Versatile Dummy Load
A resistive load for testing devices with high power output.

A device capable of high-current output sometimes needs to be tested under load. An unloaded amplifier, for example, may show normal voltage swing on an oscilloscope but have problems when delivering significant current. A power supply may read normal voltage unloaded but fall outside of range when delivering current.

Some sort of power resistor is the obvious choice. Of course, real-world loads are a bit like people: they have a mix of reactive characteristics and may not behave so neatly outside the laboratory. Any sort of load test is still better than none, but the next problem is that suitable power resistors -- 20W, 50W, or even more -- tend to be a bit scarce and expensive.

For this reason, incandescent light bulbs are sometimes popular as a cheap dummy load. A 100W, 115V bulb, for example, would have a nominal impedance around 130Ω when hot. A half-dozen bulbs bodged up in parallel gets to around 20Ω. But these are clumsy and fragile to work with, and as energy-efficiency mandates push traditional bulbs out of the mainstream, a more custom approach is required.

My thoughts became focused when I found a stock of 30Ω, 10W power resistors at a surplus store. Four units of 30Ω, wired in parallel, give about an 8Ω value, with a 40W free-air power handling. Then add a fan for even more fun. At this point we have basically recreated a ceramic space heater, and it might be more practical to just cannibalize one and feast upon a delicious primary element in the range of 10Ω. But with a suitable spare parts collection, perhaps the DIY approach will be cheaper.

A typical resistive element exhibits PTC behavior, meaning its resistance increases with temperature. Hence, for a parallel application, nominally-identical resistors can be combinedwithout precisely matching values. A resistor that heats faster than its neighbor will increase in resistance and the situation self-balances. This one can be explored at home: connect an ohm-meter to a resistor, and raise its temperature with a convenient heat source. The resistance will change noticeably on the display.

The other issue is versatility. If a dummy load is designed around two 8Ω banks, it can easily support 4Ω and 16Ω single-channel operation if built accordingly. It should also have some sort of current-interrupting mechanism available just in case a sensitive device is being tested. The obvious way to do this was to design a two-channel load with a good terminal block and exposed fuse clips.

My dummy load turned out like this:

Stereo dummy load
Figure 1. A medium-power configurable dummy load.

The resistors ought to be standing above the board for true free-air operation, but I wanted them to be nice and stable, and provided a 12V computer fan on tall PCB standoffs with the usual finger guard to prevent "Will it blend?" moments involving stray fingers and shop tools. The blue terminal block provides fan power.

The larger, black terminal block in front interfaces to the 8Ω banks. These can be used continuously at around 40W each, or more with the fan running, or jumpered into a 4Ω or 16Ω configuration with 80W handling. The fuses can be changed to any value or just slugged, depending on what sort of protection is needed (or not).

The base is "plastic wood" from someone's previous deck project. The substance is basically recycled soft-drink bottles compacted and recast into a dense block. Conveniently, it is heavy and makes for a nice, stable dummy load that won't skitter around the workbench, merrilly cooking and eating test leads as it goes.

And that's all there is to it. Assembly time was about two hours. The unit has variously been used for stability and maximum-power tests of amplifiers, loading tests of power transformers, and the occasional deep-cycling a nickel-chemistry battery ahead of a reconditioning charge.

There are, of course, more exotic ways of doing this (which I would go on to explore later). For the novice or low-budget hobbyist, though, this approach is an inexpensive path to owning another useful bench tool. §

WARNING: HIGH VOLTAGE ELECTRICITY IS INHERENTLY DANGEROUS AND CAN CAUSE INJURY AND LOSS OF LIFE OR DESTRUCTION TO PROPERTY. The presentations on this website are given for informational purposes only and are not guaranteed for accuracy or fitness to any use or purpose. Consult your local standards and codes before building or modifying any mains-connected equipment.
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