limit to pwr supply filter C?

[KU7I]

I am curious. I see some 10,000uf 450volt electrolytic caps for sale on Ebay. 8 of these would give you 3600volt capability at more or less 1250uF total capacitance. is this too much. Are there any dangers using this much C in a filter for a PS? In the past I have used eight 560uF caps and other values similar to this. Just curious, thank you. Lane Ku7i

[W6RMK]

"3600volt capability at more or less 1250uF".. that's over 8 kilojoules of stored energy. If you have a flashover or accidental short, it's going to be pretty darn exciting.  If one of those capacitors develops a fault, and the energy dumps into it...
Cap banks like this have really high peak current capability. peak powers of many kilowatts, if not megawatts are possible.

I'd look at the age of the capacitors. They're electrolytics, and they don't have infinite life.

You'd certainly need some sort of scheme to limit the energy that can dump to the tube (a few Joules is the typical limit).

[AA4HA]

You will also need to consider the pranging that those caps will put upon your transformer, rectifier and choke during the initial charge-up. They will appear to be almost a dead short across the supply as it charges.

You will either want to current limit the initial power-up surge or incorporate some sort of step-start circuit into your supply.

For example the Harris RF-350K uses a 60,000 uF capacitor in the DC supply section. They use a step-start circuit that takes a second or two to come completely up to speed.

I would make sure that I had bleeder resistors across those caps so they don't hold a charge until next Sunday.

At the worst you will have a great arc-welder.

Tisha Hayes
AA4HA

[W1QJ]

Lane, according to Bill Orr there is a "law of diminishing returns" on power HV power supply "C".  As I recall the maximum was like 88uf and after that there is little to no advantage to increased "C".  The only thing further increased "C" will cause is problems like the other poster says with the massive energy available.  In other situations of filtering high "C" is desierable but in HV PS there is this limit.

[KU7I]

Ahh, yes, I remember reading about this in his books. I have many different copies of the West Coast Handbook just not any with me right now since I am currently out to sea on USS Peleliu LHA-5. I am concerned with the potential for a bad outcome so I will need to investigate further. Lane Ku7i USS Peleliu LHA-5

[KF6QEX]

Sometime ago, I wonderered why unregulated  low voltage power supplies have huge capacitor values in comparison to  HV power supplies.

I had to find out because it just bugged me.

Capacitors in power supplies have only one job. and that is to reduce ripple.
"A couple of volts" in a low voltage 13.8 supply is a lot (*)  of ripple.
The same couple of Volts in a 40 V power supply is still "real" ripple but less significant.
Now ...a couple of Volts in a 2KV or a 3.6KV supply is not ripple, it's a rounding error in the ripple calculation !:)

For a given current, the higher the output voltage the less capacitance is "necessary".

[WX7G]

Stored energy above a couple hundred joules can be quite dangerous. I've had exciting times with big cap banks in industrial equipmment. Best to keep it as low as ripple allows.

[WB2WIK]

Those caps must be quite large.

No real advantage.  Causes lots of surge current for the rectifiers and dangerous discharge currents.

My 3kW 4-1000A power supply has no visible or audible ripple under load (almost 1A at 4400V) with only 32 uF filtering (four 8uF, 5kV oil filled Pyranol caps).

Although they're well bled (TC = 1 second, fully discharged in <10 seconds), I have discharged them sooner (before they bled down) a few times when working on the power supply, and it's very exciting...kind of like the Fourth of July.  The "shorting stick" hook welded itself to the chassis more than once.

I can't even imagine what the discharge enegy would be like from as much capacitance as you're discussing here...maybe enough to eat a hole in the chassis.  Silly.

[WG8Z]

>3600volt capability at more or less 1250uF total capacitance. is this too much.
---
For a HAM amp.
Probably just a tad overkill. HIHI
For destructive testing and U-tube videos might be exciting.
Not sure I'd want to go there even with a 10ft chickin stick.
73
Greg

[KU7I]

I am canning the idea, thanks to all who replied, very informative. Lane Ku7i

[W8JI]

I am canning the idea, thanks to all who replied, very informative. Lane Ku7i

Killing the idea is a good idea.

There is a curve of increased regulation vs supply transformer and capacitor ESR. With normal transformers and power mains, about 15-20 uF would be on the start of the flat "no change" part of the curve with a 3000-4000 ohm load.

That's why semiconductor supplies, with a high current low voltage low load resistance, need high capacitance.

The lower the transformer ESR and load resistance, the more capacitance you need for ripple filtering so you have to consider that also. But again 15-30 uF is probably enough.

All things considered though, going more than 20 uF isn't likely to be advantageous and higher capacitance comes with a whole lot of extra problems and danger.

[KF6QEX]

The lower the transformer ESR

Who left the door open and the transformer ESR sneaked into the equation ?

[K8AXW]

KU7I - Thank you for this post!  I have been wondering about this very question for about a month.  I bought a bank of 8 caps at a hamfest that would give me around 80ufd @ 3600V.  I bought them for a wild-hair dream of building a solid state amplifer one of these days but then started to entertain the idea of installing them in my homebrew 1500w amp.  I thought that this amount of capacitance would "stiffen" my voltage doubler power supply some.  But, finally stopped enteraining that idea and shelved them.

Reading the various posts here gave me an education in an area that I had never studied before.

Al - K8AXW

[W0BTU]

There's been a lot of good answers to this question, but here's one more reason not to use an astronomically high value for a filter capacitor: the larger the input filter capacitor, the larger the instantaneous peak diode current. You can damage the diodes by using too large of a filter cap.

73, Mike
www.w0btu.com

[W8JI]

The biggest danger is what happens to things downstream if the amp developed a high voltage to ground fault.

1000 uF charged to 3500 volts is like a stick of dynamite. If the ESR is very low, which it probably will be, it would easily blow a big screwdriver blade in half. :-)

I'm thinking of doing something like that to get rid of the armadillo problem I have.