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Old 28th February 2004, 02:34 AM   #1
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Default Oil Capacitor needs forming ?

I have got these nice big 2uF 3500V oil caps which I want to use in series for a 5kV powersupply for my new direct drive ESL amp.

But I don't know if these caps need to be formed when using at such high voltages.

thanks in advance,
Dick.
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Old 28th February 2004, 03:18 AM   #2
SY is offline SY  United States
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In a word, no. They are not polar caps.
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Old 28th February 2004, 08:47 PM   #3
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Thanks Sy,

does this mean I can use it right away i.e. applying full voltage at once without any explosions ?

Dick.
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Old 28th February 2004, 09:05 PM   #4
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AFAIK you need to add resistors in parallel
to the caps if you put them in series.

I don't really know how you calculate the
values of "safe" resistors, I presume the
purpose is preventing excessive voltage
across one of the capacitors.
And that this practise is only applicable to
power supplies, it can't be applicable to
general series capacitor use.

AFAIK only electrolytic capacitors need to
be formed before being placed in series
to form "bipolar" capacitors.

I'm sure if you reposted under tube amps
you''d get a definitive answer, hopefully.

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Old 28th February 2004, 10:00 PM   #5
SY is offline SY  United States
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What sreten said: you do need equalizing resistors across the caps if you're going to put the caps in series. Given the low leakage of caps like the ones you're using, they can be pretty high value resistors; figure a milliamp or so of draw to be perfectly safe, though you could probably get away with less. For a milliamp at 2500 V (your B+ cut in half), 2.7 meg is the closest standard value. They'll need to be 4 or 5 watts and able to withstand the voltage. Frankly, you could probably use 10 megs, which would cut the power requirement down by a factor of 4.

If there's a dielectric resistance spec available for your caps, figure that any resistance a decade lower will probably do.
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Old 28th February 2004, 11:33 PM   #6
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... and the resistors need to be rated for the voltage. Otherwise you get a break thru.
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Old 29th February 2004, 11:01 AM   #7
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Thanks for the answers, guys.

I have high voltage Caddock 20Mohm 1% 4 or 5 Watt resistors which I will put in parallel with the C's to divide the voltage neatly between them.

My concearn is mainly that these oil C's are NOS and so never used. Does these big metal can oil C's have a shelf life or in other words do they need a special treatment, like electrolytics, before applying power for the first time.

Because these voltages are so high and dangerous, I want to be on the safe side.

Dick.
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Old 29th February 2004, 11:23 AM   #8
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Oil caps seem to have a very, very long life. The ones in my proto DD-ESL amp originated in my old ham rig; they were old when I built that rig 35 years ago.

Check the voltage rating of those Caddocks. You don't need precision resistors, but it's not like that will hurt anything, it's just a bit of a waste.

And you're 100% right to be cautious every step of the way. This is more voltage than Florida uses to execute murderers.
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Old 29th February 2004, 06:50 PM   #9
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Quote:
Originally posted by SY
This is more voltage than Florida uses to execute murderers.
What voltage are they using? AC or DC?
Just curious....

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Old 29th February 2004, 09:23 PM   #10
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It doesn't matter because they're using silver interconnects.
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