Tantalum Capacitors – Swapping Polarity

Learned the hard way why my power supply circuits weren’t working.

Here’s the circuit:

Pretty straight forward, 12V in 3.3V out. Built a couple of these before and they all worked without hassle. Not this one!

Switches on, works for 1-2 seconds and then the voltage drops over the input terminal and the LDO (LM1117) starts heating up. The same happened on a regulator (78L05ACP).

What’s wrong you might ask? After fiddling a bit with the capacitors, I found that they would start heating up when you swap them in and out of the circuit. Turns out I had the polarity wrong. Haven’t used SMD components a lot and on SMD caps the solid line usually indicates the positive terminal, unlike through-hole caps where the line is on the negative terminal’s side.

These were tantalum; guess if it was electrolytic stuff would have gone pop. Interesting symptoms though, a reverse polarity tantalum starts off okay and then avalanches into a dead short. This in turn heats up the capacitor and if it is the capacitor at the output terminal of the LDO, the LDO also starts to heat up pretty quickly. Doesn’t look like there was any permanent damage done. The LDO has built in short-circuit protection and the caps didn’t show any signs of wear and still tested 10uF after multiple rounds of heating up.

All sorted now. Power supply up and running. Now back to the original project: Temperature regulated polystyrene cutter.

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