> Plate modulation transformers and output transformers for tube audio
> amplifiers will transmit energy up to at least several tens of kHz,
> and there is nothing very different about mains transformers of
> comparable size. Therefore there is every reason to expect very large
> high-frequency spikes reaching the diode strings, from time to time.
Most HV transformers, if well designed, saturate at about 1.3 or 1.4
times the normal peak voltage.
Very few of them have good high frequency response.
This is quite different than audio transformers, which often have
interleaved windings, operate far from saturation, and also have very
thin laminations to improve HF response.
I've never used MOV's, and can't recall ever having a diode failure.
> >In order to get anywhere with this line of reasoning, we need to
> >adopt a (or some) generic models of transient pulses and look at the
> >possible results. I think it is futile, however, because of another
> >practical factor. In a typical full-wave rectifier, one leg of the
> >rectifier is always in conduction.
>
> This is not true for the capacitor-input supplies that we use, so the
> rest of the argument doesn't hold up.
With a full wave rectifier of any type, one string of diodes is
available to clamp any surges on the power line. As a matter of fact,
I use that very method to protect control lines going out to my
antenna switching system and it does a much better job than MOV's!
For my switching lines which are pulled high by transistor emitter
follower switches, I have negative polarity diode clamps. For the
positive going pulses I have a 2200 UF electrolytic bypassed by a
.1uF disc, with the capacitors going to ground. A regular 1N5408
diode goes from each control line to the capacitor combo.
Normal control line voltages keep the capacitor charged, and
transients from lightning dump into the capacitors.
If I remove the capacitors, despite having low voltage MOV's, I have
repeated transistor failures. With the capacitors in place and MOV's
removed I NEVER have a transistor failure during storms.
This system is not much different than normal operation of a full
wave rectified power supply.
> In a typical capacitor-input supply, both strings of diodes may be
> non-conducting for almost all of the time! In the first quarter-cycle,
> the "forward" diodes will not start to conduct until the applied AC
> voltage swings up above the DC voltage on the capacitor.
So? When the transient appears, one set of diodes will turn on and
clamp. There is no way a transient faster than the turn on time of a
conventional diode will make it through the transformer, so the
diodes have plenty of time to turn on. Transients BELOW the charged
voltage level of the filter caps are harmless. Transients above hat
level will turn the rectifiers on.73, Tom W8JI
W8JI@contesting.com
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