A Simple Yet Feature-Packed Programmable DC Load

If you’ve got the hankering to own a lab full of high-end gear but your budget is groaning in protest, rolling your own test equipment can be a great option. Not everything the complete shop needs is appropriate for a DIY version, of course, but a programmable DC load like this one is certainly within reach of most hackers.

This build comes to us courtesy of [Scott M. Baker], who does his usual top-notch job of documenting everything. There’s a longish video below that covers everything from design to testing, while the link above is a more succinct version of events. Either way, you’ll get treated to a good description of the design basics, which is essentially an op-amp controlling the gate of a MOSFET in proportion to the voltage across a current sense resistor. The final circuit adds bells and whistles, primarily in the form of triple MOSFETS and a small DAC to control the set-point. The DAC is driven by a Raspberry Pi, which also supports either an LCD or VFD display, an ADC for reading the voltage across the sense resistor, and a web interface for controlling the load remotely. [Scott]’s testing revealed a few problems, like a small discrepancy in the actual amperage reading caused by the offset voltage of the op-amp. The MOSFETs also got a bit toasty under a full load of 100 W; a larger heatsink allows him to push the load to 200 W without releasing the smoke.

We always enjoy [Dr. Baker]’s projects, particularly for the insight they provide on design decisions. Whether you want to upgrade the controller for a 40-year-old game console or giving a voice to an RC2014, you should check out his stuff.

14 thoughts on “A Simple Yet Feature-Packed Programmable DC Load

  1. I love the VFD display choice.

    Anyone know where you can get the translucent material to cover those VFDs so they look really badass? Particularly the type/color that gives the “silver” look to the digits…

        1. Ah, OK. I’ve seen those white ones, which I understood to be a combinations of phosphors to produce white (like a “white” fluorescent tube), though they may be (as [RW ver 0.0.1] suggests) just a complementary filter over a standard broadband phosphor.

        2. I’ve found these films available on Ebay: not only white, but also yellow, orange and blue. Seems to be only a filter film. Looks a lot like professionnal lighting filter films.

    1. Noritake makes color filters for their VFD displays. They aren’t the metallic ones you’re talking about, but they do greatly improve contrast and readability.

      There’s a simulator to figure out what it will look like at the link below. Unfortunately, Noritake rarely has these in stock.

      The one that offers the best contrast is the blue-green one. It’s closest to the color spectra the VFD emits. The other colors work well, but they will attenuate the color of the VFD to greater degrees. This means you have to run the VFD at a higher brightness, which means its brightness will degrade faster, and it will be more susceptible to burn-in.

      I bet you could use stage-lighting color filters to get the same effect.. Maybe laminate a stage color filter between two thin glass microscope slides?

      If you really want to achieve a metallic color effect, perhaps you could source some thin glass/plastic that has a semi-reflective coating on it.

        1. Check out your local DJ house: full sheets of filters of virtually any colour can be had for around $10. They’ll often have a Rosco or Lee sample pack for free or cheap that has a huge variety of gels that are big enough to be useful for this purpose. B&H Photo has had them, as has Amazon too.
          I have not checked out Rosco, but Lee publishes filter curves for most of their filters on their website https://www.leefilters.com/lighting/colour-list.html

    2. My gut was in a good mood after lunch, so I asked it, and it says it reckons a light orange filter would trim back the blue-greeny component a bit to make it more monotone silvery.

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