Capacitor Physical Size...I Still Don't Know

[N4HZ]

Hi Gang:

I had on hand and used several 300mfd, 450V computer grade type "blue" capacitors with screw terminals in my 4300 volt power supply for my 4-1000 amp.  Physically they are around 6 inches or so tall and around 2-3 inches in diameter and work great.  I'm considering building another amp using a 3000 volt supply.  I have on hand about a dozen new 470 mfd, 450V capacitors, the little ones, about 2 inches tall and maybe 1 1/2 inches in diameter.  I know cap sizes have shrunk but just seems I must be missing something.  Seems odd that these little guys would do the same work as the big ones.  The amp I'm considering building would be using either two 8122's or two 3-500's.   What  do I need to know?  Thanks for a great forum.

Roger
N4HZ 

[N2DTS]

Modern materials.
They make very small surface mount caps that have a lot of capacity for their size, and I think the new thing is ceramic electrlytic caps.

The voltage rating is the voltage rating, if they say its good for 450 volts, it should be.
I always try and get the ones rated for 105 degrees C.

For some reason, I never liked electrolytic caps in high voltage power supplies.

[W7TFO]

For some reason, I never liked electrolytic caps in high voltage power supplies.

Me neither.  Iron Henry takes my ripple away, I don't build with big C needing soft starts.

73DG

[flintstone mop]

now oil filled caps are not a problem

[WQ9E]

Check both the temperature rating (105C strongly preferred) and its suitability for high ripple service.   Many modern capacitors are not designed for high ripple service and the spec sheet will indicate the suitability.  Sitting at idle with the supply unloaded the caps may run cool but once loaded then the caps not designed for high ripple will heat excessively with the continuous charge/discharge that is part of high ripple service.

The oil filled "energy storage" caps have the same issue and generally aren't suitable to be run at full rating in high ripple service.   They will also develop a lot more heat in loaded high ripple circuits when used in their original intent as low duty cycle storage and discharge devices.

[Jeff W9GY]

The "C X V" product determines the size of an electrolytic capacitor.  Over the years manufacturers have learned to make the same C X V product capacitor smaller.  As in an earlier post, the voltage rating on the capacitor is the voltage rating.

[WQ9E]

CDE provides a very well written and easy to understand application guide that is worth a read: 

http://www.cde.com/catalogs/AEappGUIDE.pdf

[N4HZ]

Thanks everyone for all the good info.  I know more than before I posted.  Found the spec sheet on my caps.  They are Nippon Chem-Con series KMH rated at 105c and with ripple spec of 1.78A rms @ 105c, 120Hz.  Haven't finalized building plans yet but think I'll try these caps when I begin.  Have a great day and thanks everyone.

Roger
N4HZ

[W9ZSL]

Regarding ANY filter capacitors, check for leaks.  I mean PHYSICAL signs.  Oil leaks are a red flag obviously for the "oil filled" types.  I just threw out a bunch of old electrolytics because I noticed some bulges the size of match heads on one end.  Some even had globs of "stuff" which resembled hardened pine pitch or caramels that had oozed out.  If they have been in the junk box for any great length of time, they should be reformed to make sure they are safe.  I have a Home Brew solid-state bridge DC supply that gives me 0-750 volts with a variac.  Just bring the voltage up slowly over several hours until you peak out under the rated voltage of the cap...50 to 100 volts.

[pa0ast]

Reforming Elco's is making a stronger aluminium oxide skin on the surface of the plate as isolation. Criterium is the fluid inside shall not boil and make pressure. To do that You have to limit the power for the process so that its not heating up. Simply a resistor of 330 kOhm or so  in series will do,  the current should be  than lower < 1mA . Tats all,  This the way manufacturers work.
The moment that the capacitor voltage nearly reaches the powersupply voltage the Elco is ready. Can take Hours or even days.
Anton