[Milsurplus] [ARC5] Hello Smart People Again:

[Richard Brunner]

Yes indeed.  I don't like to blow transistors and intuition said don't
go too low on resistance, so I never reached that point.  All my
applications have been receiver-size power supplies, and it would be
interesting to try a large one. This circuit also has the advantage of
NO RFI hash to trouble the receiver, in contrast to vibrator supplies.

Richard

On Fri, 2010-08-06 at 13:28 -0700, J. Forster wrote:
> Richard,
> 
> If you make the base drive resistors too small, the transistors are driven
> waaay to far into saturation. When the circuit switches, it takes a while
> to get the excess space charge out of the base region, so both transistors
> are turned on during the switching transient. This can cause one
> transistor to be fully turned on, but with double the DC supply voltage
> across CE, resulting in very high dissipation and operation outside the
> SOAR. This blows transistors and puts big current spikes on the power
> lines (making EMI).
> 
> There are better base circuits, but they are a bit more complex.
> 
> I spent months studying this on Space Qualified power supplies and blew
> several $500+ transistors.
> 
> -John
> 
> ================
> 
> 
> 
> > Transistor vibrator replacement:
> > I have used a simple version of this circuit many times which works
> > without fail, usually when I have tried to restore a vibrator and have
> > damaged it beyond help.  Two 2N3055 type transistors with collectors
> > connected to the ends of the LV transformer winding, two 5000 Ohm
> > resistors connected base to opposite collector, emitters grounded, + LV
> > to the transformer center tap.  The other parts in the referred diagram
> > are superfluous.  Buffer capacitors installed in the power supplies may
> > contribute to stability, for example in the DY-105 there is a .047
> > across the primary, and .008's across each half of the HV winding.  The
> > base drive resistors are not critical, and a little experimenting is in
> > order - if too large it won't start, and if too low the transistors are
> > driven hard.  This circuit is slightly more efficient than a vibrator.
> >
> > Richard, AA1P
> >
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