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Good ideas with bad execution, or good execution of what should be bad ideas - an analysis of inferior, off-beat or malfunctioning products, and how other people's failures can help us design better stuff.

Power Inverter has more than a screw loose

power-inverter-1.jpgIt’s always great to receive reader submissions for Made By Monkeys, so thanks to Mike Meakin for flagging this one – a blog post which details “a shining (and smoking!) example of poor engineering!”, involving a Power Jack PSW3500.

Mike writes:
“Just how hard can it be to design a switch mode inverter power supply? Read this article that I came across, weep, then worry.”

The blogger homo ludens electronicus begins…

In late 2008 I needed to buy a high power true sine wave inverter, for an alternative energy scheme. Scared away the very poor quality I had seen too many times when asked to repair Chinese inverters, but not willing to pay the outrageous prices asked by  manufacturers who are known for their quality products, I settled for a Taiwanese made inverter. It was the Power Jack PSW3500, which is rated at 230V 50Hz output, 12Vdc input (at several hundred amperes, of course!), 3500W continuous output, 7000W surge capability, with a true sine wave output. In addition, it has a built-in 50A charger, and the necessary detection and switch-over circuitry to make it work as an UPS, if desired.

And what follows, in detail, was an unhappy experience. No output, but lots of smoke…

Problems found include no voltage balancing, no current sensor, popped electrolytic capacitors, spills of capacitor juice, loose screws, heatsinks installed much too low, and more.

power-inv-3.jpgFor example, he continues:

And here is yet another photo [right] showing how these people tried to solve the problem of lousy thermal contact! There is not a trace of grease between the parts and the rubber. There is just air there. The grease is outside the voids.

I then did the complete troubleshooting, and found that all 24 MOSFETs were burned, with the sources open and the drains shorted to the gates. The fuses were fine, thank you. Along with the MOSFETs, the 24 gate resistors also failed. Then there are the two popped electrolytic caps, and also two overvoltage protector diodes (transil). These failed in short circuit, on the side where the capacitor still was fine.

Read the full blog post >>

Love the name, by the way – homo ludens electronicus. I had to look it up, but ludens is Latin for play, apparently – sort of ‘Man at electronics play’. I really should have paid more attention in my Latin class, instead of monkeying around…

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