-=> Mike Ross said to All
 -=> about "TRANSISTOR RECTIFIER" on 09-22-03  21:19.....

 MR> Recently I mentioned to Roy that a transistor can be used as rectifier
 MR> if its B-E voltage breakdown was high enough to withstand it. Greg
 MR> Mayman remarked that it needed a squarewave drive at the bases to make
 MR> it work.

Well, no, I believe that what I said was that it needed some
separate drive to make it work and that a square wave was usually
used.

 MR> I tried working out some variations but the simplest I found
 MR> was to have a transformer with extra taps. Here's an example:

Yes, and another way would be to have separate windings for each
transistor connected between base and emitter, with a series
limiting resistor of course.

OTOH because it is a sine wave drive it won't turn on the
each transistor for their full half cycle.

And the need for limiting the base drive current at the peak
means that the transistors will be turned on and off "softly",
that is, they won't be in the saturated state except for a small
part of each of half cycle.

Adding an amplifier between the extra winding and the transistor
base allows the transistor to be driven to saturation much
earlier and to turn off much later.

Then if the amplifier has high gain its output will saturate
and limit the drive current to safe values over the whole of the
active period.

This is how the square wave drive I spoke of comes about.

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

... If it doesn't matter, it doesn't matter.
___ Blue Wave/386 v2.30

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