Jim wrote...

>I'm designing a variable frequency drive, so rectifying and filtering a
>120 volt AC wave. This results in about 160 volt's DC. When I turn it
>off I bleed the voltage off through a couple of 1/2 watt 100k ohm
>resistors. This seems to take for ever to bleed. Is my best option to
>just reduce the ohms and increase the wattage to speed this up. Any
>other simple solutions? Thanks.

Only thing I can think of would be to use a much smaller bleed
resistance (small enough that the time constant of the resistor
and your DC filter cap is around one second) connected through
the NC contact of a 120VAC relay.  When AC power is on, the relay
is pulled in and the bleeder is disconnected; when the AC is
disconnected, the relay drops out and connects the bleeder,
rapidly discharging the filter cap.

The resistor won't need to be rated for very high wattage but it
should probably be a wirewound type, something that can stand the
high transient power that will occur during discharge.  Also, it
might be a good idea to put some sort of visual monitor (an LED +
resistor) directly across the filter cap as further insurance
against unknowingly going poking your fingers into a hot circuit.

If you want to get really paranoid, use a DPDT relay and parallel
the two NC pairs of contacts for redundancy.  (I learned paranoia
many years ago when I was assigned to a group that was developing
a 330 KVA VFD for a light-rail system; 480 VAC made me REALLY
paranoid).

Hope this helps a bit...

Dave D.

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