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How to determine the voltage rating of an unknown capacitor?

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smashedProton:
Blow the ass out of it!   :-+

(In)Sanity:

--- Quote from: smashedProton on January 23, 2013, 07:50:29 pm ---Blow the ass out of it!   :-+

--- End quote ---

I would much rather do that then throw them away.

Alexei.Polkhanov:
Hmm, when I am not happy about ESR of an electrolytic capacitor I usually try to replace it with one of same capacity but higher voltage rating. Usually it works so that 6.3V cap will have twice ESR of 16V one and so on. It is true at least for manufacturers that I usually order/use (Panasonic). Higher voltage rated ones are usually (not always) bigger.
Reverting this dependency should give a clue about voltage rating knowing the ESR of similar cap of known voltage rating. Also longer taller caps tend to have lower ESR - I am not sure if there is any such dependency for chip capacitors, but those with higher profile (same size i.e. 3.2x1.6mm) tend to have lower ESR as well.

saturation:
Multilayer ceramics range from 30um to 1 um between plates.  With air as a cheapest dielectric, at 3MV/m then estimated breakdown voltage such caps would be ~ 3V to 90V.  Since dielectrics in reality are rarely air, then most generally purpose caps can be used in 5V applications.  Always best to test it empirically as its is possible to get a ~ < 3V capacitor.

http://my.execpc.com/~endlr/ceramic.html

From the equation, as Vbd ~ d, then physically bigger capacitors, are more likely to have greater Vbd too.  Thus, through hole types are thus likely suitable for 5V applications compared to SMT. 

Older style disc capacitors or electrolytics have larger d, you may be able to see the plate distances if you decapitate one with a dremel then use a micrometer to measure the plate distances. Luckily, larger caps like these still have readable Vbd values printed on them.



--- Quote from: westfw on January 23, 2013, 06:35:04 pm ---
--- Quote ---Then guestimate the plate separation distance, d, in meters.
--- End quote ---
Yeah, right.  How do you propose to do that, for a modern ceramic or electrolytic?


--- End quote ---

(In)Sanity:

--- Quote from: SeanB on January 23, 2013, 04:04:56 pm ---Very simple, you make a test jig to place the capacitor in, with 2 very good capacitors to provide isolation of the cap under test from the test voltage. Generally i have seen a 10 or 15uF 250V unit used here ( a bloody big capacitor and most likely the largest film unit you can buy) in series with the leads to the DUT, and the power supply is connected via a pair of 100k resistors. You measure the capacitance at )v 9 you are measuring 3 caps in series, so it will measure low) then slowly wind up the voltage applied till you either are approaching the isolation capacitor voltages or the voltage across the capacitor drops due to leakage ( measure via a second set of 100k isolating resistors to the voltmeter) so you know how the capacitance changes with voltage and how leaky they are.

The isolating caps unfortunately will have to be good ones, polypropylene foil, ( polystyrene is best but a 10uF one is the size of a large shoe box), polycarbonate or polyester foil. Not electrolytic or ceramic. 100V, 250V or 400v best. higher voltage bigger size. If you cannot get 10uF use 3 4.7uF in parallel instead, will be easier to find.

--- End quote ---

Doesn't this just equate to indirectly increasing the voltage until the leakage current starts to increase ?   How does the test isolation capacitors keep for example a tantalum from catching fire ?    Also are you saying the isolation capacitors need to be very high quality to reduce leakage current?    Sorry if I'm being a bit dense on this one.

Thanks,

Jeff

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