Re: MMC resister problem

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

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

Having a design that by its very nature leaves charge and stored energy in
the system, and is impossible to discharge externally, makes me a bit
nervous.  If you were potting the caps permanently in epoxy or something it
would be different (although, still dicey..)

One of the beauties of a MMC is that if something goes awry and you DO lose
a cap, you can replace just the one.  Therefore, serviceability of the
arrangement is desired.   The plastic cover over the cap connections seems
a "removeable service panel". If it's serviceable, it's accessible, and you
should have separate bleeders. Alternately, you could be prepared to spend
a lot of time with a pile of suitable clip leads, a carefully thought out
sequence of applying the jumpers, and some discipline to follow your
procedure, so you can jumper out all the caps one by one.

I recognize that commercial energy storage caps (e.g. from Maxwell) are
built up from series/parallel combinations of smaller caps, and, do not
have internal bleeder resistors, however, they are not intended to be
serviced (they're in sealed steel or plastic tanks). If I were dismantling
one of these things, I'd be real careful during the process.


A typical MMC component cap stores less than a joule, so it's not likely to
kill you, but one should be careful of unforseen situations where a failed
(open) cap (or a broken solder joint, more likely) causes the remaining
caps in the string to retain charge, or where parallel strings can store a
very significant amount of charge. While the individual cap may only store
1/2 a joule, an MMC made up of 100 could store 50 Joules, which CAN kill
you, or, less dramatically, cause a significant muscle contraction leading
to mechanical damage or burn a nice little hole in your finger.

There's a reason for those design rules in the NEC and the "good
engineering practice" of separate bleeders for each element in a capacitor
string.



Tesla list wrote:
> 
> Original poster: "Justin Hays by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<pyrotrons2000-at-yahoo-dot-com>
> 
> Hi All,
> 
> (snip from Jim Lux)
> > A single resistor across a series combination of capacitors does
> > not provide a safe bleeder function.  It is entirely possible that
> > there may be significant charge on the capacitors.  "Gedanken
> > Experiment":   two capacitors, equal values, charged to 1kV.  Hook
> > them in series, with polarities opposite.  Voltage across series
> > string is zero volts, so putting any resistor across the entire
> > string won't result in any current flow, yet the capacitors are
> > still charged.
> >
> > For a MMC, the only safe approach is to put resistors across each
> > capacitor.
> 
> There is more than one safe approach, which I mentioned in my
> previous post. Again, aside from using resistors across all
> capacitors, you can do the following:
> 
> 1). Use a single HV resistor across the entire MMC
> 2). Shield the open connections with plastic so there is a physical
> barrier.
> 
> Doing both of these will keep you safe. Not one or the other, but
> both. If you omit the plastic, you could take a shock from the
> residual charge on the inner cap's. If you keep the plastic but omit
> the main resistor, you could take a shock from other coil components
> that are connected to the MMC...(that is, if and only if the
> transformer secondary winding goes open circuit,,,in conventional TC
> setup).
> 
> Take care,
> 
> Justin Hays
> KC5PNP
> Email: justin-at-hvguy-dot-com
> Website: www.hvguy-dot-com