# can anyone tell me what a spark gap capacitor is?

Discussion in 'Electronic Basics' started by [email protected], May 3, 2005.

1. ### Guest

i went to an electronics store today looking for high voltage
capacitors for building a marx generator,the lady behind the counter
was as confused as i am by the name spark gap capacitor. can anyone
tell me how these are any different if at all from regular capacitors,
and would they work for a marx generator?

2. ### Don BruderGuest

No difference in function, but maybe a difference in construction - A
cap specifically intended for spark-gap use might be built a bit
"beefier" to avoid frying it with the high voltages that a spark gap
wants to run at, or with different materials so that its "dump" speed is
faster, but aside from that... <shrug> A cap is a cap is a cap, at least
as far as basic function is concerned.

Basically, it sounds like you're looking for a high voltage rating, high
capacitance unit. What voltage range are you expecting the spark gap to
fire at? Use that as a base, and look for something rated for at least
that high a voltage, if not higher, in the mFd/pFd range your tuning
calls for. Your main worry is probably going to be arcing between the
plates and the possible resulting "pop" (or maybe "KABOOM!", depending
on how much power you're running) that can result from exceeding the
breakdown voltage of the cap's dielectric material.

3. ### SioLGuest

Maybe the answer is hidding in this last sentence, it might be a cap with
two safety electrodes (air gap) that discharge when voltage comes close
to maximum allowed voltage for that cap thus protecting the cap from
explosion/destruction.

4. ### colinGuest

yes thats what this is, its a capacitor with a built in spark gap in
parallel, it often looks like a totaly ordinary ceramic capacitor with a
slot machined into the top of it.

theyr often used for protecting high voltage electronics where even higher
voltages are used such as in CRT where the high voltage EHT might arc over
to other electrodes ocasionaly. youl find them on the base of most CRTs

not sure they have any use for a marx generator though.

Colin =^.^=

5. ### John LarkinGuest

It's usually a ceramic disc cap that has a slit sawed a bit of the way
down from the top edge of the disk. Imagine cutting a small
rectangular slice out of a pie. When the voltage gets high enough, an
arc jumps between the exposed metalization layers, protecting some
other circuit component, or the cap itself, from overvoltage.

John

6. ### JamieGuest

just two plates close together that will arc..
also they made ceramic caps at one time that
has a spark gap cut in them but i don't think
that is what you want.
check a guess.

7. ### Guest

thanks for all of your help. i plan on running this generator at the
low end of high voltage, the caps they had were rated at 2 kv. as for
the capacity, 1nf. i am not planning on blinding 2 foot arcs here, just
a little high voltage generator to "play" with. while wre on the
subject of capacitors, when using dc pulses and a capacitor to even
them out do capacitors maintain voltage while amperage drops between
pulses or the other way around? thank you so much.

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