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Re: [TCML] mmc cap protection by spark-gaps-any ideas?
Gary:
On Aug 19, 2008, at 10:31 AM, Lau, Gary wrote:
The resistors you discuss in DC power supplies are used for an
entirely different reason than why they're used in MMC's. In MMC's,
the power is AC, and the voltage division is determined solely by
the ratio if the capacitances. The bleeder resistor values are far
higher than the AC capacitive reactance, so the resistors have no
bearing on voltage division ratios.
When you connect a series string of resistors across a voltage source,
a.c. or d.c. the voltage drop across each resistor depends upon the
current through the string and since each individual resistor is in
parallel with a capacitor, the a.c. or d.c. voltage drop will be the
same across both capacitor and resistor.
The wire wound resistors used in the radio PS need to be high power
(which makes them more failure-prone) because the leakage current of
electrolytic is extremely variable over time and temperature, so the
resistor must be chosen to be significantly lower in resistance to
guarantee that IT determines the division ratio and not the variable
leakage resistance. That's just not the case in MMC bleeder
resistors.
The bleeder resistor across the output of a power supply is chosen as
part of the filter system and in many cases the filter system
consists of nothing but resistors and capacitors in a typical a.c./dc
cheap plastic radio. I don't think you noticed that the voltage
involved in the accident was 6 kv. That in itself should indicate
that in so far as the accident as concerned it did not involve
electrolytic capacitors. The capacitors involved were General
Electric, Pyranol, (oil filled) 1 mfd. 10 kv. working. The bleeder
resistors were there to help the regulation of the power supply. Just
what do the resistors do in an MMC? So far as I can see the MMC is
the equivalent of a single capacitor with low dielectric resistance.
If it might diminish your horror, consider that the likelihood of
_ALL_ of the many bleeder resistors in an MMC failing open is very
remote. The consequence of just one failing isn't that great. And
even if they did, the discharge path formed by the NST or other
transformer would ALSO have to fail for there to be a genuine hazard.
I have no horror of getting killed. As a matter of fact, I don't know
you and you don't know me so neither of us should have any horror of
the other getting killed. Of course if enough of us get killed
playing with Tesla coils we can expect Big Daddy to step in and outlaw
them as toys. That would be a disaster for a lot of nice people. So
out of common courtesy we should observe Safety First.
Any electrical shock any time and of any magnitude has the potential
of being a hazard. What can go wrong will go wrong.
It's not clear what you are proposing as an alternative. If you are
suggesting that folks not use bleeder resistors and instead use a
manual shorting stick every time they turn off their coil, I would
counter that:
1) Shorting just the ends of the MMC won't guarantee that the
individual caps are all at zero charge.
2) I would bet any money that users are more likely to neglect to
use the shorting stick, than there would be massive failures of all
bleeder resistors.
I am not proposing any circuit changes. The use of resistors in a MMC
is wholly justified but for good electronic reasons and NOT FOR
SAFETY. The MMC itself is a work around to keep the cost of the hobby
down. Nothing wrong with that.
I believe that it's far safer to have a 2nd tier safety mechanism
(bleeders) that is always present (the 1st tier is the presence of
the transformer winding), than to have a 2nd tier mechanism that
requires considerable diligence and is likely to be ignored.
My statement is that bleeder resistors across capacitors are not there
for reasons of personnel safety. There are many readers of this list
who have little or no experience with electronics in general nor high
voltage in particular. To hold out the concept that it is safe to
turn off the power to a Tesla coil and then get your fingers in it
strikes me as being just plain bad practice. Your are of course
right that safety devices are often ignored. Some people have been
known to park a car on a hill with out setting the emergency brake nor
putting the car in park. I doubt you are advising people not to
provide a shorting or grounding stick because they might not use it.
There is no doubt that bleeder resistors do provide some safety, but
they are not designed to be either fail safe nor infallible.
It should be pointed out that the retained charge in a capacitor is
d.c. They do not hold an a.c. charge (if there were such a thing). As
to a Tesla coil operating at a "high" frequency having anything to do
with it, I would remind you that "High" in the case of the Tesla coil
is only relevant to the 50/60 Hz. line voltage. Most operate within
the low or medium frequency range.
The mention was made in another post of the fact that capacitors with
a high voltage rating are often shipped with their terminals shorted.
Who ever shipped them was looking out for the next fellow. Capacitors
can frequently be charged by static electricity while in storage or
transit.
Frankly, I am wondering why we are having this discussion. How many
times have people on this list commented on the fact they have been
"bit" by a turned off coil and they should have waited longer. Any
thing we can do to keep some one from being hurt is worth doing.
I still want an explanation as to what you think the resistors do in
an MMC capacitor bank.
Safe coiling
Ray
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