PCB Assembly Services - Screaming Circuits: Fun With Electrolytics


Fun With Electrolytics

I was fiddling with one of my robot boards the other day - popping some passives on and off and checking out subs and alternate values. I was doing this on a couple of boards at the same time. Everything was going along fine until I started to do a power-on test. The first board was fine. The second one would briefly light the power indicator LED. It would start a full brightness and then fairly quickly fade out.

My first thought was that I had been too agressive with my soldering iron and had burnt something out. (who has already guessed what really happened?). Turns out, that wasn't the case. I put it aside and came back to it a few days later. This time, I gave it the finger test and discovered that my regulator was hot. Darn. Next, I found a hot tantalum cap. Nothing looked out of the ordinary/ I stared at it for a while. The + side was on the left in both parts and... The plus side was on the left in both parts. One was supposed to be on the right. Oops. The cap had a high enough voltage rating that it didn't blow up. It just pulled down the supply until the over-current protection in the regulator shut it down.

I've heard a number of folks recommend that you try and keep all of your polarized parts facing the same way. It's not always possible, but it can certainly reduce opportunities for errors like I made here.

Duane Benson
Left, right. Left, right. Left, right. Left, left. Left, right...


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Speaking of tantalums, I had the strangest thing happen a good while ago.

I had built a filter for a computer case fan which was generating a heap of EMI, and had tacked that on - but the computer would no longer start. Since the computer was in a less than accessible location and hooked up to a flaky UPS, I had the idea to try plugging it straight into the outlet.

Well, turns out the tantalum was shorted out, and the UPS was preventing the computer from drawing enough power to actually blow it up - but it didn't actually blow any fuses in the UPS or computer PSU. Once it got what it needed, there was a loud bang and smoke streaming out of the no longer filtered fan. And some choice comments being said about it too.

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