Re: Transformer coupling question

In article <ndBJf.1644$VI6.9@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
Robert Baer <robertbaer@xxxxxxxxxxxxx> wrote:

I have a nice toroid transformer oscillator that works from
about 600mV supply and up; very ligit load; made with two
transistors, collectors drive CT primary with 10 turns each side
and 3 turns feedback to the bases (biased with 4.1K to supply).
The output waveform looks like a square wave but has definite
overshoot on each edge and a little droop; did not measure the
frequency; the toroids are made with some kind of powder ferrite.
Ten turn secondary to a FWB gives about 450mV at 600mV supply,
about 1.6V at 1V supply and about 5.5V at 3V supply.

A popular dc-dc circuit in the 1950's Robert. The variant
you describe above is one where the whole of the base
current is supplied by the 4.1K, and the 3+3 turn base
winding simply steers that current. Using the saturating
o/p transformer to control base drive always had the problem
of both transistors being ON at switchover.

In about 1957 the circuit was simplified. No base winding,
base drive being derived via cross-coupled collector-base
resistors, with speed-up caps in parallel. This speeded
up the crossover, and it was claimed that the spike was
much reduced. Possible -Vbe probs with npn Tr's though.

What i want to do is run a common "driving" secondary winding
from this toroid oscillator to three or so other toroids (same
size and type) that each has ten turn secondaries and that FWB
load (1meg, 0.1uF).

My old boss was into this sort of thing, but driving
only one or two other toroids at a time. When he drove
two toroids he half-wave rectified each one, on the
opposite polarities.

Do i need to put a resistor in series with that "driving"
winding to prevent excessive loading, and is it OK to use one
turn for that or better to use ten turns?

The last thing you need is any resistance in series
with the coupling loops. One turn is possible, but
leakage inductance is the thing that dominates these
coupled cores.

If you can measure the leakage inductances it should
be possible to do a reasonable model in LTspice.

--
Tony Williams.
.

Quantcast