Building an Electronic Load

One of the tools that I sometimes need but haven’t bought yet is an electronic load for testing circuits such as power supplies. Of course you can make do with fixed resistors (and I have so far), but you practically never have the right value/wattage to hand when you need to test something (in my case, usually on a Sunday afternoon…grr!).

The solution is a programmable electronic load, or basically an adjustable current sink (or a “reverse power supply” as some people call it) that can simulate the load from any fixed resistor within reason. I haven’t bought one of these yet, partly because I didn’t feel I needed it enough to justify the expense, and partly because I am rapidly running out of space to store instruments that are only used occasionally.

Some weeks ago I started toying with the idea of building one myself to at least get started. I’d seen some nice designs on Tindie, but wanted something that was capable of higher power and something which I could more easily tweak for myself. A bit of googling turned up a few promising pages, most notably this one on Kerry Wong’s (excellent) blog.

I liked Kerry’s design as a starting point, mostly because it is relatively well documented and the control code is Arduino (which I can work with). I therefore started revising the circuit to suit my needs and laying out a PCB for it as well.

Key changes from the original:

  • I’ve scaled it down from three pairs of MOS-FETs to two because that is what I could fit on the Eagle board (being constrained by the freeware version).
  • I’ve replaced the parallel LCD connections with I2C to simplify the PCB layout and free up Arduino pins.
  • I’ve mounted the controller (an Arduino Nano v3) onboard. That wasn’t the original plan, but the space was there so why not?
  • I’ve broken out a pair of analogue Arduino pins, a pair of digital Arduino pins plus a second I2C-connection that can be used for other purposes. Top of the list for me would be a real-time clock (RTC) module for data logging purposes and some sort of thermal sensing and fan control, but I am sure there are many other potential uses (web-interface anyone? 😀 ).
  • plus a bunch of other minor tweaks 🙂

This is still work in progress, but I have received the prototype boards in hand and I have started the assembly as you can see from the pictures. Still to do:

  • Do the mechanical work on the heat sink (in progress)
  • Rewrite the software to work with the I2C-display (also in progress, but might take me a while)
  • Test whether the damn thing works! 🙂

EDIT 13th March 2016: The schematic for my v1.0 PCB can be found here. There are at least two know issues that need to be corrected. 1) The “sense” pin (A3) is connected directly to the output but should actually have space for a voltage divider. 2) The spare opamp (IC1B) should have pin 6 and 7 connected together and pin 5 connected to AGND.

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7 Responses to Building an Electronic Load

  1. Rob B says:

    Nice work. I too am building the Wong load. I came here after reading of your exploits on the site. I have not gone so far as a PCB since I want to keep my options open and to nail stability. In testing I found stability can be an issue so if you have one be sure to test using a oscilloscope under a range of load scenarios.
    One thing I notice with your prototype is the power resistors. Ideally these need to have the lowest temp coefficient you can afford and be well cooled. Hot high tempco resistors will lead to inconstant performance if you are chasing some sort of precision. In an effort to really kill the tempco issue go overboard on the resistor power ratings. Oh and as to your question on the A5 analogue input, don’t forget you will need a voltage divider to keep the volts into the Arduino at under 5V. One of the ideas I was contemplating was to implement an auto ranging divider network to maximise resolution at all load voltages.
    Good luck with the build.
    Rob

    • theslowdiyer says:

      Hi Rob,

      Thanks for stopping by and thanks for your comments. Good point on the resistors. The ones I have chosen are 5W, but actually it is possible to fit up to 10W types in the same footprint. For the kind of precision I am aiming for here I think that should do 🙂

      As for the A5 divider, it’s one of those things that is easy to overlook in an existing schematic (because I tend to believe those…) but of course very logical when you think about it. I can airwire something for testing purposes, but then i’ll have to think about a way to implement it on the board as well.

      /U.

  2. Tim says:

    You did a great job. I was searching for a electronic load and found your design. Very compact. Is it working? I could not any further Information about your design on your website. I would appreciate any Information to rebuild your load.

    • theslowdiyer says:

      Hi Tim,

      I haven’t actually finished this yet, but I hope to get to it during my xmas break. There are a couple of issues I know about with the board and probably a few that I don’t know about yet, but the biggest challenge for me will be converting the software. I will update the site as soon as there are any news to report.

  3. Justas says:

    Hi, have you finished this project yet? I would like to see schematics of this board.

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