I recently replaced a bad capacitor on a computer component. But after doing so, I realized I totally forgot to consider possible polarity. I just soldered the thing in, and turned on the computer, which seemed to run happily for about 30 minutes (I shut it down for the day after that).
As you can see it is 1500uF, 6.3v, electrolytic. I don’t know much about electronics, I figured it was an easy replacement but totally spaced on the polarity issue. My question is: given what I described (i.e., the computer seemed to run fine for 30 minutes), did I luck out and put it in right? Or does it not matter at that voltage? Or is it a potentially ticking time bomb and I need to fix it?
not necessarily blow up, but still possible, even likely. reverse biased electrolytic caps exposed to a voltage> 1-1.6 V risk failure of the dielectric, sometimes catastrophically. if the voltage its less than 1V, it won’t work but probably won’t explode.
electrolytics are the most dangerous however…they are amost always polarized. they are always marked with some symbol and/or have one leg longer than the other. the negative lead is the short one.
ok, thanks. What exactly am I looking for on the board that shows which end was supposed to be negative? Am I looking for a plus sign or negative sign somewhere, or some other kind of marking?
When I was in college, the Electrical Engineering courses I took had labs, one of which I had to build a DC power supply. Despite numerous warnings from the TA to be sure to get the correct polarity on the capacitor (as well as the bridge diodes. and the zener, which is supposed to be reversed), I managed to get it in wrong on the capacitor. It blew in about 5 seconds. Although I passed the lab, I did have to buy a new capacitor (which was a significant expense to a poor engineering student).
The capacitor’s package usually has a stripe or arrow containing negative signs. Something like this:
The stripe containing negative signs points to the capacitor’s the negative terminal.
The silkscreen on the PCB will usually have a plus (“+”) sign to indicate the capacitor’s *positive *terminal. So when inserting the capacitor into the PCB, the terminal associated with the stripe/arrow containing negative signs on the capacitor’s package should go in the hole that does *not *have a plus sign next to it.
on som****e PCBs, one hole is square, one is round. Negative lead to round. remember, electrons (negative particles) travel through (round) holes. admittedly, they do that in diodes, not caps, but that’s the mnemonic I use.
also, if there is you can also look for other polarized components in series - diodes, transistors, etc. this isn’t definitive but can help point you in the right direction.
what is the equipment. is a schematic available online?
Yea, aluminum and tantalum electrolytics are pretty unforgiving of reverse voltage.
In one of Bob Pease’s books, he describes an electrolytic capacitor that is impossible to insert incorrectly. The capacitor has three leads in a row. The outside leads are the negative terminal, and the center lead is the positive terminal (or vice-versa). Interesting idea, though I’ve never come across one.
Hmm. I’m reminded of the phrase “make something idiot-proof, and the universe will provide a bigger idiot.”
I can easily imagine a circuit on a solderless breadboard, with two leads going to the capacitor–you could place the third spare lead to the left or right (since the breadboard allows the leads to go anywhere), and the polarity will vary based on your choice.
I have seen tantalum capacitors made that way on some computer motherboards (386/486 types), although I wonder if that was the intent or they were connected as bypass capacitors (I find filters made the same way, consisting of two ferrite beads and a small ceramic capacitor in a three pin package, with the outer two pins power and in series with the supply and the center ground).
Anyway, as to the OP’s question, electrolytics can withstand some reverse voltage (and even act somewhat like a capacitor, if poorly), thus if the voltage was much lower than its rating it might not immediately fail, or even after a while (an oxide film can build up in a similar manner to the initial forming process; in fact, I have read you can increase the voltage of capacitors by subjecting them to a controlled high voltage). The exception is solid electrolyte capacitors, tantalum or aluminum polymer - one bit of reverse polarity or overvoltage and POOF (normal electrolytics act like Zener diodes when over/undervoltaged; that is, they don’t short but current rapidly increases as voltage rises, so a brief overload usually doesn’t permanently damage it as long as the pressure didn’t rise too high).
In some cases, especially power supplies, the ESR (equivalent series resistance) may also be important in stabilizing the regulation loop (if unstable, large oscillations in output voltage can occur); some spec sheets advise not to use low-ESR capacitors for this reason, so replacing a capacitor with a a better one may cause problems (higher voltage capacitors often have lower ESR due to a larger plate area). Also, most capacitors have a pretty loose tolerance (+/-20% is typical, have seen up to +100% tolerance on some old filter capacitors), so it might work if you use a different value, as long as it isn’t lower and too far off (except for tuned circuits and the like where an exact value is critical, but motherboards aren’t any concern here).
