Capacitor pop - why?

So... back to the old drawing board. I built a PAiA Theremax theremin and plugged it in to tune and test, etc.

The LED (attached to the power switch) did not light when the switch was turned on. Nothing, in fact, seemed to happen. I waited and watched to see if any heat, smoke, etc.

After about a minute I went to switch it off. A loud cracking POP interrupted me, though. I switched off and examined. Couldn't see where it came from, but eventually saw that the top of C1 (100 uF electrolytic capacitor) was bulged and cracked open in the top vent.

Any idea why this would have happened? And -- clearly it's getting power (too much?) -- so why did the LED not come on? Everything seems to have correct polarity for the wiring...

That sounds like exactly what happens when you put an electrolytic cap in backwards.

It can act like a short-circuit (thus no LED lighting up). In this case, it is drawing a lot of current through it (not sure all of the exact chemistry going on, but it is no longer "2 plates separated by an insulator" in these cases) and ultimately so much heat builds up inside that it explodes.

If they are installed correctly, this sort of thing never seems to happen. The most common failure mode when they are installed correctly is that they no longer provide the same amount of capacitance, so you will hear a loud hum in your audio output (presuming they are power supply capacitors).

Thanks... I was baffled, because I thought that may be the reason; I double-checked for correct polarity on all electrolytic capacitors and diodes before even plugging it in, and I checked again after the mishap.

Scott Lee (customer support for the Theremax and a super-helpful and awesome guy) responded to my email with the same information as you, but more insight... he said it sounds like, instead of the capacitor being backwards, that the power supply polarity wiring may be reversed (having the same effect)... this makes more sense, because the wall-wart they provided had no indication of whether the white-striped wire was + or - (it only indicated the tip/sleeve polarity for the connector, but that gets cut off to make the Theremax, and the wires get soldered in)

I assumed that they used a black=+ configuration, but maybe I was wrong, so I will probably need to disconnect the power wires and test them out to see the polarity. (and replace the capacitor)

You mean, you really soldered the power supply wires without checking the polarity before??? :rolleyes:

Yep... being only my 2nd electronics project, not having a good way to test the polarity (I should've waited until I could obtain an LED, as the one I was supposed to have was missing) and doing research which ALL said that the positive wire is always black, I assumed it was the black wire.

Lesson learned, though.

DMMs (digital multimeters) have gotten very inexpensive these days. I have seen them as low as $3.00.

You don't need every whistle and bell on it. It just has to be able to measure DC and AC voltage. For theremin troubleshooting you need 0-2Vdc and 0-20Vdc ranges. And the ohmmeter feature is of course handy for tracing wiring with the power off.

I hadn't planned on doing a lot of electronics work, so I didn't invest in one, but that is a very good suggestion to get the multimeter (especially if I plan on modding the Theremax later -- which I do -- or for troubleshooting if something else goes wrong... which very likely could happen)

Even if you will spend 20$ for a multimeter, it will be less expensive than grilling most of the semiconducturs in a theremax...

Yep... I'm lucky that the reversed-polarity shouldn't have harmed more than the one capacitor; in a different project, it may have fried practically the whole board.

So... thanks for the advice, guys. And I just ordered a $10 multimeter.

I just had the same problem. Only in my case the LED came on solid, and bright. In a few moments, I started to hear a sizzle, and noticed the C1 Capacitor top bulging. I touched it, and it was hot enough to burn my finger nearly instantly.

I switched off at once.

The C1 is installed correctly, as are all other components. There is no bridging, or shorting.

The Wallwart leads are connected with the black to pluss.

According to the manual, there was supposed to have been a note that came with the power supply to indicate polarity. There was none. Guess I made the same mistake of assuming which was which. Stupid thing is, I have a multi meter right here.

My board is revision D. I wonder if something in this revision could be incompatable with one, or more parts.

Guess it's time to extract the burned out capacitor, and board.