Leaking Electrolytics

Well, the 'perfect cap' in series with an ammeter will cause an initial surge of current which will taper off over time and eventually settle at zero. But a leaky cap will continue to pass a small amount of current, I would assume, indefinitely? Again, I would guess that a real-world cap in good condition would continue to pass a tiny amount of current - a negligible amount?

Good point.

It wasn't able to measure the capacity at all. It did report a very low ESR commensurate with a large value electrolytic. But that was all it was able to measure.

Indeed.

Just spotted the meter only covers up to 22,000uF! we

How would you test for leakage, then? Those Peak meters aren't cheap and they've never let me down before. I suppose technically this one hasn't let me down either since I was attempting an out-of-range measurement. Perhaps the later models have expanded ranges; I'll have to check....

The better capacitor testers will test the capacitor under its rated voltage, especially the electrolytics.

If they have not been used is a long time it takes the rated voltage for them to reform. The factory usually forms them at a higher voltage than they are rated for.

The Peak metrs are only expensive because of the name. China has the same thing on ebay for around $ 20. The China ones have multifunctions instead of just one or two the Peak meters have. That is so Peak can sell you two or three meters that are really the same but functions missing.

What you have wrong is the imagined association between leakage and ripple.

The "ripple" on the voltage across a capacitor reflects the charge that is being taken out of and fed into the capacitor over the mains cycle. More capacitance means smaller ripple.

Leakage is just the current flowing through the oxide layer on top of metal conductor surfaces inside the electrolytic capacitor. If the capacitor has been un-used for a long time, some of the oxide layer may have diffused away, making the capacitance and the leakage current higher. Applying the working voltage will re-form the oxide layer by making it a bit thicker.

If an electrolytic capacitor hasn't been used for some time, it's going to be leaky. If you want to measure how leaky, measure the direct current flowing through the capacitor as you increase the bias voltage across it. It should drop as the oxide layer re-forms, perhaps over hours.

Eh? I did say leakage was probably about the least likely cause of ripple!

But then how do you determine - given that electrolytics come in all sorts of votlage and temperature ratings, capacitance values etc - how much leakage current in each case is "too much" leakage current rendering the cap unsuitable for use?

Seems pretty definitive. Thanks, Liz.

That's not how cap testers work, though. They take virtually none of the following into account:-

No circuit involved. The question relates to generic smoothing caps of linear power supplies.

Where in a circuit is a "smoothing" cap? Oops, sorry, there is no circuit.

The error comes from imagining that the two were connected.

This might have been the information you needed, if you knew enough to process it. The placement of your response suggests that you don't.

Capacitors usually come with manufacturer's logos and part numbers.

Google should be able to find you the relevant data sheet. For a given size, voltage rating and technology a broad-line distributor can usually throw a similar part, and it's data sheet. <snip>

Yes, there's no circuit. Marconi/Aeroflex never published one and and a lot of their designs are still under wraps. It's a real PITA.

How's the weather on Planet Sloman today, Bill?

Your remarks were so trite they didn't bear a response. Everyone here knows electrolytic caps need to be re-formed if they've been unsused for any significant length of time. It's vintage scope repair 101 and even you must be aware restoring vintage scopes is my prime interest in the subject. God knows I've posted enough about it over the years!

But you didn't mention it in your original post, and clearly don't appreciate what it is actually doing.

I'm sure you know about it as a ritual you have learned to perform. You didn't actually comprehend what you were doing, any more than you did when you acted out what you thought was researching the climate change literature, and were actually performing a trawl through the unreliable early results to cherry-pick those few results that suited your daft hypothesis.

If you are repairing the units, you should be able to trace that bit of the circuit. It isn't going to be complicated.

Ordinarily it would be a piece of cake, but they seem to have gone out of their way to make access as difficult as possible.

Ah, that must be it. Over 60 years of hobby electronics and I never knew that. Thanks for spoon-feeding me that vital tidbit, Bill. How would we ever manage without you?

Sorry, Bill. I'm not biting today. You'll have to get a bit more inventive if you want to suck me into another of your pointless pissing contests.