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Transformer coupling question

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. 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). 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?
Reply to
Robert Baer
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I kinda got lost in the middle of your explanation here - would you draw a picture of what it is you're trying to do here? It kinda sounds like you want your first toroid to have a sort of tertiary winding, which then goes to the primaries of three more toroids.

How close did I guess? And what's your end goal?

Thanks, Rich

Reply to
Rich Grise

That overshoot and droop is caused by running the magnetization current up till the core saturates and the transistors run out of current gain and come out of saturation. At that moment, since the core is saturating, the turns become poorly coupled and each winding acts like a fairly separate, variable inductor.

Why not run the voltage from the two collectors directly to these other transformers, so that the first transformer acts only as the frequency determining base drive mechanism, instead of also having to do duty as a power transformer?

(If the cores are all the same) you need to have a higher turns count on the other transformers, so those cores see less volts*seconds per turn and so, do not experience as large a flux swing per half cycle and do not saturate. Then they will not draw a big current spike at the end of each half cycle.

Reply to
John Popelish

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.

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.

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.
Reply to
Tony Williams

Hnatek's book has a fair amount of detail on this type of converter

Cheers Terry

Reply to
Terry Given

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