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DRSSTC Tank Capacitor

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ScotchTapeLord

Thu Mar 04 2010, 03:22AM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
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Posts: 635

I know there must be a thread on this but I couldn't find it.

I'm working on a small DRSSTC, and so far I've collected 17 CD942's. This collection is growing. My question is, what is their official voltage capability? Most people rely on the DC rating of 2000, but I've recently heard of people claiming that they will fail at that kind of peak voltage in AC. So that leads me to trust more in their AC voltage rating of 500V. This, however, according to the datasheet, is the rating for 50/60Hz. Since a DRSSTC often operates at more than 1000x this frequency, for what kind of voltage can it be held reliable?

So basically, I want to know the safe way to figure out how to determine the voltage that the capacitors will be taking (can this be done experimentally with a fast diode+capacitor across one of the capacitors in the bank?) AND what voltage the famous CDE942C20P15K are good for.

ALSO
I rigged up 10 of these capacitors in series as a tank capacitor for a 9kV neon sign, used it a few times, going by the DC rating. Will this have affected their integrity? Would it show if I measured the capacitance?

Thanks!

Luca

Thu Mar 04 2010, 08:51AM

Registered Member #2481 Joined: Mon Nov 23 2009, 03:07PM
Location: ITALY
Posts: 134

ScotchTapeLord wrote ...

My question is, what is their official voltage capability? Most people rely on the DC rating of 2000, but I've recently heard of people claiming that they will fail at that kind of peak voltage in AC. So that leads me to trust more in their AC voltage rating of 500V. This, however, according to the datasheet, is the rating for 50/60Hz. Since a DRSSTC often operates at more than 1000x this frequency, for what kind of voltage can it be held reliable?

Hi!

I have found that a good way to determine voltage rating is to take the peak expected voltage and use a DC rating which is at least two times the expected peak value. Example: if you have 6kV peak then use strings that are rated at least at 12kVDC (6x 2kV in series)...
AC rating is useless for this application since it is referred to standard 50/60 Hz voltage.

The way to determine the maximum peak voltage is simply Vp=Ip*Z, where Ip is the maximum peak current in the primary (you set this value with the OCD threshold) and Z is the impedance of the primary at resonance (sometimes called "surge impedance")

Regards,

Luca

ScotchTapeLord

Thu Mar 04 2010, 10:17AM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
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Posts: 635

Thanks!

I'm just really bothered by the AC rating being 1/4 the DC rating, at only 50/60Hz for this capacitor.

Mads Barnkob

Thu Mar 04 2010, 03:05PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

This thread from page 2 should cover all your questions:

ScotchTapeLord

Thu Mar 04 2010, 03:16PM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
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Posts: 635

Those formulas are helpful, but I was more curious about the voltage capabilities and specifically the relationship between the CDE942's AC and DC ratings. 2000VDC and 500VAC confuses and troubles me.

Arcstarter

Thu Mar 04 2010, 09:13PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

ScotchTapeLord wrote ...

Those formulas are helpful, but I was more curious about the voltage capabilities and specifically the relationship between the CDE942's AC and DC ratings. 2000VDC and 500VAC confuses and troubles me.

The CDE 942C caps you are speaking of indeed have a 2kvdc 500vac rating. As coilers, we normally use them at a bit below 1kvac each. For example, a 12kv 30ma NST coil would likely have 16 caps in series for 9.375nf, and 32kvdc. This means 6kvac, minus the safety margin of 4kvdc. We simply say it is a string rated for 12kvac, because we use them at double the rated voltage.

This goes for drsstc's, as well. Just take the DC voltage of the string of the caps, and divide it by two, plus a safety margin of a few kvdc. That is how we do it.

Hope it sheds some light on these caps that we terribly abuse

. But hey, i have never managed to kill one!

Matthew.

ScotchTapeLord

Thu Mar 04 2010, 10:14PM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635

Thanks, well I guess it just gets summed up with: Divide DC rating by two and use that for peak voltage, with margin. Works for me... and it looks like I'm going to have a larger surge impedance than originally planned.

Steve Ward

Wed Mar 17 2010, 06:18PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

So basically, I want to know the safe way to figure out how to determine the voltage that the capacitors will be taking (can this be done experimentally with a fast diode+capacitor across one of the capacitors in the bank?)

Ohms law (applied to impedance) works pretty exactly. Measure the current and frequency and math does the rest.

For awhile i operated my DR-2 with 8kVDC rating on the 942C caps, but i pushed it to about 9kV peak in operation. It probably lasted about 20 hours of total run time and then failed pretty hard. Generally the capacitance drift is miniscule on a failing cap until it really tanks, so measuring *ANY* change in capacitance is a pretty big warning sign!. Many of the caps in my old DR-2 MMC still measure within spec, a few of them measured about 15% of the original C... not 15% out of spec, but 15% of .15uF.

The answer is that there is a lifetime curve vs voltage. Provided you dont over-volt them so bad that they fail from mechanical reasons, then the lifetime vs voltage is a pretty steep curve. That is to say, reducing the applied voltage just a little makes a huge difference in lifetime. The question to ask yourself is, how long do i want this thing to last? If it were a display coil, expected to be reliable, id go with the 500VRMS AC rating of the capacitor. If its just to get some experiments done, and you wont be too heart-broken to see it fail, then i might just run it at the DC rating and have it last a few hours to tens of hours, knowing it WILL fail eventually.

BTW, keeping the capacitors running cooler should help improve their lifetime too, though im not sure how dramatic the effect is if you are well below the melting point for PP, maybe it doesnt matter then.

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