#electronic-load

I`m going to use the esp8266 modules I made in the previous post. Esp needs 3.3v communication so straight COM port voltage levels will not work here. After a closer look at the PCB near the communication port, we can see the isolation line going thru some chips. U23, PC10, and PC1 are used for digital signal isolation.

U23 is 1201ARZ which is Dual-Channel Digital Isolator and is used in the UART path here. This IC is responsible for protecting the device from voltages that might be present on communications port. The separated area has its own isolated power supply.

That is a good thing to know and we going to use this power source so that our communication hack won`t upset the device itself. Following the traces showed that this zone has its own regulator, rectifier, and even transformer winding.

A capacitor on the edge of PCB is used for the communications power supply.

It has about 18V DC unregulated so it can be used with esp/buck module. Note that I connect to the power before regulation as I don`t want to put extra heat on the small regulator for communications area which is not meant to drive such spiky loads as esp.

After dealing with the power problem I connected RX and TX to the isolated side of U23. For that, I reused 4.7K resistors what was used as pull-ups. Now, these resistors would be used for current limiting on data lines in case existing communication circuitry would do something funny.

Now I can place esp module in the front plastic grill as other parts of the case are made from metal.

And glue wires in place to stop them flapping in the breeze.

First things first. I took the load apart to inspect everything and replaced the heatsink so that fan could be mounted in a different orientation. Not sure if it helped the cooling but with this setup load still can pull 35W for an hour or so.

I started by adding a buck regulator for a 3.3V rail as the power supply I used for the device was 12V.

After carefully adjusting output voltage I added the ESP01 module and soldered data connections to the board. Everything was attached by double-sided sticky tape.

And a layer of Kapton tape to insulate from shorting out to the display board.

Everything fits nicely in-between two boards and can be mounted back to the original case.

I designed and laser-cut a simple box so that it would resemble a real bench instrument. Before putting too much effort I decided to test thermal performance in the box. The results were less than 80 degrees C after about an hour of 35W dissipation.

I finished sanding the box.

And assembled everything inside. The thing in the green heat-shrink is a mains power supply to 12V.

Power socket with a real fuse and the fuse is rated for a load (that’s rare in DIY projects).

As the hardware part was finished I started working on the software. I was interested in testing the real capabilities of various cheap buck and boost converter modules.

I definitely could just adjust the load, measure voltage, write it down somewhere and repeat until I have all the measurements. Then I could put the data in some kind of spreadsheet and generate a visual representation of current and voltage correlation.

Buuuuuuut, there’s a problem - it sounds like a lot of work and I should repeat that for every module I want to test and maybe for several times if I want to make sure I made no mistake taking so many measurements.

Did I mentioned that I hate “a lot of work”? So I decided to do “a lot of work” and wrote a utility that lets me automate “a lot of work”. Here it is: https://github.com/makerspacelt/fz35-cli

Now I can simply connect buck or boost of interest and start the test with a simple command after which I get my results plotted on a nice graph with a resolution of my choice.