You set a limit to the current that the fet will conduct - then switch it off. As the fet is on or off the dissipation is low. ----- Original Message ----- From: "Dave Dilatush" To: Sent: Sunday, February 24, 2002 6:33 AM Subject: Re: [EE]: Switching low-voltage AC Edson Brusque wrote... > Now, how about using an IGBT? I'm thinking of using this circuit to make >reverse dimming control (a topic that was discussed last year) in 100-240V >and I'm afraid I will burn some MOSFETs. Maybe IGBT would be a better >solution as they're made for high voltages, but the only way I can think is >to put the IGBTs on the MOSFETs place on your circuit and use a pair of >diodes. But then we'll have a large voltage drop. What's reverse dimming? Is that where conduction begins on the zero crossing and then shuts off partway through the cycle? I've never used IGBTs and I only know a little about them. But from what I've read they should function in this circuit, provided you put a diode in parallel with each one so current can go around it during its "OFF" half of the cycle. This circuit, using MOSFETs, would certainly not be very good for high voltage applications because of the relatively high Rds(on) of FETs with high breakdown voltages. Also, I would expect real problems--maybe insurmountable ones--with the huge current surges that occur during lamp burnout in high voltage circuits. Maybe IGBTs would alleviate these problems; I don't know. Dave -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body