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Voltage ratings of ceramic caps

 
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Re: Voltage ratings of ceramic caps

Post by PYE625 » Thu Apr 02, 2015 11:41 pm

Hmm, tricky because some 100n ceramics that I have are rated at 12vdc for transistor sets.
I would think that the lower values should be ok for typical voltages in valve radios, but a cautious approach is called for I think.

 
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Re: Voltage ratings of ceramic caps

Post by crackle » Fri Apr 03, 2015 12:41 pm

A good question, I am looking forward to the answer.

Mike

 
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Re: Voltage ratings of ceramic caps

Post by hamid_1 » Fri Apr 03, 2015 12:47 pm

Those brown disc capacitors are fairly modern. They are marked with a value either in microfarads or picofarads with a multiplier e.g. 104 = 10 + 4 zeroes = 100000 pF = 100nF = 0.1uF. This is similar to resistor codes. Very small capacitors under 1000 picofarads do not use this system. Instead, the actual value, in picofarads, is used, such as 10 (for 10 pF). So the disc capacitor marked 20 KN is 20pF.

What about the other letters? The letter after the numeric value is usually the tolerance. J=5%, K=10% and M=20% so in your examples
0.01M = 10nF +/-20%
0.05uF KCK = 50nF +/-10%
20 KN = 20pF +/-10%

Working voltage? This is a hard one to answer. Some manufacturers add extra letters or symbols to represent the working voltage as well as other characteristics like temperature stability. I'm not sure if there is a standard for this. Sometimes the working voltage is written directly (e.g. 250V or 1KV with an AC or DC symbol after it) in which case there is no doubt.

If there is no working voltage obviously marked, I would assume them to be low voltage (say, less than 50v). Those brown disc capacitors are commonly found in low voltage consumer electronics, like Made in Hong Kong pocket transistor radios which run on 9 volt batteries. I don't think they will be high voltage capacitors unless clearly marked as such. I've found the red tubular ones in a Vidor battery valve radio (90v H.T.) so those may be good for slightly higher voltages.

 
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Re: Voltage ratings of ceramic caps

Post by a4000bear » Sat Apr 04, 2015 6:04 am

Here are a few 0.0047uF ceramic caps. The ones on the right appear to have what could be voltage ratings printed on them.
55, 63, and 75. The 55V one is hard to see due to lighting. It's interesting to note that the 75V one is smaller than the others, though of course the dielectric could be made from a different grade of ceramic. The others on the left have no indication of voltage, but do show the wide variety in size.
Attachments
0047.jpg

 
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Re: Voltage ratings of ceramic caps

Post by pclover » Sat Apr 04, 2015 8:39 am

I would just throw them out ! Caps are cheap.

Gary

 
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Re: Voltage ratings of ceramic caps

Post by Panrock » Sat Apr 04, 2015 11:53 am

ppppenguin wrote:For comparison's sake I took a 47n 25V disc ceramic from my stock and it read short circuit at 500V. At low voltage it's now an approx 200k resistor.
Have you just discovered a new way to make Voltage Dependent Resistors? :qq1

Steve

 
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Re: Voltage ratings of ceramic caps

Post by Michael Watterson » Sat Apr 04, 2015 2:33 pm

All my 50V and 100V ceramic caps pass on 600V test. I imagine they might fail later. They are not older types.

The 1960 & 1970s 0.1uF Z5U caps were about 20V.

For same size, a smaller capacitance has higher voltage.

Most tubular types are well over 250V.

The very small multilayer type are lower voltage. Disc type are usually single layer and thickness vs dielectric type is a guide Z5U are worst in every respect.

I have 2M ohms fed from 300V driving a 50uA meter and also a 600V flash gun electronics with 2M Ohm and neon (with capacitor in parallel). On "solid" = fail. If it's flashing you can estimate leakage from 10M Ohm to 1G Ohm depending on speed as the flashing cap is ultralow leakage.

 
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Re: Voltage ratings of ceramic caps

Post by Doz » Sun Apr 05, 2015 11:43 am

I had some cheap 1KV types whose capacitance decreased as the voltage rose. There was a note in the datasheet to that effect.

VDC? Voltage dependent capacitor?

 
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Re: Voltage ratings of ceramic caps

Post by johntheboffin » Sun Apr 05, 2015 1:29 pm

Some capacitance bridges have the facility to apply DC bias so that you can see if the value is affected by voltage; an example is the old Marconi TF2700, which can take up to 500 V bias:

https://sites.google.com/site/marconiin ... sal-bridge

Useful for polarising electrolytics whilst measuring the capacitance, too.

 
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Re: Voltage ratings of ceramic caps

Post by Kalee20 » Sun Apr 05, 2015 8:20 pm

Some ceramic types, particularly the so-called Hi-K types, are quire sensitive to voltage (and temperature). Mainly used for non-critical, low-voltage decoupling.

They may not break down at 500V, but the capacitance could be very low. It's a bit like inductors saturating at high currents, these capacitors' dielectrics effectively polarise fully at high voltages.

I agree, markings often do leave a bit to be desired...


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