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RE: Cap life testing - Re: MMC cap bank - ion inception testing
Original poster: "Lau, Gary by way of Terry Fritz <teslalist-at-qwest-dot-net>" <gary.lau-at-hp-dot-com>
A couple of times, you suggested that a streamer strike to an MMC cap might
be the cause of trouble. I'm having trouble imagining a scenario where an
MMC might be (re)charged to a higher potential than Vgap. Am I missing
something? I just hate to propagate imaginary failure modes.
Regards, Gary Lau
Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>
Normally, we don't overvoltage caps to the 5500 volts needed to start
punching holes in the dielectric. Our MMCs will probably never see a
voltage punch though. But if we do have a problem like a short in an array
or a streamer hit. Our MMCs will laugh it off ;-)) Josh's filter will all
the popped caps was pretty cool! Far cooler was the fact it still worked!!
The dissipation factor may go up some over time, but most of the current is
in the solid plate area where the ionization is far less. We might not see
much dissipation increase from dielectric damage just at the foil edges.
A zero voltage crossing solid state relay may fix the line transient
problem right away. It would prevent full voltage from going across
discharged caps during turn-on or a momentary loss of AC which might blow
the fragile fuse prematurely. There is the HV transformer there too, but I
think it would still work.
I was thinking about how long of life we want... Assuming a museum coil is
run for 1 minute out of every 10 minutes, 12 hours a day, 365 days a
year... That is 438 hours/year for a very heavy use. For the home hobby
coiler, we are probably looking at maybe 5 hours/year.
We probably would like our hobby coil to last 50 years at which time "we"
won't care anymore ;-)) So that is 250 hours total run time.
For the museum, assuming the coil is real popular... The capacitors
probably have to last till a big overhaul at which time the gap motor,
relays, switches, etc. need to be replaced. I am thinking every 5 years
our museum coil will need a pretty big OEM overhaul which could include a
new primary cap bank... Maybe DC or another pro coil maker could tell us
if that is reasonable to assume. So we could say 2500 hours.
So we have a "wish" for 250 to 2500 hours of life. Not too bad and pretty
easy to test. Basically 10 to 100 days. Of course, we could make an MMC
to run at far lower cap voltage too vastly extending life. But then it
starts to get so expensive you might as well buy commercial caps unless you
need an odd value, ultra reliability, or like to hit the caps with
streamers ;-)). If you made a cap with only 500 peak volts per cap, the
life would be... just about forever... It would be interesting to compare
the price of a commercial cap and an ultra over-designed MMC which could
easily outlive it.*
BTW - Don't let Mark and I steal all the fun here. If you Ken (or anyone
else) wants to do this experiment too, feel free!! When it really gets fun
is when we all get different numbers :o))
If CD does not already know these numbers, just watch them use "our"
numbers themselves too ;o))) Maybe we can get some free caps out of it
;-)) Then watch them send us other caps to be tested too!! Vendors love
this kind of free testing ;-)) (as long as the results are good...)