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Re: [TCML] A few questions, any help would be great
Brian Hall wrote:
Hi,
I sent these a week or so back and never got any replies, maybe the
message didn't go through. I have done lots of research and recieved
much guidance in building my TC, these items that folow are some of
the gaps that I would like to fill in, to get the best, and safest,
performance out of it.
Folks get busy, etc.
How does one calculate a good bleed resistor Ohm and Wattage rating
for each cap in an MMC?
Whatever is cheap and seems about right. OK, that's kind of glib.. The
idea is that after some "reasonable" time, the voltage will have been
reduced to where it's not "dangerous". The electrical code (which
doesn't apply here, but can provide guidance) says, get below 50V in
some moderately short time. So, if you have a 20kV charge on a 0.01 uF
capacitor, and you want to get to 50V in, say, 60 seconds.. ln(20000/50)
= 6 (that is, it takes 6 RC time constants to get from 20000 to 50). so
the RC time constant needs to be about 10 seconds. 0.01uF and 1 Megohm
is 10 milliseconds.. 0.01uF and 100 Meg is 1 second. If your cap is
actually 10 0.1 uF caps in series, then 10 Meg across each one should
work nicely.
BUT wait.. you don't want to be dissipating huge power in your
bleeders.. you could go bigger. 20kV/100Meg = 0.2 mA, so it's not going
to load down your NST much.. 20kV * 0.2mA is 4 Watts divided among your
10 resistors, that's 0.4 Watt.. that's a fair amount (you'd want to use
2 W resistors)... Might want to go a bit bigger resistance.. get the
power down to say 100mW or less.
There's also the voltage rating of the resistor to consider.. those
little 1/8watt resistors are probably rated to 250V.
Let's say for example, connected to the 15/30 NST, I have 20 caps,
each one 2000 volts and .22 uf, all in series (for 40kv @ .011 uf
total)
OK.. 20 caps, 20kV, 10Meg on each cap would be a total series of 200Meg,
so the current is 0.1mA.. not too bad. 2kV is pretty high for a single
small resistor (unless they're in oil or potted).. maybe a couple or 3
10Megs in series for each cap..
Is there some kind of equation that I can plug those numbers into, to
get the right value for the bleed resistor between the leads of each
cap in the MMC? Does SG size get involved?
How fast do you want to bleed down to "safe"?
How much power do you want to dissipate in your bleeders?
SG size doesn't matter at all.
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