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On 2/13/18 2:07 PM, dennis otmaskin wrote:
re: Bleeder resistor for Maxwell cap
Again, thanks everyone for the info.
Jay I have a final question on your method shorting the capacitor through a
*grounded* 100ohm 100w resistor. Not sure I understand why you'd need to
ground the resistor. Wouldn't it work to just put the resistor across the
capacitor terminals to short it? Do you have it grounded for extra safety
in the event the resistor fails?
Thanks, Dennis Hopington MA
in some circuits, neither end of the capacitor is connected to ground,
so while shorting the cap discharges it, it could still be "floating"
above ground potential. ALso be aware that if there's no other circuit
to ground, connecting one terminal of a cap to ground doesn't make it safe.
From a safety standpoint, what you want to do is follow this sequence:
high wattage 100ohm resistor to ground from Terminal 1 of capacitor.
Leave it connected.
Connect a zero ohm jumper from ground to Terminal 1.
Remove the resistor discharger from Terminal 1.
Connect resistor to ground and Terminal 2.
Leave it connected.
Connect zero ohm jumper from ground to terminal 2
Remove discharge resistor.
In a high current power supply, if you accidentally turn on the AC, with
the capacitor grounding wires(plural) grounded, you'll pop the fuse or
breaker.
In a NST power supply, it's more dangerous - the NST will happily supply
current to the jumper. If you remove the "grounded" end of the jumper
first, then it will suddenly jump up to full potential.
So, when removing jumper wires from capacitors - remove the connection
on the capacitor first, then the connection to ground.
My HV experimenting supplies (where I actually work on it from time to
time) have permanently connected ground leads That way, there's never a
possibility of having a safety jumper that's hot on one end and open on
the other.
On my quarter shrinker the circuit goes like:
AC power: variac: NST: diode bridge: shorting relay, both HV terminals
to ground: 600 ohm high current resistor: 20 uF energy storage cap.
This way, if AC power fails (or the AC power to the NST sticks on), the
shorting relay shorts the output of the diode bridge and shorts the
capacitor, through the 300 ohm resistor, discharging it quickly (but not
spectacularly - 20kV through 600 ohms is 34 Amps, for a few
milliseconds. But at least I'm not discharging 4kJ in microseconds,
like would happen if I just shorted the cap. It makes a "little bang",
and all the wiring stays intact.