Sebastian Harnisch's Slick Programmable Power Resistor Handles Up to 400W

Built to help with calibration of a power source and analyzer, this impressive piece of bench equipment can cope with the load.

Gareth Halfacree
1 month agoHW101 / Debugging

Maker Sebastian Harnisch's bench equipment collection has grown again with the design and development of a programmable power resistor, capable of handling a 400W peak — and, once again, the result is something that looks like it could easily have come from any one of the big test equipment makers.

"For the performance checks and calibration of my Agilent 6811B AC Power Source/Analyzer (375VA) I need a 20 Ohm resistor with a ton of power handling capability," Harnisch explains. "The original plan was to mount 8 wire wound 50W resistors on a heat sink and be done with it. Then I thought: why not reuse all the code I've already written for the programmable resistor and build a programmable high power resistor that can complement my DC load for AC applications."

As Harnisch writes, back in July last year he showed off another programmable resistor design, which he self-deprecatingly described as a "glorified decade resistance box." This provided the base for his latest creation, which takes the same concept — a digital device that can be programmed to deliver the required resistance on-demand — but offers support for a high electrical load.

"In this application the focus is on power handling capability, not so much a high resolution (high number of resistance values) or precision," Harnisch explains. "This is why the topology used in the programmable precision resistor is not as suitable for this application as it has been before. Also my ELMA cases are really small (and I mean really small, so thermals will be a challenge on its own), so I can't simply increase the number of power resistors and relays in the process. That's why I'll stick to the original plan of using eight power resistors."

The device itself is split across two boards: a relay and interface board, which includes the relays required to switch the current through the required combination of power resistors to deliver the chosen resistance value; and a user interface and controller board, which an STMicroelectronics STM32G441RBT6 microcontroller — the only one in the build. Like Harnisch's earlier desktop thermometer, it also provides a Standard Commands for Programmable Instruments (SCPI) interface over USB, in addition to the programmable resistor's front-panel controls and on-board display.

"I have some ideas for a second revision with some minor improvements and adaptations (like integrating the PWM-to-voltage circuits), but also one new feature: A simple voltage measurement," Harnisch writes of a potential upgrade to the design. "This not only allows for a handy display of the voltage, current and power, but also additional fan control options and protection features."

The project is detailed on Harnisch's blog in a series of posts going back to early March this year.

Gareth Halfacree
Freelance journalist, technical author, hacker, tinkerer, erstwhile sysadmin. For hire: freelance@halfacree.co.uk.
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