Safe Ratings for "Energy Storage" Type Oil Capacitors?

[W9GT]

I picked up a couple of nice Sprague oil caps last weekend at the Ft Wayne Hamfest.  They are labeled "energy storage" and were probably used in a large strobe flash or laser power supply.  I know I have previously used this type of capacitor in power supply continuous duty filter applications, however, some de-rating of the voltage is in order for safe operation.  These caps are rated at 35 Mfd @ 2500 VDC peak.

Any one know what a "safe" voltage rating would be for regular power supply filter applications?  Would a series-connected arrangement yielding 17.5 Mfd would be OK to operate at 2.5-3KV??

73,  Jack, W9GT

[WQ9E]

Jack,

I spent a lot of time trying to get an answer to this 9 years ago and the best I could come up with was running them at 60% was very safe and 75% of rated was OK under conditions of reasonable ambient temperature.  I used 8 of the GE units rated for 7.1 uf at 4 KV in parallel on a supply producing 2200 volts no load and they have worked without fail or noticeable temperature rise.  One of my colleagues is a former engineer from GE and the numbers I provided came from one his former coworkers.

So it sounds like yours should be fine for your described conditions.  I would suggest running a few tests with the power supply loaded for increasing periods of time and see if the capacitors are heating up when exposed to higher ripple.   If they aren't heating up when the supply is  loaded to rated current then they should be fine.

[Bill, KD0HG]

"Energy Storage" means they have low internal resistance to as to be able to deliver large peak currents. Nothing to do with voltage ratings. Energy storage caps will have more internal tab connections to the aluminum foil plates than is customary to lower the internal Z. Or they might even use thicker foil electrodes.

I use a pair of 2,500 volt Aerovox "Energy Discharge" oil-filled caps in my big homebrew rig, and haven't had any issues with them in over 15 years.

[W2PFY]

Found this on the net. I have used them for additional capacity in my BC-610 running 2000 volts and had no heating. The additional capacity seemed to give me more wallop on mod peaks


http://www.dei2000.com/pdf/ESecscr.pdf

[WQ9E]

Bill,

The possible problem with these energy storage caps is that they are designed to be used on relatively pure DC and some of them heat up excessively when used in power supply filter service as opposed to a storage/rapid discharge device for pure DC.

The GE units I use are about the same size as traditional oil filled caps and probably could be run right up to their rated voltage without problem.  But a lot of these put a lot of capacitance into a little space and some of the units I looked at several years ago had specific warnings not to use them in high ripple service.

You are right that it isn't the voltage that creates the problem but the higher ripple current and resultant increased dielectric heating associated with the higher voltage.  De-rating the voltage is just a simple rule of thumb providing a conservative rating for these when used outside of their intended application.  The same capacitors used with a choke input filter would be much happier.