# RE: MMC resister problem

```Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Gary,

I too wonder how the commercial caps made from a series of "packs" manage
to keep the voltages equal.  Having them all enclosed may help some as
opposed to MMCs with exposed leads with corona.  They could also make the
oil slightly conductive but I don't think they do.

In MMCs, the unequal charges would come mostly from corona at the ends
(that is a rectification process to a point).  Other near conductive
objects would also have capacitance to them.  Of course, construction and
"dirt" in the assembly can do all kinds of things too.  The bleeder
resistors solve all that and make them safe too for only 20 cents more...
I think every MMC plan uses bleeder resistors from day one so were all set.
But MMC caps CAN arc internally without harm.  I do worry about the
commercial caps, since they don't recover from an internal arc...

Cheers,

Terry

At 08:59 AM 6/4/2002 -0400, you wrote:
>I can vouch from experience that a charge sufficient to make a healthy
>"snap" when shorted exists on individual MMC caps after the power is turned
>off, even though the MMC end-terminals are shorted by virtue of the NST
>secondary.
>
>It's not at all clear how these asymmetrical residual charges came to be.
>If individual caps have slightly different capacitance values, then their
>respective voltages will scale inversely with their value, but there's no
>(apparent) way that they can develop a voltage reversal with respect to
>others in the string.  Same thing with unequal leakage resistance.
>Something non-linear must be going on.  Could there be some rectification
>occurring as the corona inception voltage (AC voltage rating) of the caps is
>exceeded?
>
>But the fact that such unequal residual voltages does exist has some
>troubling implications for the MMC's voltage rating.  Say we have a string
>of ten 2000V caps and we want to charge the whole thing up to 20kV.  Fine,
>that's 2000V per cap.  But if one cap has an initial charge of 500V, then
>we've exceeded it's rating by that much.
>
>So, it may be that the value of individual bleeder resistors goes beyond
>personal safety and benefits the well-being of the caps.
>
>Gary Lau
>MA, USA
>
>
>Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>"
>
>
>----- Original Message -----
>From: "Tesla list" <tesla-at-pupman-dot-com>
>To: <tesla-at-pupman-dot-com>
>Sent: Monday, June 03, 2002 8:36 PM
>Subject: Re: MMC resister problem
>
>
>> Original poster: "david baehr by way of Terry Fritz <twftesla-at-qwest-dot-net>"
><dfb25-at-hotmail-dot-com>
>>
>>
>> I think some of the large commercial made caps use many' packs' of caps in
>> series inside them........do they put resistors on each pack in these
>> caps???????
>>
>Nope, but as pointed out in a subsequent post in response, they are in a
>sealed container, not intended for access at any time after manufacture, so
>the safety implications of voltage on the individual units are negligible.
>The external user cares not what the inside does, since all they can see is
>the exterior terminals, and those CAN be discharged to zero with a short or
>bleeder.
>
>This might have life implications though... keeping a capacitor charged puts
>mechanical stress on the innards..... Such things are probably one of many
>of the factors affecting rated life of commercial pulse caps.
>
>
>
>
>

```