[Syonyk] has been acquiring some large load banks to test power supplies and battery packs. These devices consist of a big current sink, a measurement device, and a fan. He picked up two similar-looking boards from the usual Chinese sources, both rated for 150W, both for about $30. Upon closer examination, though, he found that one was really a bargain and the other was likely to blow up.
The loads are rated for 60V and as you can see from the photos, appear virtually identical at a glance. They offer a configurable cut-off voltage and even use 4-wire measurement to avoid problems with voltage drop through the power cables.
Tearing the devices apart, he found an Intel 8051-style microcontroller. The unit that [Syonyk] liked agreed with his bench power supply by about 0.15% which is actually quite good. The other unit — the one he calls a fake — came in at just under 5% off. In some cases, the measurement was almost 10% off.
Ok, so a $30 device is 5% off a bench unit. That’s not a surprise, and you might wonder if he just happened to get a randomly very good example and the 5% number is more typical. But that’s not the case. It turns out that the fake unit also randomly shorts out the load which — mercifully — blows the fuse. It also appears it might be related to trying to use it without the two additional voltage sense wires — something the good until can do, although it reduces accuracy, of course.
Close examination of the board shows the two units are slightly different. The good board has some markings and differences in traces. The fake units also have a different firmware with fewer features. It is possible that the “fake” units aren’t fake at all, but just an older revision with some firmware issue that was later fixed. However, if you are in the market for one of these it would be wise to get the seller to verify you are getting the newer board.
[Syonyk] also did a review of a different load bank that didn’t work very well, either. He calls it the “MOSFET destroyer” which might be a new class of test equipment.