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| How to determine the voltage rating of an unknown capacitor? |
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| G7PSK:
If you have a quantity of the same capacitors you could just ramp up the voltage until they blow, then run the remaining ones at half or three quarter voltage of that which blows them up. |
| SeanB:
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. |
| saturation:
In theory, the breakdown voltage is characterized : http://en.wikipedia.org/wiki/Capacitor#Breakdown_voltage For general dielectric strengths, Eds you can find them here: http://en.wikipedia.org/wiki/Dielectric_strength Then guestimate the plate separation distance, d, in meters. This is taken from Paschen's law but actual construction and conditions to the capacitor matters, so in the end destroying a few is your best and most simple test. Using 1/4 or 1/3 of the Vbd as a derating is also just overall good practice for high reliability systems. http://en.wikipedia.org/wiki/Paschen%27s_law#Basics Bolded below are 2 techniques I use. One can get 'grab bag' bulk capacitors in the 100s from someone, even if marked, but could have been QC rejects. Its good to test those caps are to some spec, including Vbd. Also, watching them blow is also more fun than measuring capacitance :-+ so you have incentive to characterize you're salvaged caps ;) --- Quote from: jimmc on January 23, 2013, 03:24:55 pm ---Should mention that not all ceramic capacitors have a significant voltage coefficient. Those using a Class 1 dielectric (NPO, COG) do not change significantly with applied voltage or temperature . This dielectric is used for lower value capacitors say less than 1nF give or take a decade. A quick blast with a hot air gun will show if you have a Class 1 dielectric - if the capacity doesn't change significantly with temperature, it won't change with voltage. For example see http://www.vishay.com/docs/23140/geninfo.pdf pages 6-8 The only way I can think of testing these is to apply a proving voltage of 3 times the voltage you want to use them at (through a current limiting resistor), if they survive for 60sec then you should be OK. e.g. If you apply 48v through 47k for 60 sec and no significant current flows then they should be safe at 16v. Jim --- End quote --- --- Quote from: G7PSK on January 23, 2013, 03:53:25 pm ---If you have a quantity of the same capacitors you could just ramp up the voltage until they blow, then run the remaining ones at half or three quarter voltage of that which blows them up. --- End quote --- |
| westfw:
--- 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? |
| olsenn:
The simple answer to this one is, you don't! Throw away your unknown caps and buy a new batch that provide you with a rating. However, electrolytics usually imprint the voltage rating on the cap sleeve itself, and other caps are usually pretty standardized. |
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