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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
Without using any expensive special devices, is there a way to tell if a capacitor is good? I am able to verify with a capacitance meter that the reading is correct, but I am not sure that it is properly holding its charge. The uF rating is low enough that it loses charge quickly. Would I have to test it on a scope to monitor the charge decay, or would this still not be sufficient?
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
A capacitor will have three main defects (there are others), series inductance ESL, series resistance ESR, and shunt or leakage conductance causing it to "lose its charge". Only th elast one will tend to change over time, the first two are "by design".
If you really want to test the leakage, and the voltage drops too fast into your test gear, then charge it and leave it disconnected 10s then measure, and charge it and leave 60 seconds and measure. Any significant difference in the kick when you connect it indicates leakage.
You can get some indication of the ESR by connecting an inductor, and sweeping the frequency response with a scope and signal source - I don't know whether these count as special or expensive for you. Be warned though that for a halfway decent capacitor, most of the circuit loss will be dominated by the test inductor, not the cap.
You can get a indication of ESL with several inductors of different value, but again the caveat applies that you will probably be measuring the test inductors and the test setup more than the capacitor.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
I consider three main parameters for capacitors; 1) Capacitance - if the capacitance of a film cap decreases it indicates it has 'self-healed' many times - if the capacitance of an electrolytic decreases it indicates loss of electrolyte.
2) Series Resistance - for electrolytics an increase in esr is a good indication of end-of-life - film caps rarely change esr
3) Parallel Resistance (leakage resistance) (best considered as leakage current) - the leakage current of an electrolytic increases when left discharged, ... after a few minutes at rated voltage the leakage decreases - film caps usually only leak after 'self-healing' ... polystyrene capacitors sometimes get 'leaky' due to moisture ingress.
You can check the leakage current of a film cap with a dmm on ohms range the dc resistance of a film cap should be MOhms to infinity.
Larger film caps used as snubbers or across a dc bus are worth checking at or near rated voltage, at work I use a 'megger' for this.
The leakage current of an electrolytic can be checked by using a voltage supply of less than or equal to the rated voltage, charge the capacitor via a 1k resistor, leave it on charge for a minute or more, then measure the voltage across the 1k resistor to determine the leakage current.
Ah! .. Dr. Slack beat me to it! (an early-riser too)
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061
I have a very expensive capacitor here which appears to be bad. Its a 215uF 2.5KV one, but it won't hold any charge at all. Yet it measures perfectly well on my LCR which is very strange. Ideas? It was going to be used for a TC driver but as it failed initial tests there wasn't much point.
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061
Can't tell, but appears not. I know that charging it up using six CCFL transformer units in parallel didn't work at all, highest voltage was around 70V.
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Is the capacitor good?
