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On 2/11/18 1:26 PM, dennis otmaskin wrote:
Subject: Bleeder resistor for a Maxwell cap
I do have a question: in lieu of putting together a string of 39 resistors
as Gary has, I'd like to see if I can safely get the same value in a single
High Voltage resistor.
That's tough - real HV resistors are really expensive. As a general
rule, a 2W resistor has a max voltage of 500-1000V, limited mostly by
the creepage distance over the case, with a healthy margin.
Free air breakdown is 70kV/inch, 30kV/cm. But that's in a uniform
field. Between needle points, it's less than 1/3 of that. And over a
surface, it's less than that.
So a 1cm long resistor is rated at 1kV (or maybe 2kV, if it's really clean).
There's also a power dissipation issue - 1kV with 1 Meg is 1 Watt of
dissipation, and that's hot. 1kV for 10 Meg is 0.1 Watt which is more
reasonable.
For the usual sort of "several hundred watt" power supply (i.e. a NST)
you don't want the resistor string dissipating 10s of watts.