-=> Roy J. Tellason said to Greg Mayman
 -=> about "VEHICLE LED'S" on 09-12-03  04:06.....

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

HA! I wish!!!! But that would only make the transistor act like a
diode, more or less.

No, what I am talking about is a driver circuit that will start
putting enough base current into the transistor to saturate it
over the whole half-cycle of the input wave. Often this is a
square wave drive to the base, as being the easiest to generate.

And this circuit will also require power to run it and as a
source for the base current, usually by stealing some from the
main supply.

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

True. And since the drop is lower, the heat generated is less.

OTOH, it is a lot more complicated than a diode. And the diodes
seem to do an acceptable job in this application.

 RJT> I have a board here that has a bunch of power FETs on it,  gotta dig
 RJT> up some specs somewhere.  (Grabbing board)  There seem to be four
 RJT> marked K707,  and five marked K799.  They're all mounted to heatsinking

Nope, that means nothing to me. I can't keep track of these part
names these days, and I can't afford to buy the books that would
tell me :-(

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

Yep, I agree. Once you find the specs of those parts, you'll find
uses for them.

 RJT> with _ten_ 12v batteries in it) and I'm guessing the board with the
 RJT> power FETs was the inverter... 

That's very likely! FETs are very good as drivers in an inverter
as they have a lot faster switchoff than bipolars and this means
lower dissipation in the switches.

IIRC those losses are due to reverse currents flowing during the
switchoff time, or was it more complicated than that.....

From Greg Mayman, in beautiful Adelaide, South Australia
   "Queen City of The South"    34:55 S  138:36 E

... Semantics is the propagandist's worst friend. - Stuart Chase
___ Blue Wave/386 v2.30

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