Re: Check Your MMC Bleeder Resistors?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Sundog by way of Terry Fritz <twftesla-at-qwest-dot-net>" <sundog-at-timeship-dot-net>

Hi All!

I got a ton 'o MMC here.  I checked all the resistors I could reach (i
didn't really count on having to measure 'em), and aside from the dead ones,
they're all within specs.  They measure a little low, as normal, but the low
reading is consistant across 'em all (i measured over 100 resistors).  As
for the level of abuse they've gotten, everything from a 4/30 all the way up
to a 14.4kv pole pig at 8kva, when RMS current got the best of a few
strings.  Oopsie!

So my un-professional opinion, the resistors (I'm using the Digikey ones
Terry sent with the panasonic caps) are good to go till they physically
smoke-test.  Mine only died as the strings died and at that point, the
fireworks showed through :)
Some considerations,

- make your solder joints as smooth as possible.  I had some arcing because
of the little points I neglected to smooth on my solder joints.
- twist the leads.  Yes, it *IS* a PIA for a big MMC, but I've had some
crazy current across my MMC, and it was enough to de-solder the primary tap
on long runs. (wholy cripes!)  I would not want my MMC to start falling
apart.
- put a resistor on every cap.  I tried to cheat and put one every other
cap.  I got jolted for my trouble.
- do keep some space between the caps, about 1/4" or so.  I crammed some of
mine together to save space, but it makes visual inspection hard when you
see and hear a failing string, but can't find the caps that are split open.
and the panasonics can have most of their shells split open and the core
melted and *still* test good.  Trust me on that one.

I think that's got it all :)

Shad


----- Original Message -----
From: Tesla list <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Saturday, July 07, 2001 10:42 PM
Subject: Check Your MMC Bleeder Resistors?


> Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
>
> Hi Ed,
>
> At 11:36 AM 7/7/2001 -0700, you wrote:
> >
> >Terry
> >
> > I still worry about deterioration of those resistors due to operating
> >at much beyond their rated voltage.  Has anyone ever measured any after
> >a few hours of operation?  In my experience they usually fail open, an
> >unsafe thing if you're expecting them to discharge the bank.
> >
> >Ed
> >
>
> I just checked the resistance values on my coil's MMCs.  The big coil has
> not gotten much use in recent years but my small coil with it's EMMC
> certainly has.  The resistance values are all within 2% of their markings
> and they are 5% to begin with.  So, I cannot detect any deterioration.
>
> However, maybe it is a good idea to ask ALL users of MMCs to check their
> resistors just to be sure and see if there are any problems out there.
>
> One can simply ohm across each resistor with a meter that reads resistance
> high enough for the resistance being measured.  The values you read will
> look lower than spec since they are in circuit with the rest of the
> resistors in the array.  I think the formula is:
>
> Rm = 1 / (1 / (Rr x STRING / (ROWS - 1) + Rr * (STRING - 1)) + 1 / Rr)
>
> where:
>
> Rm = Measured resistance of a single resistor
> Rr = Rated resistance of a single resistor
> ROWS = Number of rows in the MMC
> STRING = Number of strings in the MMC
>
> Of course, the resistance tolerance rating gets into that too...
>
> That failing, the resistance will look "around" 5 to 10% low...  If any
> resistors measure "above" their rating given the tolerance, they are
> definitely bad.
>
> I would ask all users of MMC with bleeder caps to please check them to see
> if the resistors are holding up ok.  If your's are good or bad, please let
> me know either way at the e-mail address below so we can all get some idea
> of the number of people that did this and their results.  I will report
the
> numbers back to the list in a few days.  Also include a brief description
> of your MMC and how much use it has had.  Hopefully, we can get a real
good
> idea of how they are doing and if we need to be concerned or not.
>
> terry-at-hot-streamer-dot-com
>
> Cheers,
>
> Terry
>
>
>
>