General > General Technical Chat
How to determine the voltage rating of an unknown capacitor?
<< < (10/11) > >>
JackOfVA:
To return this to the original question, if you have sufficient number of capacitors of a particular type to test to destruction, apply a current limited DC voltage to a representative sample of the lot, increasing the voltage until the dielectric ruptures, calculating the max, min, mean and standard deviation. 

Apply a reasonable safety factor to the result, perhaps 3:1 or 4:1 from the mean.

That strategy won't be useful if you have 5 or 10 parts of each particular type, but if you have 100 or 200, sacrificing 5 or 10 for a destructive breakdown test isn't too bad a price to pay.

With a knowledge of a reasonable maximum recommended operating voltage, you can then look at C versus V over the rated voltage to get an idea of the dielectric class. NP0/C0G ceramic will have next to no change in capacitance with applied voltage. X7R will have a lot and Z5U even more. (I assume these are ceramic capacitors.)

If you want, I'll can run a destructive test on a few parts for you - at least up to 1KV, don't think I can easily go beyond that point without kludging up a HV transformer and a Variac and I don't have time for that. But, I can run up to 1 KV with a standard lab HV supply. Contact me directly if you are interested. I can't run hundreds of different parts, but be glad to look at a dozen or so samples of four or five different values and/or capacitor types.

With respect to surface mount ceramic caps, most every C0G/NP0 part I purchase is  light gray color whilst the X7R types are brown or tan. That's not universal and I don't suggest it as a definitive test for dielectric class, but I can say that I've never seen a light gray ceramic surface mount cap that was not a C0G/NP0 type and that I've never seen an X7R of light gray color. I have seen a few C0G/NP0 parts that are brownish, particularly in values of a 100 pF or less. 

 
(In)Sanity:
I have no problems with 1000V AC or DC.   I've only done DC destruction tests so far.  I can try AC of various frequencies.   I'll see what I get.  My focus has shifted to trying to fix a 60 inch TV with back-lighting that acts up after 10 hours.   But that's another subject.

Jeff
Babak7:
I like both idea of VCO & LCR and isolating the device under the test ( i.e. the capacitor) with high voltage capacitor.

However, I have an idea that might be closer to actual physics of cap.  As the voltage increases the leakage of the cap increases.  So let us use a current source, connected to very high voltage say 500V, or more if you want to use Vacuum tubes  to create the current source -- I know we have transistors that can handle up to 1000V--,  Let's adjust the current source to have current of about 10nA or whatever leakage value is acceptable to you.  Then start charging the capacitor from 0V.  At the beginning the capacitance voltage increases linearly with time, but at some point leakage current becomes equal to current source current and the voltage does not increases anymore, and that voltage is you Capacitor's maximum voltage.  Here are some physical derivation:

Q=CV ,

take derivative of both sides.

dQ/dt=d(CV)/dt

dQ/dt is definition of current  or I  (I=dQ/dt)

so

I= d(CV)/dt 

so

V(t) =(1/C) I t

However as voltage increases the leakage current increases and at some point leakage current = to the current source and voltage across capacitor does not increases with time anymore and stays constant.  you can choose this value as voltage rating of your cap.  So, you could say voltage rating of my cap is defined, when the capacitor have say 10nA of leakage current.

To generate high voltage you could use Tesla coil, boost convertor, or voltage multiplier ( voltage multiplier ladder) using diodes and capacitors.  For current source you could use high voltage transistor, or vacuum tube connected to this voltage multiplier circuit.
james_s:

--- Quote from: AndyC_772 on January 22, 2013, 09:55:36 pm ---For many ceramic capacitors, the capacitance starts to fall off quite sharply as the dc bias is increased. Perhaps you could devise a circuit which measures capacitance with a gradually increasing dc bias on the cap under test until the capacitance falls by, say, 20%, or you chicken out - whichever happens first!

--- End quote ---

I don't think this will work. I am not an expert on this topic but I remember reading that the capacitance drop is more closely related to the physical size of the capacitor than the rated voltage, so for example using a 50V 0603 cap instead of a 25V 0603 part won't solve the problem, but using a 1206 cap will reduce the effect.
shapirus:
...yes I know this is a very old topic :).

I am here to leave a link to an article that describes a clever technique and proposes a very simple device that allows to measure capacitance under dc bias relative to zero-bias capacitance: https://www.analog.com/en/technical-articles/how-to-measure-capacity-versus-bias-voltage-on-mlccs.html

Having that one, and a device such as DY294 or other that is able to ramp up the voltage until a leakage current is detected, one could characterize unmarked caps without any (or without trusted) specifications, which can be parts salvaged from discarded boards or bought in kits from aliexpress etc.

I'm going to build it some day soon.
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod