Test BIG capacitors? 25v/47mf(47,000uF) - Page 1
Forum section: electronics / beginners

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: September 16, 2015, 07:25:57 pm
content: Ive got an old power supply and once another member explained to me what it was and how the transformer in worked I want it to keep it working in its original fashion.https://www.eevblog.com/forum/projects/old-psu-i-like-the-case-restore-or-makes-something-new/msg753234/#msg753234The link above has pics of supply and explanation of why it was thought to be "bad" although I dont think I ever mentioned at some point the original switch was replaced with a rocker that doesn't lock in to position. I plan to rewire everything and have the case powder coated. I love the case and what the supply does is definitely usefull.To the point I pulled the big black cap out to test, unfortunately my meter doesn't go to 47mF only 40mF.... I have charged it with a 12 volt supply and it blew some huge sparks when I shorted the leads with a long screw driver. To me the cap looks fine but it is one of those caps with a plastic cover on the top so im not sure if you can tell by looking at it, if its starting to bulge or notThe point is I can only find 1 105c cap on digikey this size and its about 40 bucks, So if the cap has to be replaced im going to parallel a bunch of 4400uf caps togather which would give better ESR anyways, but I have other things to spend money on. So whats everyones opinion on this cap? #2 Im still waiting on my real siglent scope but I have a PC scope and function gen that will go to 1mhz before distorting the signal gen may acually go way higher but I dont know what the waves look like after 1mhz my meter will read up to 5mhz out of it.So is there anyway to test this cap like shoot the signal gen into the + side and use the scope to count what comes out the -? Or would I just be AC coupling and blocking all the wave forms. I know that there is some way people do this with pico farad caps.

author: Fank1
author_rank: Regular Contributor
timestamp: September 16, 2015, 10:38:38 pm
content: That's a quite large cap. Just totally discharge it check it with a voltmeter, then charge it from a 12 volt supply thru a 1000 ohm resistor.Using a stop watch and a voltmeter on the capacitor measure how much time it takes to charge to 7.56 volts.The capacity ( in farads ) is that time divided by 1000.P.S. It looks good to me.

author: singapol
author_rank: Frequent Contributor
timestamp: September 17, 2015, 03:12:28 am
content: Do not use screw diver to short/discharge capacitors,you can damage them. In this case the cap probablysurvived. You can use oscilloscope to test and calculate capacitance if you don't have a meter that reads such a big value.Search youtube for the tutorial. Get the UT-139C multimeter, it can measure up to 100K uf.and is true rms for AC voltage up to some limit of frequency. It would be better to use one cap than pararelling several caps even if it's 85 degree Celcius. OK... 5 X 10000 uf? Easier if it's 25V rated.BTW use 1K - 10K ohm 10 watt white ceramic resistors to discharge. If it becomes slow ( to discharge, assuming the majority of the capacitance is discharged. )them then use a lower value while monitoring with a multimeter. It should be safe once the voltage gets to below 5 volts. This way you avoid thev dramatic spark discharge in exchange for a liitle spark.

author: macboy
author_rank: Super Contributor
timestamp: September 17, 2015, 01:21:27 pm
content: Measure another large cap within the range of your meter, record the value, then place this one in series and measure the series combination. Then calculate the value of this one (caps in series combine like resistors in parallel: 1/Ctotal=1/C1+1/C2). The value of the second cap should be as close to the limit of the meter as possible to reduce errors.You should also measure self-discharge, which will likely improve after a proper re-form. You can measure ESR too if you think it might be a problem. However, if you got a big snappy spark, it likely isn't.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: September 17, 2015, 05:59:21 pm
content: I think franks idea is good, I thought about using an RC rime constant. I just bought a UT71D and UT61E(which I screwed with acetone) so no more meters! The 40mF it can measure is MORE than enough on a regular basis!Im curious if it were bad, or even if I just wanted to why not parallel 10 4700uF caps? Ive always been taught this will give much better performance/esr in psu circuits. Whenever I can I parallel caps instead of using one big one (when it comes to electrolytic type) I.E if a linear regulator wants a 22uf electro Ill usually just parallel two 10uF caps... If the regulator requires 10uF or under I just throw an x7r Ceramic or tantalum cap in instead, unless the data sheet specifically calls for an elctro to stop oscillation. Are these bad practices?