capacitor question- multimeter is showing a negative voltage - Page 1
Forum section: electronics / beginners

author: kq702
author_rank: Newbie
timestamp: November 05, 2012, 09:56:32 am
content: Hi I have a bunch of small computer capacitors that I harvested from dead motherboards (so its posable some may be bad allthough none have swelled yet) and I was going to test them with my new ESR meter, but the manual for the meter says to make sure the capacitor is discharged first. So I hook them up to a voltage meter and most show very low voltages, lower then 0.1 volts.But a few of them show negitive voltages, such as -0.02 volts. I have them hooked up so the black terminal is on the side with the - signs printed on the capacitor. So since it has a (-) value does that mean it is bad?Also if one of the capacitors has a small ammount of voltage and has to be discharged, I do it by touching the leads with two speaker terminals (its a very small cell phone speaker that is used for discharge purpose only). Is that a safe way to discharge?

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 05, 2012, 10:08:20 am
content: Quote from: kq702 on November 05, 2012, 09:56:32 amyes but not for big capacitor

author: kq702
author_rank: Newbie
timestamp: November 05, 2012, 10:32:43 am
content: ok, it shows -0.02 V and is sanyo branded, rated at 10 V 1500 uF. Would you say that is big?

author: digsys
author_rank: Unknown
timestamp: November 05, 2012, 10:34:53 am
content: Quote from: kq702Not really, down that low I wouldn't worry about it. A simple quick test is to use the ohms test method. (Use lowest range)The OHMs range generates a fixed voltage / current to calculate resistance. Connect +/- to the capacitor. You should see asmall "kick" (maybe), starting at around 0ohm then, then the resistance increases as the capacitor charges. Once it hassettled, reverse the probes. You should now see a big "kick" and the same process, starting from a negative value.This indicates that the capacitor is probably OK. It's an old quick test method.Quote from: kq702If you don't care what happens to the speaker, you'll probably get away with it, but it's not much effort to do it properly !Make up a discharge kit. Soldersay6" leads to asay1ohm-10ohm 1W-10W resistor (or make a couple different ones)Soldersay2-3" stiff wires to the ends (if you have old probes, even better). Heat shrink all exposed and soldered parts.That way you always have a safe discharge set. Note: Always calculate how much power the resistor may need to dissipate !!

author: kq702
author_rank: Newbie
timestamp: November 05, 2012, 12:34:30 pm
content: Good idea- Making the discharge kit seems like a much better way to go then using the speaker.What would be the ideal size resistor to use? (just wondering because the ranges you gave seemed pretty wide). The highest voltages I would be discharging are capacitors that hold up to arround 16-20 volts.

author: digsys
author_rank: Unknown
timestamp: November 05, 2012, 01:21:55 pm
content: Quote from: kq702Start with a 10ohm 1-3W. The resistance only sets how FAST you discharge the caps. 1ohm will dump them damn fast. Even 100ohm is still fast enough.It becomes more important when you get up to 30,000-50,000uF 50-100VDC.Make sure only a ~few mm (1/8") of the discharge tip is exposed. That way you can fish around inside equipment safely.

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 05, 2012, 04:25:01 pm
content: Quote from: kq702 on November 05, 2012, 10:32:43 ammedium, not enough to burn your speaker coil when fully charged. big is something like 330V 1500uF.

author: c4757p
author_rank: Super Contributor
timestamp: November 06, 2012, 12:41:21 am
content: Quote from: kq702 on November 05, 2012, 09:56:32 amBTW, the (-) mark doesn't somehow mean that sidewill benegative, it just means it should be used as negative in a circuit. (A reverse bias damages the oxide insulation layer that is formed electrochemically in the capacitor) By the time it's been harvested from a dead board, there's no telling what voltage it will have on it.

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 06, 2012, 04:53:42 am
content: since nobody chime about reverse polarity, let me give my shoot. few milli -ve voltage register is not an indicator the cap is damage or not, there must be slight chemical tolerance that resulting that. the proper way to test a capacitor is using capacitance meter (mode in DMM) and esr meter, better if you have proper and more expensive LCR meter.