Homebrew a DC Power Supply Load

I thought you all might be interested in a recent thread on the HamRadioHelpGroup mailing list. One guy had just purchased a 40A power supply and wanted to be sure it could supply the full 40A before he connected his transceiver to it.

He sheepishly described his first attempt—using a 100A car battery tester as a load. Predictably, it tried to draw 100A, and the power supply promptly cut the voltage so as to not overload the supply. I told him that that was probably a pretty good test. He now knows that his supply will not burn itself out if he puts too big a load on it.

He then thought about using headlamps as the load, but couldn’t figure out how to do that exactly. I said that might not be a bad idea, and gave him some advice on how to figure out how many he needed. Headlamps have a power rating, just like any lightbulb. You can then figure out the approximate current draw with the equation

I = (power rating)/13.8

Connect as many in parallel as you need to draw 40A.

Rod, KD0XX, then pointed us to this homebrew power supply load (pictured at right). This is basically a homebrew, air-cooled, power resistor. This is a fairly ingenious solution, imho.

The solution that the guy finally adopted is this variable load battery tester from Harbor Freight. It’s certainly a more versatile load than the homebrew version, but at 50 bucks, it’s also a lot more expensive.

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