Post by thread:Whats an alternative to bleeder resistor

> 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 -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.