the solder mask of the target PCB. However, using the PCB hotplate as a preheater could work fine, with the addition of a hot air gun assist.

I would like to use this hot plate to solder and test complete prototype designs, with packages that are hard to hand solder, particularly leadless stuff like QFNs. I find solder fumes to be irritating (and there are more at higher temperatures), and so I am going to try using the bismuth solder, at least for prototyping. For leaded work, I’m pretty sure the hot plate would work fine, and it will be a good tool (combined with hot air) for reworking SAC alloy lead-free boards. I’d also like to take up less closet space than a toaster oven, so I think the project does have some value. It also fits in nicely with the USB-C powered soldering iron ecosystem.

Electrical Stuff:

In the previous posts I shared some choices around the electrical system. The two concerns I had were not shorting out the power supply, and preventing inductive spikes from exceeding Vdsmax.

The Vgs, shown above, was totally fine since it was limited by the free-wheeling diode. Even with a 1ms charge time, the inductor failed to exceed 25V and there was very little ringing. The Vdsmax is about 30V, so there is plenty of margin.

To test power integrity, I wrote some code to turn the heater on different power levels for 10ms, starting at 100% power. As you can see, 100% power reduces the output voltage just a bit, to 19.5V. This is well within the PD specification. When the PWM begins, the power supply seems to tolerate some relatively large spikes/dips as the heater (inductive load) is switched at 25kHz.

Electrical + Control Improvements:

To get a truly performant hotplate, the resistance at 200C needs to be less than 4 ohms, so the full 100W (or close to it) can be used to maintain the target operating temperature. Doing this is tricky, because it means that at 25C the heater can’t be operated at 100% duty cycle. Basically, power is not a function of duty cycle, but of duty cycle, temperature, and board characteristics.

There are two approaches to solving this problem. One is a per-heater lookup table, that lists a reasonable safe duty cycle per temperature. This is limiting since it would need to be generated per-heater, but its not unreasonable to do that given that I will likely stick with a single heater, and most heaters should be pretty similar.

Another approach would be to add a current monitoring to the USB PD supply, and cut the heater power off once you get close to the limit. This requires a relatively high bulk capacitance to provide current, even at 25kHz, but its certainly achievable. Overall power could be modulated by adding a dead time in between heating cycles, in order to follow a temperature profile. Higher frequencies could be considered, taking into account the switching time of the low side FETs. This is basically a current-controlled buck converter.

I’ll likely stick to the first approach for now, since doing current limited control requires more hardware, and it is unlikely to really improve the outcome of my soldering.

Mistakes + Takeaways

There were a few minor circuit goofs, and some things that went surprisingly well. Here is a small list, that might help you or me in the future:

GPIO9 and GPIO8 on the ESP32 C3 are bootstrapping pins– they control what the ESP does when it boots. I connected one of them to a switch, as an input for the program. This meant that the ESP would not boot into my program, but it would boot back into bootloader mode (and accept new programs). Toggling the switch made the ESP behave normally. Note to self: use these pins carefully.

The pinecil is a pretty beefy iron. It had trouble soldering to the aluminum PCB, but with boost mode and patience it had enough power to solder directly to a 3mm wide trace on an aluminum substrate. It had NO trouble soldering XT60 connectors to large pours. Don’t let the small size fool you- it is a real iron.

It is a good idea to put indicator LEDs on things like the actual power rail and the actual HEAT signal. It makes it unambiguous that something is really happening, and that the heater is getting hot.

I added a solder jumper to the overtemperature cutoff circuit. I accidentally swapped the inputs on the monitor chip, so it didn’t work. Cutting the jumper made it really easy to test with this protection turned off.

All my temperature sensors gave wildly different readings, even when co-located. I’m not sure why, but I do trust that the delta temperature measurements were ok since they were from the same instrument with two of the same probes, and they had the same readings when physically connected.

USBC-PD can deliver serious power, and its easily obtained with USB PD trigger boards.

Next Steps:

During testing, I learned what this type of soldering setup is really appropriate for, and what the capabilities of the heater are. I want to try some of this low temp solder, and I want to do some hot plate soldering to figure out how it feels, before going back to take another pass at the heater element. I suspect targeting an even lower heater resistance will allow me to hit 200C easily with the aluminum plate.

I also have some work to do on the UI (there is a screen- I just didn’t plug it in), and on the controls. If I find it useful, I will probably build an enclosure for it to protect the electronics and to make it easier to use.

PCB Hotplate Controller

The PCB heater for the hotplate obviously needs to be driven by something. I could have put more circuits on the heater board itself, but separating them has some advantages:

  • if the heater element is bad, or needs reconfiguration, it can be replaced
  • better opportunity for a heat break between the boards
  • opportunity to try different heater substrates or even different heaters/heater shapes

My goal for the hotplate is to keep it simple. I decided that I really probably need one profile at most, and a manual-temperature mode. I’d rather make that profile on the computer, instead of on the device, and be able to play it back from the device. I’d also rather get information back from the plate on the computer instead of a tiny display. Hopefully these simplifications will allow a hotplate to be up and running sooner rather than later.

Here is the architecture I came up with:

There are two usb C connections- one for programming the micro, and one for power, equipped with a USB PD “trigger” chip. My new favorite controller, the ESP32-C3 is in charge of the show (and native USB C). A single thermocouple is used for feedback, and an RTD is used as a safety cutoff- basically if it goes above some target temperature, the micro will no longer be able to drive the power FET. This won’t prevent the board from getting super hot, but it does provide a hardware safety against exceeding some set temperature if the controller spaces out…assuming the thermistor does not fall off.

Additionally there are indicators for PD power, DATA power, data transmission (heartbeat), heater power, and safety over temp, hot surface, mode, and (finally) a temperature readout. These are diagnostically useful, and they should satisfy my desire for a cool looking annunciator panel.

One annoying thing is that the ESP needs to be driven at 3v3 while the rest of the stuff is happy at 20V. While a cheap, integrated buck would be a neat solution, an LDO is so much cheaper. I decided to go with the buck, since I wanted to avoid burning extra power when operating near the 100W limit of the power supply.

The heater itself will be driven by a gate driver like the NCP81071B, which is 20V tolerant. 20V will just barely allow us to use something like the AOSD32334C, which is a dual N FET. Paralleling the FETS will reduce the RDSon to almost nothing- close to 10 mOhms.

The heater element here is represented by a resistor and an inductor- I expect the trace to have some very real inductance (100s of nH?), which will cause a large voltage spike when the FET turns off. This could easily exceed the 30VDS of the FET, and cause it to break down. The flyback diode will hopefully prevent this. Additionally, a capacitor Cbulk will be chosen to prevent over-drawing current from the power supply. At 20V, the 3.3 ohm heater will draw 6A, while the maximum allowed current is 5A. By PWMing this, the average current will be below or at 5A, but when the heater is on that current needs to come from somewhere- the bulk capacitance C.

To connect to the heater itself, I am using an XT60 connector. These are low-resistance and they seem easy to get, due to their use in drones.

Safety + Temperature Monitoring

There are a lot of ways making a big heater that is capable of destroying itself can go wrong. Ultimately this device falls under the category of things you should unplug if its not in use, and definitely not something to leave running unattended. However there are a few details I added as a nod to safety.

The first is a thermal trip based on the MIC841N The HTH input is fed by a NTC resistor divider that exceeds the 1.24V reference at about 200C. This SETs the latch, so the inverting output goes low, pulling down the enable pin of the fet driver. This is the case until either the whole circuit is reset (and the thermistor has cooled off) or until the reset line is pulled down by a button. Resetting while hot should result in an illegal state where ~Q is still 0.

Note: this circuit totally fails if Rtherm is removed, and it wont work if Rtherm gets disconnected from “the hot part”. If I wanted to add more logic, this could be resolved, e.g. inverting the logic of the resistor bridge. However, this is currently just a secondary protection. If it works nicely it can be refined later.

Temperature monitoring (for feedback and control) will be done with a thermistor dropped either onto the heater, or onto the target PCB. This allows for point control, near the stuff that you care about. For the time being, this will be made out of a MAX31850 thermocouple breakout that I have lying around from years ago.

Next Steps

With the heater and the controller designed, its time to get them made and then patiently wait for them to arrive!

PCB Hot Plate State of the Art

I have started seeing a whole bunch of PCB hot plate builds, of varying success. I kind of want one now, since they seem very practical for one-off small PCBs, and most off the shelf solutions are quite large. However, I also want to improve on the state of the art of PCB heaters. These are useful for all kinds of stuff, from PCR machines to 3d printer hot plates.

Builds I researched:

CarlBujejas V1/V2 hotplate videos, files

AfterEarthLTD Solder-Reflow-Plate git, video coverage

electronoobs video

There are a lot of challenges to overcome to make something that does not suck. Ideally, it could even be USB powered. Below are some calculations and considerations for making a pcb hot plate. You can see the WIP (scripts, pcbs, etc) on github.

PCBs are not a good material:

Using a PCB as a heater is not a new idea- however, in some ways, they are not good material candidates for high soldering temperatures:

  • The board will be at/above TG for a long time
  • solder mask/silk can discolor
  • could de-solder itself/the controller
  • repeated stress on copper tracks from board warping while heating

That said, several people have made them and they seem super-good-enough for this process.

Thermal Expansion

A lot of people worry about thermal expansion with pcb heaters- it makes sense because at the length and temperature scales that the heater will experience (100s mm, 200 C), real growth/shrinkage of the pcb will happen. With those temperatures and lengths, a FR-4 PCB will grow about .28 mm! An aluminum one will grow even more- .4mm! copper has a similar coefficient of linear expansion (alpha) as FR4, which means that the copper will need to stretch about .1mm/100mm of length. That is a strain of about .1%. That should be in the plastic deformation range for copper according to some stress strain curves I found, but it could cause cracking in more rigid parts of the assembly.

To further mitigate any expansion issues, I meandered the heaters on both sides, which should hopefully prevent pulling “straight” on any piece of copper. hopefully having a longer track (albeit, glued to the substrate) will help distribute the strain along the track- in the same way that a meandered track in a strain gauge rosette works.

In addition, I cut a slot between the hot part and the cold part, and put the connector thermally far away from the heater, while adding a lot of area for cooling. This should keep the connector from desoldering.

Low Voltage Efficiency Challenges:

A lot of these builds use lowish DC voltages to power the plate. This is not a great idea, because these things take a good chunk of power. Since the voltage is low, this drives up the current in order to deliver enough power. This results in losses in all the cables and connectors between the power supply and the heater.

As the impedance of your load approaches the impedance of the source of power, the power supply starts to eat a lot of power. I am considering all parasitic resistances here to be part of the “source” driving just the heating coil.

n is efficiency- as Rload gets to be about the same as Rsource, the efficiency will be about 50%- that means 50% of the power is going to get burned in whatever is supplying the power. This means that the rest of the circuit- connectors, fets, etc. need to be very low impedance compared to the load.

For example- to get a 100W heater at 12V, the Rload needs to be about 1.2 ohms This means that even a 100mOhm RDSon will drop the efficiency about 8%!

What would be ideal would to have the Rload to be greater than the R of everything else, by a lot. This is possible, but it comes with a catch- the current will be lower, and therefore, the power delivered to the heater will be low. Lets compare two cases, RL = 1 and RL=9, for Rs=1, and a source voltage of 10V that can output up to 5A.

Casen, efficiencyCurrent (A)PtotalPheater
RL = 150%550W25W
RL = 990%110W9 W

An interesting (and not very important) aside is that for a particular heater power, and an infinitely powerful supply, there are usually two operating points for a given heater power. Check this out:

This gets turned into a very ulgy polynomial:

Lets look at a case where we claim Rsource = .5 ohms, V = 24v, Pload = 80 w.

If you crank this through the quadratic equation, (with some rounding) you will get R load of about 6 or .04 ohms. two very different resistances, but if you look at the same table you will see:

Casen, efficiencyCurrent (A)PtotalPheater
RL= 692%3.788W80
RL = .047%44A1000W80
yes- I rounded a lot but this is about right

Obviously, a heater with .04 ohms of resistance is impractical – the heater would be tiny (since the resistance has to be low), it would get SUPER hot because of the low thermal mass, and we would need to draw a kilowatt to use it.

Low Voltage Workarounds:

The good news is that the voltage does not need to be very high. Even a voltage of about 20-24 V (commonly available) is much better at delivering power than 12v.

Boosting to a higher voltage can help too. This won’t help as much as not having a low voltage at all, because the loss in the wiring/connectors in the low voltage section will still exist. However it could give more flexibility in the design or choice of heater elements. e.g. a 12W element that runs at 12V will have a resistance of 12 ohms (and a current of 1A). running this at 24V will result in a quadrupling of output power to 48W.

Ok, how much power do we need?

If we look at the existing hot plate examples, one figure of merit seems to be the steady state temperature at a given input voltage, and therefore power. My suspicion remains that connectors are eating a lot of power here, because the numbers do not add up. For example, at 3:10 in the hot plate V2 video we see the power supply is at 12V 7A, for a total input power of 84W. The temperature is 196C, with a room temp of about 25C. The heater diameter is .01 m2.

If the temperature is stable, that tells us energy in = energy out:

Using these numbers to back-solve for h, the convection coefficient, gives us about 44W/(m2k), which seems high. Given a similar delta T and the 70*50 area of the after earth board, the coefficient is about 10 W/m2k. These numbers seem high- I do not think all this power is getting delivered to the board, since a typical number for h is closer to 10-20 W/m2k for a pcb application. Its possible I am missing a few mm2 of area, or a lot of radiation or conduction, but these pcbs seem reasonably thermally isolated and appear to be in still air.

These designs come out to 5-10W/in2, or about 10kW/m2. I have seen a lot of heaters that claim to be 5-10W/in2, so that seems like a reasonable power to target.

Doing the math forwards (delta T of 175k, A= .01m2, h=25W/m2k), gives a desired wattage of 44W. I will aim to overshoot that, because it will reduce the heat-up time.

How fast does it need to get hot?

A good rate seems to be about 3C/sec, based on some reflow profiles. however, this rate needs to be done not only at the lower temperature, but near the top end of the reflow temperature- lets say 200c. This is important because if the heater is designed so that the max temperature is the reflow temperature, the heating rate will be slow as it approaches the max temp asymptotically.

drawing of typical newtonian heating

This is semi easy to estimate- we can make some assumptions, like that our heater is infinitely thin, massless, and is heating a plate that it is in very good contact with. the plate is also thin, so we don’t care about the gradient across it.

This basically says Temperature is equal to thermal mass * energy in an object. Taking the derivative with respect to time gives us a heating rate that depends on the heat into/out of the object. If we assume mostly convective cooling, and choose Qin, we can get a rate of temperature change. The delta T chosen tells us what the rate will be at a given temperature above ambient.

assuming a 1mm aluminum platen, with .01m2 of area, the volume is 1e-5 m3. with a density of 2.7e3 kg/m3, and a specific heat of 900 j/kgk, the platen has a thermal mass of about 73j/k.

for h=25 w/m2k a= .01 m2 dt=175, the convection term comes out to 43.75W (this is the same as the power needed to heat to this dt)

Tdot should be 3c/s,

so solving for Qin gives an power (at the heater) of about 100W. This is a rough SWAG, and it comes with the caveat that it will likely be even slower than that, due to delays in conducting that heat to the PCB.

Using USB C PD:

A USB C power delivery brick can, in theory, supply 100W. Really getting 100W out of a USB PD charger seems to be a bit of a stretch. There are tolerances on the actual voltage a given supply must provide, plus an allowable voltage drop (and power loss) in a to-spec USB C cable. Per the usb C R2.0 spec:

This Vmax and Vmin represent +/-5% of the allowable voltage. At 0A, there is no drop on the cable, but at 5A there is an allowable 750 mV drop. Interestingly, this drop is not symmetrical- only a 250 mV drop is allowed on the ground connection (even though it should have the same current flowing through it). This voltage drop at 5A means that a USB cable is allowed to burn about 3.75W.

This means the worst case voltage to come out of a 100W PD supply and cable at 5A is really 18.25 V, which is really closer to 91W. However, as much as 20.25V can be delivered at 5A, for 101.25W (with an ideal 0 ohm cable).

This means to get the full power out of the lower allowable voltage chargers, (18.25V), the heater resistance will need to be 3.65 ohms. however, that will draw too much current on the high side. the solution seems to be to cater to the lowest voltage, and then PWM the heater (which I will do anyway) so that it does not exceed the average current draw for higher voltage/lower resistance supplies. Designing around the higher voltage supplies will result in a under-powered design, since we will not get the full 5A at lower resistances.

Additionally, there needs to be some allowance for other resistive circuit elements, e.g. the switching FET, solder joints etc. Estimating a few 300 milliohms for those seems ok, so so the real target heater resistance should be about 3.3 ohms at room temperature.

Resistance Temperature Sensitivity:

Resistance of materials increases with temperature. Copper increases about .0039 ohms per degree ohm. With a delta T of 175K, and a target resistance of 3.3 ohms, this means that the resistance will increase by 2.2 ohms! This means that at higher temperatures the maximum output will be closer to 72W.

It is easy to calculate what original resistance would give us 3.3 ohms at 200C- this turns out to be about 2 ohms. However, the bulk capacitance required to smooth out the ripple at 10kHz is about 1000uF, which is expensive to implement. Using an electrolytic capacitors incurs a pretty steep power loss in parasitic resistance, and ceramic capacitors are too expensive for the design. I will stick with 3.3 ohms for the design, and just loose some power generation at higher temperatures.

Heater Design Math:

What follows is a detailed design of the heater traces. An important concept is sheet resistance- this is the concept that the resistance of any square of material in a homogeneous sheet has the same resistance. 1 oz copper has a sheet resistance of about .5 mOhms, per square.

As we know from our previous calculations, the resistance of the heater should be about 3.3 ohms. We can use this equation to get the number of squares:

From this I calculated that the number of squares needed is 6600. This means the trace for the heater (if its made of 1 oz copper) to be 6600x longer than the width.

I want the heater to be about 100×100 mm, or less. This will give us an area of about .01m2, which was used in the calculations above for heat transfer. Ideally, the heater would be a little smaller, in order to hedge on the side of performance. Another practical consideration is putting the power terminals next to each other- this will minimize the length change between them when hot, and it will make the wires going to the heater short (preventing a wire run from another edge or corner of the board).

In order to meet these requirements, I wanted a rectangular pattern that would fill in the heater rectangle. I opted for what I decided would be called a double zig-zag, which can be cut anywhere on the perimeter so that the power terminals are next to each other. The extra long trace should also help prevent from stretching the copper under thermal expansion, since the whole pattern should act like a spring.

This drawing shows the units broken down- two long bars Ltarg long of width W, and several units that would each be connected to each other. Each unit has these dimensions:

The ideal distance between the tracks, H, should be very small, because it will result in more coverage of the board in heater tracks, and therefore more even heating. I chose to start H at about 6 mil/.16mm. H needs to be varied so that the total number of units, N, is an integer. having a half-unit means that the spacing will be off or that it will not be possible to terminate the pattern properly. The next thing to do is to come up with a couple relationships for the geometry of the pattern. The first is that the pattern must be a certain number of squares long. This sum is the top and bottom width full length bars plus N units * squares per unit. The Number of units, N, also has to be equal to the target height of the units (Htarg – 2w) divided by the unit height. Here h is the spacing between units, and H is Htarget.

These first two expressions combine, after some work, into this ugly polynomial. we can now input an h, spacing between traces, and get out w. w allows for computation of N, the total number of units, which must be an integer to be valid. see plot below to compare red dots (N) to track width (mm) vs spacing, h (mm), as the spacing is increased.

With this computed, it is easy to choose the best outcome, where the track thickness is maximized, and spacing is minimized, to create an evenly heated area.

Final design:

I decided that the heater dimension should be 100×76, since it is a little smaller than .01m2, which should give better performance at 100W. It is also a reasonable size for most PCBs I would make and under the price break dimensions for aluminum PCB.

Since the pattern can’t go all the way to the edge, and because the mounting is somewhat TBD, I decided to subtract a 1mm border on the long (100mm) dimension and add 4mm border on the short dimension. This makes the final dimensions for the board 100×100 (including connector), and the pattern dimensions 98×78 mm.

For a target track resistance of 3.3 ohms, the calculator reports that the width should be .996 mm, with .267 mm spacing, and 38 repeats.

Kicad Plugin

Of course drawing this trace is tedious and difficult to get right. A plugin was designed to draw the trace meander. Details can be found on git.

Next Steps:

The next thing to do is to ship the design off to be fabbed. I’m planning on trying both FR4 and aluminum substrate PCBs. FR4 has the advantage of having a similar linear expansion coefficient as copper, and being an ok insulator. Aluminum on the other hand, is possibly more rigid than FR4, and is a good conductor/heat spreader- this would be good to prevent heat buildup in any one trace. I will also need a controller for the heater, with a micro, a sensor for feedback, etc (post coming soon).

Color Temperature Tunable, High-CRI LED Lamp

In my last post, I discussed the basic requirements and layout of my lamp – I wanted it to be able to control the color temperature and brightness, and I wanted it to be really bright. I experimented with a single strip, and detailed how the control board would work. If you want to try to build your own, the code/boards are here, although it is not very well documented.

Here is the control board all wired up. It packs an ESP32-C3, because I had one on hand, and because connectivity is important for this project. In addition to manual over-the-air control, I want to be able to put the lamp onto a simulated daytime color temperature loop- a real-life version of f.lux, which turns up the warm temperature light on your display as the day progresses. I also would like to add a way to sync it to some kind of live color temperature data.

This wiring leaves a lot to be desired- this is because the boards and mechanical assembly were developed at different times, and the mechanical side of things ended up going to “plan b” aka laser cutting. It might be worth revisiting in the future, especially because a lot of ports/connectors ended up unused. For now, I am just happy to have a lamp.

Here is the control page that the lamp serves up. It lets you choose a mode: SYNC, MAX, or NORM.

Norm is a smooth light intensity with changing color temperature- the output never exceeds the brightness of having all warm or all cool lights on, so when the temperature is “balanced” the output is 50% brightness warm, 50% brightness cool.

Max allows for the overall maximum light intensity to vary, but keeps the mix “smooth”. So at full warm, 100% intensity, the brightness of the warm strip is 100%, and at “balanced” temperature, 100% intensity, the output is 100% warm 100% cool.

SYNC will take over both intensity and temperature and to create a profile that mirrors a day, which is still TBD- to be developed. There are many ways this could be implemented, from having the lamp look up the weather/sunrise/sunset, to building a weather station, to having a server on a main home automation controller that tells the lamp what to do. Its also not totally clear to me what the ideal profile would be- for example I would like to have nice bright light, even in the early evening, so I don’t want it to totally follow the real “day” profile.

Mechanical Notes + Details

This lamp, despite not achieving one of my crazy tensegrity ideas, fancy woodworking concepts, or brass-pipe dreams, does use one of my favorite tricks: using orings as precision rubber bands. The LED channels are very thin, and therefore fairly hard to attach to with screws. They are meant to be mounted with supplied clips, but I didn’t feel great with those overhead, since they are not closed on the bottom.

Instead of clips, I used the aforementioned fancy rubber bands, with two per side (four total) per LED channel. This means that if one breaks, I will have time to notice and replace it. Since they are flexible, the overall alignment of the strips matters less and the hole placement does not need to be spot on. They are also very thin, so they don’t block light.

Another interesting fabrication note is how I removed the scorching from the laser cut cross-bars. These were completely covered in soot as-cut. Usually cleanup of parts like this is annoying and time consuming because the soot is ingrained in the plys of the wood. This means the parts need to be sanded back significantly in order to remove the soot, causing the overall shape of the part to change. In this case I had access to a media blasting cabinet- a light blast with glass media removed most of the scorched material, saving time and preserving the shape (and sanity).


This is the kind of project that could go on forever, but for now I will enjoy having a nice lamp. Here is a short list of improvements for “someday”:

  • use websockets for more responsive control of the lamp
  • make lamp status persistent
  • find a nicer looking power supply
  • add a setup page/wifi STAtion mode as a fallback in case it can’t get on the network
  • finish daylight color tracking/sync mode
  • redo the PCB +other components to make it less of a noodly mess
  • add a display (of some sort) to show the IP address
  • use the neopixel on the dev board to indicate connection status
  • etc.

LED Strip Dimming Driver

After listening to my Make Your Own Gnome, I decided I wanted to make my own led lamp. Specifically, I want a lamp that:

-is dimmable from “nightlight” to “worklight”

-is color-temperature adjustable from warm to cool light

-has some kind of smarts- daytime color temperature tracking, live outdoor color temperature matching, remote control

LED Choice

I decided the “hybrid” LED strips from waveform would be nice based on having a high CRI, and having a mix of warm and cool LEDS on separate channels. These LEDS strip lights are “DC constant voltage” strips meaning that you put 24 volts across the power/ground of each to light them. Each color temperature has a separate ground, for low side switching. Each rail (one per color temperature) has in parallel several sets of 6 LEDs in series, with current limiting resistors. The number of LED units in parallel is controlled by the length of the strip, with 6 LEDs per color per 4″.

Since I want to have worklight-bright intensity from either a warm or cool color, I need about twice as many feet of LEDs than I would with only one brightness. Empirically, this is about 4.5 feet of LED strip (per color), or about 2000 lumens, or a total of 9 feet and 4000 lm combined.

System Architecture

I decided to have a main control board and several strip driver boards. Each control board has switching supplies for 3v3 and 12V, and an ESP32-C3. The control board has enough connectors for three channels of output consisting of warm white PWM, cool white PWM, and driver enable, as well as a connector to distribute 12V. 12V is used to run the FET gate drivers.

At higher currents (single- digit amps), the inductance of the wires from the power supply (30 cm or so) and the switching can start to cause some ground bounce, so having local decoupling is needed- and reducing the overall strip length also reduces the total inductance per-strip. Above is the drain of the fet switching the warm channel- as you can see switching what is essentially two inductive loads causes some voltage spikes, which fortunately do not exceed the VDS of the FET.

This modular architecture also lends itself to reuse for a two or one strip lamp.

Switcher Board Detail

A FET driver is way better than driving the FETs directly from the microcontroller, because it drives the gates at a high current, higher voltage, and without the inductance of long wires coming from the controller board (which could be several feet away). It also reduces the requirements for the FETS, because they do not need to have low RDSon at logic level drives.

Driving the FETs fast is important, because the longer switching takes, the longer the LEDS will be operating with additional current limiting, and therefore far from the target current. This change in current can cause the leds to shift their output spectra, which defeats the purpose of having really nice high CRI LEDs. Additionally, the longer switching time can lead to frequency limitations e.g. if the switching time starts taking up most of (or exceeding) the period.

I want to switch at 25kHz- this is above the audio threshold, so any components vibrating due to switching (capacitors) won’t be audible. This is also well above what a person should be able to notice for flicker (120 Hz) and it seems like its also well above the frequency where it would affect a phone camera with a rolling shutter. 25 kHz means that the period is 40uS. I estimated that I want at least 10-100% duty cycle, so a minimum on or off time of .4uS. I estimated that the total switching time should be no more than 10% of this minimum time, which gave a rise/fall time of 200nS.

The FET has a gate charge of about 9nC, and the driver can source on the order of 1A, so the switching time should be a on the order of 10 nS- well under the 200nS limit. This means that the minimum on time, allowing for 10% of the waveform to be rise/fall time, is 200nS. This would allow dimming to .5% full brightness, which is very dim indeed.

As you can see here, the real rise time is pretty much the predicted 10ns, with a symmetrical falling time, so the driver should be suitable for this application.

Concluding Thoughts

The switching board works fine, and I’ll certainly use them. The light is GREAT and being able to control the color output is awesome! Of course, in the process of this design I have come up with some improvements for a next iteration.

First of all, the efficiency of these lighting strips is Not Great. Per-color channel per-section (4″) they consume almost a watt (.96 W). The problem is that the LEDs consume only 70% of that energy, and they are not 100% efficient. 30% of the energy is burned off as heat in the resistors, which are wisely distributed along the strip. While this is much better than an incandescent bulb, it makes the strips get pretty warm, with the outer aluminum shell reaching about 40C in 20C ambient temperatures.

The reason for this voltage design decision seems to be wanting to run the strip off a higher voltage to reduce IR drop along the length of the strip, and 24V being a ubiquitous supply voltage. Since my strips are relatively short for this type of application, IR drop is not a huge issue. Given 6 LEDs in series, dropping down to even 20V would result in an improvement to about 85% efficiency.

Going a step further, a current-controlled switching supply could be used per-section of LEDS. While this is more expensive, I expect it to be much more efficient (close to 90/95%). Given that I’m not sensitive to a small increase in BOM cost, this could be just fine.

Second, I found the WAGO connectors to be annoying to hand solder, and a bit expensive. They are really nice, but ultimately I don’t think they are worth it. Standard screw-terminal headers would be just fine.

Next up: building the lamp+controller.

ADC Linear Gain/Offset Correction

In my last post I explored the errors of unprocessed ADC data. In this post, I will show what happens if you add some simple gain + offset correction.

Arduino INL

Gain error is caused by DNL. The ideal code width is 1, but lets say that there is a predictable DNL error of +.1 LSB- that means that for every code (and there could be a lot of codes!), the measured value strays from the ideal value by +.1 LSB. At the end of the range, the difference will be +.1LSB * number of codes. This (in the example) is equal to INL. Take a look at the INL of the arduino above- the average code width is 1.08, so the INL increases by .08 LSBs/code. Therefore the INL looks like a straight line when plotted against LSB.

Corrected INL plot for Arduino

If we just assume each LSB is a little bigger, we will get a much lower INL. This will cause us to loose a little voltage resolution (.08 LSBs), but we will end up with a MUCH lower INL. This is the same data, but with gain correction. INL was reduced from 85 LSBs to about 6 LSBs.

However, the INL can still be improved- at low voltages, the most ADCs dont work very well. This causes a large DNL at the first code, as seen here in the corrected plot. This is where the line shoots straight up from 0 to about 6 at the first ADC code.

Arduino uno adc near code 0. orange line is the end of adc code 0

On a plot of DAC input-ADC output, it looks like the above – basically code 0 has a huge width. We can null this out by just adding a small offset to all the data. That means code 0 will still be wrong, but the rest of the codes will be right. because only the first code is messed up, we can usually ignore this error because we it only exists for a single code (code 0).

MUCH better

Here is the DNL for the fully corrected ADC- the max DNL across the whole ADC is closer to .8 now, except at code 0, which is off by about 6LSBs. This is equivalent to saying we cant measure less than 6 LSBs, but that if we make these corrections we will have an accuracy that is about .00084 mV (with 100x oversampling). That is pretty darn good.

What about the other ADCs?

How much can the other ADCs be improved? The ADS already has a pretty perfect gain and very little offset, so I skipped trying to fix it. Note the new LSB size and standard deviation are different than the previous values – to avoid the influence of the extremes of the ADCs, I only used the middle 90% of the ADC range to calculate the average LSB size.

ADC NAMELost Range
New LSB size
Compensated INL
INL Improvement
Compensated INL
ESP (COMP)1971.00±.7123.8*129.2.006
*this is actually much worse

Surprisingly, the ESP and the SAMD are much closer in this comparison. SAMD still wins out, but at least the ESP32 has recovered slightly from having a three-digit INL. The one caveat here is that my calculation of code widths does not account for missing codes, which produce -1 DNL each. This data is correct if the missing codes on the ESP (of which there are 15) need to be ignored for this to be valid.