[ARC5] [Milsurplus] Hello Smart People Again.. More

J. Forster jfor at quik.com
Fri Aug 6 20:18:00 EDT 2010


There is a fairly simple improvement that can be made to this kind of
inverter that's somewhat more work, but performs significantly better. It
is the addition of a Feedback winding to drive the Transistor bases.

The problem is that it requires another transformer winding. Such
"Transistor Inverter" transformers were available years ago, but are
likely to be expensive now.

If the filament transformer you are using has enough "window" left for a
small winding, you can fake it.

First, you need to figure the number of turns you need. Excite the
teransformer primary from its usual 115VAC and put a turn or two over the
existing winding and measure the Volts/turn. Disconnect.

Second, wind a Bi-Filar winding of #22 or #24 magnet wire over the
existing winding and connect the end of wire #1 to the beginning of wire
#2 to form a center tapped winding. The number of turns should be chosen
to make the new winding produce 2 to 3 VAC.

Third, connect up the cpllectors to the transformer secondary leads as
before. The secondary CT goes to +28

Fourth, connect the ends of the new bifilar windings through resistors to
the transistor Bases. The resistors are chosen so that the current into
the Bases is about twice what is needed to saturate the transistors at
average gain (hFE) and full load current. hFE of 2N3055s is about 30 or 40
at 2 Amps Collector current. The voltage across the resistors is the peak
voltage between the CT of the FB winding and one of its ends MINUS 1.2 V
(2 Si diode drops).

The CT of the FB winding is connected as follows:


       +28
         |
         R1
         |
         o---------- CT of Bifilar
         |
        --- D1
         ^
         |
        GND

The diode can be almost any low current rectifier, like a 1N4001
The R1 is chosen so that it is just enough to bias the transistors into
the operating region. 1 K to 2.7 K should do it for good 2N3055s.

In operation the junction point of R1, D1 and the CT will sit at about
-0.6 to -0.7 VDC

FWIW,
-John

============












> From: arc5-bounces at mailman.qth.net [mailto:arc5-bounces at mailman.qth.net]
> On
> Behalf Of arc5 at ix.netcom.com
>> Thanks, everyone, for your help with this.
>> Will let ya know how it works out.
>
> Best luck with it, Dave.  I'm interested as well due to the lack of rotary
> thingies for some of my equipment, my BC-348Q being a prime example.
>
> Best regards,
>
> Michael, WH7HG BL01xh
> http://www.nationalmssociety.org/chapters/NTH/index.aspx
> http://wh7hg.blogspot.com/
> http://kludges-other-blog.blogspot.com
> Hiki Nô!
>
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