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Cloth Ears 4th October 2007 02:57 AM

How would you test a capacitor?
I've got some big caps (33,000uF) that are now just sitting around, as I've decided I'm too much of a wimp to play with real electricity. They're reasonably old, but they're made by RIFA, so they might still be OK.

I've read some stuff about charging and the time it should take. For instance, if I use a 10v source and a 1000 resistor, it should take 33 seconds to get to 6.32 volts. And I guess if I use a 60v source, then it should take 33 seconds to reach 37.92v (give or take), except that I would know where to locate a 60v source.

What else should I be testing? Maybe how long it takes to discharge if they're not connected to anything - if so what parameters should I use (how much percentage loss over how many days/hours/minutes is acceptable)?

If they're any good, then I'll pass them on at cost, but I'd like to be able to check them all just to make sure I'm not selling any lemons (no, they're not yellow...:)).

Mooly 5th October 2007 07:02 AM

Hello Cloth Ears,
Sorry but you just can't test electroylitics this way.If they have been unused for long time you can gradually bring the cap up to it's working voltage SLOWLY by charging from a current limited supply.Then discharge via a 5K 2watt or similar resistor.NEVER short out directly.Doing this helps re-form the dielectric.As for checking value, what you really need to do is measure the E.S.R. This is the surest way to check "goodness".Think of it like a battery, a flat 12volt batt will still read 12 volts but its internal resistance is so high you can't draw current from it.Capacitors rarely fail through change in capacitance in fact a +/- tolerance of 50% is normal.

AndrewT 6th October 2007 01:13 PM


Originally posted by Mooly
As for checking value, what you really need to do is measure the E.S.R. This is the surest way to check "goodness".
you could try measuring leakage current by monitoring voltage drop across a large value resistor and with a steady DC voltage applied across the series combination of C+R.
You can check how long it takes to discharge from fully charged to 80% of full charge into a fixed resistor. The longer it takes the more capacitance it has. hint: 1F takes 1S to fall 1V when 1A flows. eg, 0.01F (10mF) takes 10S to fall 1V when 1mA flows. The leakage must be well below the test discharge current for this to be valid. The DMM across the supply must have a high impedance compared to the test resistor to make this meaningful.

How do we measure ESR with our meagre resources?

Mooly 6th October 2007 05:54 PM

Hello Andrewt
You're quite right saying how do we measure e.s.r. easily or cheaply, and there is no easy answer for the average
Charge/discharge currents require a constant current source/load to be meaningful.I have worked on more faulty switch mode psu's than I care to remember, and the main failure mode is "dried up electrolytics" in other words a high e.s.r. These components always show their rated capacitance value when read on a capacitance meter.However there is one simple way to compare known good capacitors, with suspect.Connect your cap (100mfd perhaps) in series with a suitable resistor say around 100ohms and drive it from a high frequency (5 to 20 khz for e.g.) square wave source.Increase the drive and monitor, the ripple voltage directly across the cap on a a 'scope.This will be small, in the milli volt region.Good quality caps will be noticably better (smaller ripple) than inferior one's.Now to show what a poor e.s.r. looks like add a resistor, 1 ohm say, in series with the positive lead to the cap.So we now have 100ohm + 1 ohm in series with 100mfd.Measure again the ripple across the cap but this time connect 'scope to junction of resistors.The increase in ripple is caused by the higher e.s.r. Taking this further if a 10,000mfd cap had 1 ohm e.s.r. it would be appalling, but I suspect that trying to measure charge/discharge currents would show nothing amiss.

Cloth Ears 8th October 2007 11:05 PM

Thank you gentlemen, much appreciated. Sorry I didn't respond earlier - I must have missed the responses.

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