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PICList Thread
'Whats an alternative to bleeder resistor'
2003\02\08@061844 by Russell McMahon

> 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.

If you don't mind slightly MORE complexity -

Provide a transistor which is turned on via a resistor divider when the
supply is up to voltage. Provide a second transistor with a collector
resistor to supply such that it doesn't exceed max rating of transistor and
load resistor. Connect collector/drain of first transistor to base/gate of
second. Bias second transistor on from supply.

Now, when full supply is present the above arrangement will draw current
only via Q1 base bias chain and Q2 bias on resistor. As supply starts to
fall Q1 turns  off, Q2 turns on and capacitors are discharged. You can make
this go all the way to ground with an extra diode and electrolytic (to
provide on bias to Q2 when supply gets very low).

This arrangement works very  well and is lowish cost.
Q1 can be eg BC337 and Q2 MPSA42 (needs to be high voltage).
Can use FETs if desired.

Ask me for cct if above isn't clear enough.

       Russell McMahon

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