MIKE ROSS wrote in a message to Roy J. Tellason:

 MR> "Roy J. Tellason" bravely wrote to "MIKE ROSS"
(08 Sep 03 
 MR> 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.

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

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

Normally when you do a multiple-battery setup in a vehicle you get an
"isolator",  which is just a heatsink with a pair of diodes
mounted on it.  But you were saying something up there about "instead
of diodes"...?  To avoid the voltage drop?

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

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

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

Hmm.  But you'd need to get specific transistors to do this?

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