Gotcha's - electronics is full of them. The little disasters that jump up and bite just when you've finished your board, checked everything, and are absolutely, positively, definitely, certain that nothing go wrong...
I've been working on a little waveform-generator PCB. It needed a 9V supply, and I grabbed a cheap one off Amazon.
Being the suspicious type, before plugging it into my board I stuck the multi-meter on it: Yep, 9V. We're good-to-go...
I plugged into my PCB and... pop! There go the electrolytic caps.
So what went wrong? Well, wall plug supplies are overwhelmingly wired to have the jack plug inner-pin positive, and the outer plug-shield ground. 'Overwhelmingly' expect for mine that is.
I'd checked I was getting 9V but hadn't paid close attention to the polarity. To be fair to the manufacturer (and just to double my embarrassment /irritation) I noticed afterwards the polarity was printed on the plug casing, but again I hadn't paid close attention. -9V was all my board needed to blow its caps, and destroy the main IC. Grrrrrr.
10mins soldering and I've swapped the wiring to give me the polarity my project needs. It still leaves me waiting a week for replacement parts however. Double-grrrrr.
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