On Thu, 8 Nov 2001 14:01:19 -0600 Lawrence Lile writes: > I can't think of a way to use FET's for AC without steering diodes, > if you > come up with one post it+ACE- > > Now, IR used to make a device that was an LED in combination with a > small > solar cell-like voltage generator. Created total isolation from > input to > output, PLUS automatically generated the 10 volts or so required to > turn on > a FET+ACE- As I remember, they were kind of slow, but might be OK > for a light > dimmer. > > I am frustrated with Triacs, they just eat up too much power and > make too > much heat. A FET could really reduce the heat sink requirements. > But with > a steering diode, how much more efficient can the combo really be? > I agree! It still seems triacs (or dual SCR solid state relay modules) are hard to beat for phase control. The IR part (http://rocky.digikey.com/WebLib/int%20rectifier/pvi5050.pdf) is interesting. It IS slow, and its output current capabilities are severely limited. I can just imagine it trying to drive an FET gate with a high Miller effect capacity... I DID do an interesting low voltage (12V) dimmer a while back using FETs. We took one of our standard 24 channel cards (used in the DM1224 and DM2412 at http://www.dovesystems.com) and drove FET gates directly. The FETs were N channel with the source grounded. The 12V loads were placed between a full wave rectified 12VAC and the drains of the FETs. The rectified 12V was not filtered, so we did not have to adjust our dimming curve (or pay for a filter capacitor). We used a full wave bridge rectifier, so losses there were a bit high (1.4V), but losses in the FETs were very low (pretty much due only to gate rise time). It worked well. However, the loads were not isolated from the rest of the circuitry, which would be required in a line voltage design, and the lamps ran on DC (though unfiltered). This would probably not be permitted in a line voltage unit. In such a case, you'd either have to go with two FETs per channel with steering diodes and messy drive circuitry, or a bridge rectifier per channel between the FET and the line to be switched, again increasing losses due to diode drops. So... SCRs and triacs are still hard to beat... Harold FCC Rules Online at http://hallikainen.com/FccRules Lighting control for theatre and television at http://www.dovesystems.com ________________________________________________________________ GET INTERNET ACCESS FROM JUNO! Juno offers FREE or PREMIUM Internet access for less! Join Juno today! For your FREE software, visit: http://dl.www.juno.com/get/web/. -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics