Greg Mayman wrote in a message to Roy J. Tellason:

 -=> Roy J. Tellason said to Mike Ross
 -=> about "VEHICLE LED'S" on 09-08-03  20:01.....

 RJT> But using them in place of rectifiers?  Normally the collector of a
 RJT> transistor would be reverse-biased.  Then you switch polarities and
 RJT> it's forward-biased?  I don't see how that's going to work real well.

 GM> You have to drive the bases of the transistors in phase with the
 GM> AC. This takes a small amount of current, but usually less than
 GM> 1/10 of the collector current, even for low gain transistors.

You're talking about something like a resistor from base to collector?  As
simple as that?

 GM> And the effect is that the forward drop through the rectifier is
 GM> down to the saturation voltage of the transistor, about 0.1v or so,
 GM> instead of about 0.6 to 1.0v for a conventional diode, or about
 GM> half this for a Schottky diode.

They'd make one heck of an isolator,  from the alternator at least --
either off or with a very low voltage drop...

 GM> It works VERY well, and even better with FETs.

I have a board here that has a bunch of power FETs on it,  gotta dig up
some specs somewhere.  (Grabbing board)  There seem to be four marked K707,
 and five marked K799.  They're all mounted to heatsinking which just sort
of bolted to the chassis,  about a quarter inch thick aluminum,  and
there's an assortment of smaller parts,  some diodes (zener?) that seem to
connect across G-S of the K707s,  a TO-220 packaged rectifier,  a few large
power resistors,  and 27 pins worth of interconnect that went elsewhere. 
Also some kind of transformer and a relay!

I figured it was worth taking out of the unit it came from for the power
FETs at least,  anyhow.

The other board also has a bunch of power semiconductors on it,  mostly
rectifiers in TO-220 and TO-3P packages and one power transistor,  TO-220
but on a heatsink that's bigger than most of what else is on there.  That
one overall looks like a switching power supply,  presumably to charge the
batteries (the unit used to be a UPS with _ten_ 12v batteries in it) and
I'm guessing the board with the power FETs was the inverter...

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