[Milsurplus] Vibrator failure

[J. Forster]

aGEnuine Ham wrote:

> John:
>
> Actually, the solid state replacements help solve the resonance issue, as
> the 'make' and 'break' events are much softer than the metallic contact
> closure.  Chapter 9 of the Mallory book which Robert mentions leads you
> through the buffer capacitor issue.

I would dearly like to see a copy of that book. Is anyone either willing to scan
their copy, or loan one so it can be scanned, so that it can be made available
to all on the Trackpads site? There is currently > 500 MB of stuff there.

> The problem arises from applying a
> square wave to a transformer.  And, not a very good square wave at that.
> The solid state replacements do not have contact bounce, the off to on
> transition is clean, repeatable, and can itself be shaped to whatever is
> desired.  The buffer capacitors tune the transformer so the field
> collapse waveform follows the voltage produced by the vibrator more
> closely, and at the same time 'clean up' some of the higher frequency
> harmonics, producing an output which is easier to deal with, and less
> likely to punch through the transformer insulation.

Boy, that a paragraph which poses a bunch of questions.

AFAIK at the moment, the buffer caps tune the waveform so that the primary
contacts open at zero current, to minimize L dI/dT arcing in the primary
circuit. There seem to be two possible SS designs, a simple one that is a power
astable multivibrator, and a more complicated one that includes zero current
switch opening.  The latter would mimic an optimum vibrator operation. A simple
astable design may well need to be 'tuned' to operate correctly.

As far as square waves through the transformer, there are several issues.
Firstly, a square wave is optimum for rectification, since its ripple content is
low. OTOH, the higher harmonic content would increase the core losses (due to
eddy currents). The possibly destructive high voltage spikes would occur if the
primary circuit were broken at non-zero current. That's why a zero primary
current switching design would be best, IMO.

Finally, in a mechanical design, the current through the vibrator coil may be
altered by the buffer caps in those designs where the coil current also flows
through the load either partly or fully. This is a real can of worms.

> Anyone interested in vibrator power supplies needs at least the Mallory
> book.  I have often wondered if there are any other texts which shed more
> light on this arcane corner of radio?
>
> 73,
> George
> W5VPQ