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Re: MMC Protection SGap current limit
Hi Ted,
The Panasonic caps I use have a internal real resistance of "around" 0.025
ohms. Thus, my 5 x 10 EMMC has a total resistance of 0.050 ohms. If I
charge it up to 21kV and arc it, the current calculates out to an
astounding 420000 amps! If you place a 30 ohm resistor into the loop you
drop this current to 700 amp which is good. That resistor has to be large
enough to hold off the full cap voltage across its terminals. Actually,
the inductances will limit the current much more than the real resistance
but that will cause all kinds of fun ringing which may tear the MMC up
also. One person was blowing big commercial caps when the safety gap fired
and adding resistance cured the problem, so it does work.
I put the safety gap basically across the main gap so that the primary
inductor of the Tesla coil will act as a current limiter. Some people fear
that the main cap may charge up to higher voltages especially in resonant
systems. However, as long as the interconnect wiring is sound, it should
not make much difference if the primary inductor is in series with the cap
and the safety gap. MMCs, if well designed, can probably take over voltage
hits well but normally they should never have to. I worry more about the
transformers getting damaged from over voltage more than the caps. I think
the recent posts about Ross Overstreet's "hot rodded" NST being successful
is due to the fact he "only" over currents it rather than overvoltages it.
He may run at 4X the rated current, but I bet he doesn't run it at 4X the
rated voltage ;-)
Cheers,
Terry
At 03:06 PM 05/14/2000 +1200, you wrote:
>Team
>
>Would anybody have a view on whether a MMC protection spark gap should have a
>resistor (around 30 ohms or so) to keep the peak current in the MMC to a
within
>the pk current capabilites of the caps used. Believe if safety fires without
>this
>limiting resistor possible to damage caps due too low an impedance discharge.
>Tnx for views
>Ted
>
>
>
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