Re: Capacitor discharge question

On Jan 12, 2:29 pm, "Mook Johnson" <m...@xxxxxxxx> wrote:
I think that you're asking for trouble using tantalum caps in pulsed
power circuits. I'm not sure what options there are at 200C. Placing
the capacitive energy storage in the hostile environment, if there are
alternatives, could be asking for trouble.

You don't mention bleeder equalization parts, or other frequency
compensating components that I would expect to see in a
series-parallel connected module of this sort. This argues against
predictable pulse performance.

RL

The short circuiting is a infrequent but possible fault condition. Im not
concerned about the capacitor being damaged. I'm more concerned if this
capacitor discharged into .1ohms what peak energy rating on the resistor is
required to withstand this without blowing open. If that .1ohm resistor
opens up, there will be hell to pay. I have as big of a resistor I can
comfortably fit there now, I just need to know if it is up to the task
without actually doing an arch test. I will do one eventially but not
right away.

Your 60uF 600V capacitor bank stores 11J of energy.
Most small power resistors I've looked at have a
transient-power maximum spec of 5x their rated power
for 5 seconds. That means, for example, a 1-watt
power resistor would be able to safely absorb 25J
into its thermal mass over 5 seconds. Presumably
this energy could be absorbed into the thermal mass
much faster than 5 seconds. A de-rating can be used,
to take into account that very rapid events, say
faster than 10 to 50us, might be absorbed entirely
into the resistance wire, which has less thermal
mass than the entire resistor with its leads.

If your capacitor bank has the 3 ohms = maximum esr,
then a 3.1-ohm discharge will have a 186us time
constant. But if in fact it's much lower, say 0.5
ohms, then the 0.6-ohm discharge time constant will
be 36us. And more of the bank's energy will go into
the 0.1-ohm resistor. However, it still looks good,
even with a small 1-watt power resistor.
.

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