"Roy J. Tellason" bravely wrote to "MIKE ROSS" (08 Sep
03  20:01:34)
 --- on the heady topic of "VEHICLE LED'S"

 RJT> MIKE ROSS wrote in a message to Greg Mayman:
 MR> SCR's or transistors can be used instead of the rectifiers as they
 MR> have a much lower saturation voltage than a rectifier's forward
 MR> voltage.

 RJT> I wonder about that...

 RJT> An SCR,  maybe.  A transistor?  I dunno.  A few years back I acquired
 RJT> a few of these transistors in stud-mount packages,  that seem to be
 RJT> capable of carrying a *lot* of amperage,  if properly heat-sinked.
 RJT> And I was thinking about using them in an automotive application,
 RJT> more precisely to control the charging or not of an auxiliary battery.
 RJT> I've never sat down and worked out the details, though.
 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.

We are only talking of about 500 milliAmperes of current and 6 volts AC.
"We don't need no stinkin' badges... with stud mounted transistors"!!!
 

Well assume you have a steering network to properly bias the devices in
time with the AC waveform, then you have most of the problem solved.

I'm using a transistor as a voltage doubling pump/rectifier and at the
same time acting as voltage regulator (with a zener clamp) in a car
radio bench supply.

A transistor can stand a bit of reverse voltage. It is limited to the
voltage breakdown of the reverse biased emitter which then acts as the
new collector. (BTW there exist transistors with the emitter breakdown
voltage equal to the collector, had some 25 volt transistors like that).
The current gain drops dramatically say to about less than 5 or mayble
slightly more but you still have a transistor with gain. If you have
zero base current then the new collector won't let current flow either,
just like in normal mode.

So there you have it: a transistor can be used as a low loss rectifier.

 Mike
 ****

... I said, E.T., call OHM; but he resisted.
--- Blue Wave/DOS v2.30