Re: Further MMC woes.

["Robin Copini" <rcopini-at-xxxxxxxxxxxxx> (by way of Terry Fritz <twftesla-at-xxxxxxxxxx>)]

Hi Gary,

 The cap code you give '940C' implies a CDE part, i.e. Cornell-Dubilier.
Well..... I have to say I disagree with your assessment of these caps. I
have a mmc
built with CDE 940C caps (7x(33x0.33uF)) rated at 1600Vdc. The caps are fairly
plain looking with a cheap white stick-on label but... I run a pole pig
rated at
33Kv/220Vin and I overdrive it with 270Vin, at over 80amps, the coil throws
arcs
over 4 metres and I cannot get these caps even warm.

Best Regards

 Robin Copini, Australia.



Tesla list wrote:

> Original poster: "Garry F." <garryfre-at-pacbell-dot-net>
>
> Well, after trying to replace the blown caps with all the spares I had I
have a
> worthless $84 MMC.
>
> I built an MMC out of 3 strings of .033 uF caps at 2kv part number 940C
20S33K
> from Mouser electronics. 15 caps per string.
>
> I placed short pieces of thick plastic tubing around every other cap by
> slitting the tubing length wise and slipping it over every other cap. This
> effectively insulated every cap from its neighbor. Then I place poly sheeting
> between the caps and the resisters and put generous amounts of silicone seal
> around the ends to make sure there was no possibility of arching between the
> solder joints. I used heat sinks to protect the caps from heat when I was
> soldering the joints as well. Within 3-10 seconds after I hooked it up to my
> tank circuit of a static air-cooled gap and a 12/30 tranny, a cap would blow,
> throwing carbon all over the place.
>
> Also, I noted that resisters would BLOW as well, arching accross the body of
> the resister as if it were a spark gap.
>
> I know I got the correct type of resisters for I got the resister part number
> from Terry and they looked identical.
>
> I noted that the resisters would blow, and then when I snipped out the
resister
> the cap accompanying that resister blew.
>
> All in all about 10 caps were destroyed as I jumpered caps and tried
everything
> I could possibly do to make this thing work.
>
> As far as I am concerned, these caps are NOT suitable for MMCs and should be
> avoided like the plague! If the caps were good, there should have been plenty
> of other caps to take up the slack of the jumpered out bad ones to handle the
> voltage easily but they blew and blew and blew. Many of them look like they
> have been nuked. I watched one cap light up with dozens of sparkles as the
> dialectric was machine gunned clear through.
>
> By contrast, the EMMC cap that Terry Fritz sent had not blown but it does
seem
> to have quite a drift after a minute of operation, the coil seems to
reduce in
> power output.
>
> The part number for the caps is a part number for caps that have worked in
> MMC's but I got a note in the packing slip that the caps had been
replaced with
> equivalent caps instead of the original brown broad teardrop caps I was
> expecting with these white tubular caps. So, it's the replacements they
sent me
> that are defective. I think that if they had sent me the caps I originally
> ordered instead of these substitutes, I would have a good MMC.
>
> To say I am disgusted is an understatement and if it weren't for a kind soul
> here that offered to put together and test a multiple string MMC, I would be
> quitting the tesla hobby. I do not think I will be ever attempting to make
> another MMC myself. I do not think it would be smart of me to try this
again as
> apparrently, I am either doing something horribly wrong (I am pretty
certain I
> am not) or I have the worst luck in the world and am doomed to failure with
> making MMC's. I will not invite the thief of bad luck to have a heyday
with my
> wallet again.
>
> I feel like a fool, I feel like an idiot that doesn't know when to quit. This
> in fact feels like what I imagine being hooked on gambling (Something I
do not
> do), as I hope the next near $100 bucks gets me something that doesn't go
up in
> smoke.
>
> My only consolation is that maybe my extreme misfortune will save someone
else
> the loss.
>
> I won't risk mucking up the market for all of you by mentioning a thing to
> Mouser about this desasterous loss so I will keep my mouth shut. I will
forever
> wonder if I had at least called and asked nicely if they were aware of any
> problems with the replacement caps that they might have replaced them or
given
> me at least a partial refund. I do suggest that when ordering caps, to
specify
> that no substitutions are to be made. I feel THIS was the cause of my failure
> of this MMC.
>
> Terry Fritz, Would you like me to send this MMC to you so you can look at it
> and perhaps find out what the big problem with this cap is?
>
>