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Doorknob capacitor substitutes

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throbscottle

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I probably need to replace a 16KV, 470pF doorknob type capacitor. They are very expensive, so I was thinking of putting 2x 10KV, 1nF ceramic disk type caps in series, which is much cheaper. Then my brain eventually ground round to the idea of hollowing out the original cap and putting the disk type ones inside (if they'll fit), kind of like an internal standoff (it has a hard casing something looking like bakelite and the back is filled with something looking like epoxy, so it may be possible).

The other alternative is to get some kind of HV poly type cap and mount it on a pillar. Maybe that's the best thing, since they do come in the voltage and value I want.

Thoughts anyone? Or suggestions of where to get one of these caps for cheap in the UK? Is there another name I should be looking for them by?
 
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Beau Schwabe

Active Member
What is the application if you mind my asking? ... I have made 30KV caps that are flat parallel plates with polypropylene as well as the same cap rolled into a tube for a 750KV Marx generator I made several years ago.
 

throbscottle

Well-Known Member
It goes in parallel with the final anode in my 45yr old 'scope. The actual voltage is supposed to be up to 11.2KV. I tried running the 'scope with it disconnected and couldn't see any difference in the trace, so I'm replacing it for completeness really (maybe it makes a difference at the fastest sweeps).
Interesting idea there. I don't have any polypropylene, but I do have some 2mm thick hdpe (well, I think that's what it is!) - do you think that would be any good?
Actually as far as I can make out from the calculator on this page: http://www.ajdesigner.com/phpcapacitor/parallel_plate_capacitor_equation.php I'd get a 17pF cap with that if it was a 2cm circle, so maybe not. I don't think I'll attempt more inventive designs for this!
Any pics of your Marx generator in action?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
It goes in parallel with the final anode in my 45yr old 'scope. The actual voltage is supposed to be up to 11.2KV. I tried running the 'scope with it disconnected and couldn't see any difference in the trace, so I'm replacing it for completeness really (maybe it makes a difference at the fastest sweeps).
It's simply an extra reservoir capacitor on the CRT, to supplement that of the CRT itself.

It helps to provide better voltage regulation for the EHT, although it's not much of a problem on a scope due to the fairly small beam current changes.
 

throbscottle

Well-Known Member
Thanks for that Nigel. So it may be better just disconnected until I can find a replacement?
 

Beau Schwabe

Active Member
Any pics of your Marx generator in action?
sure... how about a video? I ended up building this Marx up to 20 stages and it was damaged in our move last summer... One of these days i will build another one.

BTW, the caps in this Marx generator are flat and made from the side walls of kitty-litter containers. To hold back 30kV you need at least a half inch border of polypropylene from your capacitor plates and rounded corners are a must. Adhesive aluminum tape works well for plate material.
 

throbscottle

Well-Known Member
Yeah it's amazing what you throw away that now costs a premium. I once had a collection of Marvel comics including the first Spiderman stories... in the bin they went when they were simply "old".
But anyways, I'll leave it out for now I think, I guess it's less strain on the rectifiers that way, and get a replacement "for the sake of completeness" when the opportunity arises.
Nice spark, Beau. I may yet have a go at making caps... How thick is your polyprop?
 

throbscottle

Well-Known Member
Quite impressive that it stands 30KV then.
I was thinking I might saw the legs off this original cap and solder them onto the ends of a strip of PCB and mount 1 or more ceramic hv disk caps on that. But no rush. It seems getting that value/voltage combination is expensive no matter which way I do it.
 
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