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Old 15th September 2006, 12:02 PM   #1
bembel is offline bembel  Europe
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Default NOS pF capacitors voltage rating?

Hi,
I need a 500 pf / 400V capacitor (// with Ra), and got a bunch these olds (mica or ceramic stacked plates caps), flat stamp-sized covered with wax with no voltage ratings.
Will it fit securely to my needs ? Any infos ?
thanks
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Old 15th September 2006, 12:27 PM   #2
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Don't bother. Why use a dodgy cap in a circuit when a new, well-rated one costs perhaps 20 cents and is easily available?
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Old 15th September 2006, 01:04 PM   #3
bembel is offline bembel  Europe
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Dear SY, thanks for your advice

Quote:
Don't bother. Why use a dodgy cap in a circuit when a new, well-rated one costs perhaps 20 cents and is easily available?
But it seems that small caps with HV ratings are not easily available at my no so close "corner store" at least the info on voltage rating.
And I got an hundred of those NOS caps (even a few kilos of other types).
I'am actually prototyping, so I just need one witch stands the voltage (rather than go to the shop...order...wait...go again to the shop) and continue my project.

Any idea of those NOS caps voltage rating ?

Any suggestion on a cheap widely available in europe (Radio spare for exemple) HV small cap reference ?
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Old 15th September 2006, 01:35 PM   #4
SY is offline SY  United States
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There's no way to tell the voltage just by inspection.

I would be very surprised if you couldn't order the caps online and have them in a day or two; I don't know who the European equivalent of Mouser or Digikey is (Farnell?), but these are very common parts. WIMA and Vishay both make them, and I know that those brands are widely distributed there.
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Old 15th September 2006, 01:45 PM   #5
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Sy, I don't currently buy online, (but, sadly,I don't think it's worth shippement for small amount of components).

Quote:
I don't know who the European equivalent of Mouser or Digikey is (Farnell?)
the bigest ones (where I can easily order from my "corner shop" with no shippement) are Radio Spares, Farnell & Conrad (no Mouser here!).

In fact I was thinking that an "old timer" could give me some (lost?) infos about those type of caps that I see sometimes (couldn't find anything about voltage ratings on google). Also, what are their usual failures ?

Any help appreciated.
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Old 15th September 2006, 02:48 PM   #6
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Quote:
Originally posted by bembel


In fact I was thinking that an "old timer" could give me some (lost?) infos about those type of caps that I see sometimes (couldn't find anything about voltage ratings on google). Also, what are their usual failures ?

Any help appreciated.

Hi bembel

IF you have a variable high voltage supply , like my Heathkit you can try to see if the cap withstand the 450 volts. For that , you must put a 300 KOhms resistor in series with cap, for in the event of the cap short with the voltage , the current will be minimum .

If you don't the variable power supply you can use the HT tension in your amp (usually around 450 Volts) ,connect the serial circuit of the capacitor and the 300K resistor between the 450Volts and ground and see if it don't become shorted.

If there is any voltage across the resistor , using the Ohms law ,you can even measure the leakage current (if any).

Obviously , this test is not so stringent and valuable as the original ratings given by the capacitor manufacturer ...

Salut!
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Old 15th September 2006, 03:10 PM   #7
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thx for the trick Jorge ! I should have thought about that !
But then how much leakage is to expect to be the limit, let's say @1000pF ???

(effectively I haven't found yet an HV lab supply )

I'm very surprised nobody's remembers the standard voltage rating of such widely used capacitors (before transistor era) !!!

Salut !
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Old 15th September 2006, 03:29 PM   #8
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Before transistors??? Cortez brought those caps back from the Aztecs.

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Old 15th September 2006, 06:03 PM   #9
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Hello bembel,

Because I'm in the U.S. I'm not that familier with older European parts. However, we have square & rectangular mica capacitors covered in wax very much like the ones you pictured. For all these unmarked caps the standard working voltage is 500VDC. I think a rating in this range is not too far off for yours. But this is just an educated guess.

Tube_Dude suggested an excellent way of testing them using a high voltage source. Personally, I would use a 100K resistor in series just to make the math easier by simply moving the decimal point in your head. 100K ohms will still provide plenty of current limiting. For a part of this type leakage current should be very small. Being on the order of only a few microamps.

If you use a voltmeter across the resistor to monitor current, be sure to use a portable (floating) battery operated meter. This will avoid shorting out the HV if your bench meter's negative lead should be grounded through the mains.

Victor
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Old 16th September 2006, 01:08 AM   #10
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Victor, thanks for your answers

You confirm my thought, that such good ol' caps can withstand HV ( but I was not sure in wich proportion 120V-4KV? ) so 500V seems to be a work around point (especialy for those times).

Quote:
For a part of this type leakage current should be very small. Being on the order of only a few microamps
do you mean less then 10uA@1nF ?

Thx again

PS: I appreciate your safety advice, but what can be the mess with grounded supply & grounded voltmeter if polarity is OK ?
(the HV will not be shorted because of the 100k-300k resistor (in the event of cap failure) , or have I missed something ?)
anyway I use a floating voltmeter for such tasks.
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