Dear SY, thanks for your advice
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 ?
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 ?
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.
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.
Sy, I don't currently buy online, (but, sadly,I don't think it's worth shippement for small amount of components).
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.
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.
bembel said:
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!
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 !
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 !
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
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
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).
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.
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).
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.
bembel wrote:
Yes. For small mica capacitors that are NOS/unused. Mica is a very good insulator with little dielectric absorbtion.
If you work on the low side and one end of your resistor is at ground, then there is no problem. It was late for me and I was perhaps thinking you might work on the high side, as I sometimes do for convenience sake, then both ends of the resistor would be above ground. Not recommended of course!
Victor
Yes. For small mica capacitors that are NOS/unused. Mica is a very good insulator with little dielectric absorbtion.
If you work on the low side and one end of your resistor is at ground, then there is no problem. It was late for me and I was perhaps thinking you might work on the high side, as I sometimes do for convenience sake, then both ends of the resistor would be above ground.
Not recommended of course! Victor
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