High voltage capacitor question 43uF 4kV

[Chris Wilson]

I am hoping to build a large high voltage power supply that will serve as a supply for any valved (tubed) RF linear amplifier I might care to build over the next few years. I have an American friend staying nearby, recuperating from a bike accident, and he's building a couple of big linears to take back to the states once he's better, so he's sharing the build cost of the supply. I have found a schematic that, with a suitable (big, heavy and expensive) mains transformer will give 1.5 to 4 kV at 1 amp. The transformer can be built locally, but the schematic calls for a 43 uF oil filled capacitor of 4kV working voltage, and I would imagine such a beast is nowadays very rare. The photo of the built supply shows this capacitor to be a whopper of a thing, and probably war surplus, as it's an American design from 1994. The text specifies *NOT* to drop below 30uF for this cap. What are my options here? Thanks.

[Psi]

Doesn't *have* to be oil filled, you could build an array of smaller caps to mean your requirements.

The magnetron in microwaves is just a valve and works at around 2-4kV
Maybe you could make an array of microwave oven capacitors to get your 43uF.

However i question if 43uF is actually needed. They may be spec'ing 43uF due to ESR limitations.

There are some pretty low ESR high voltage caps available. Cornell Debilier sell some pretty insane film caps,
i have some in my tesla coil.
But it wouldn't be cheap to make a 43uF array of them.

[Rerouter]

for that capacity and voltage i could only fathom making your own,

this site is a good reference, http://www.sentex.ca/~mec1995/circ/hv/hvcap/hvcap.html

say mylar sheet and aluminium foil wrapped up into one massive capacitor then soaked in mineral oil or if available transformer oil, (assuming K is correct)

from the math you would be looking at atleast 50,000 square inches of plate material, or 32 square meters, just to give you an impression of how big this thing will be, and that is to break even at 30uF, (rolled capacitors effectivly have 3 plates)

you can buy all of the materials other than the housing relativly cheaply, and depending on your width 32 square meters wont seem like much, at 20cm plate width that is only 120m in length, and as both materials are only a few thou wide it will be taken up over a significant number of layers for a minimal width,

you will want your dielectric wider than your plate areas, atleast enough that the oil will not break down over the edges,

[Chris Wilson]

Yes, I was thinking of a series array of higher value caps, which seems to be how most more modern HV PSU's are done, even commercial ones. Secondaries  will be 1.5, 2.5, 3 and 3.5kV at 1 ampere. I don't fancy making one at all Only by using commercial units in an array or other tried and tested means, whilst a "roll your own" is undoubtedly possible, it sounds like a recipe for dangerous fun and frolics at my level of experience with HV Thanks for the idea though Rerouter!

[SeanB]

Go but a whole lot of microwave capacitors. They are generally around 1.0 to 2.-uF 2kv, and all have bleed resistors built in. Buy 50 of the 2uF units and run them as 25 parallel units of 2 in series.

If you get mixed values you just use 2 of the same value in series. Add a microwave fuse in the lead to each section of 5 capacitors, to provide protection if there is a fault ( these capacitors can and do go short circuit) in a section It will be a big unit, and you will need 50 of the clips and a big plate to attach them all to.

When I saw the heading I thought you were building a 1kW pumped laser....

[Chris Wilson]

Thanks, this all sounds very fiddly and messy though. Is there the slightest prayer of finding a single oil filled cap to do it as the original designer did his? How do the makers of the current  big tubed RF linears smooth their power supplies? We are happy to have the power supply in a separate housing on a wheeled trolley, because of the weight of the transformer, but another case just for the caps is getting ridiculous

[SeanB]

Contact the big transmitter suppliers, or eBay?

Big transmitters use big expensive caps. Harris, Collins and such are the makers. Filling a used microwave oven with the capacitors and the bridge rectifier is now looking better. 50 units can be packed surprisingly tight, as they can be stacked into a block. You just need the clearance at the terminal ends, and they come with a nice creepage controlled connection that has at least 2 connections, and you just have to use well insulated interconnections.

[Chris Wilson]

OK, will look into the microwave caps, thanks Sean. Have put a watch out on Ebay as well, for a big oil filled jobbie, cheers.