"Ian White, G3SEK" wrote:
>
> Unless I misunderstood Tom's point about mains transformers saturating
> at 1.3-1.4 times normal primary voltage, surely that would only apply at
> full load? The main risk to the rectifiers is more likely to be during
> RX periods, when the load is at a minimum but the DC voltage is at a
> maximum. I don't believe the saturation argument would apply then, would
> it?
>
A transformer core saturates with voltage not current. It will saturate with no
load
at all. It is easy to see. Take a transformer with no secondary connection and
put
an ac ammeter in series with the primary (or clamp on meter) and hook it to a
variac. watch the current as you turn up the voltage. You will see a definite
knee
in the current. When the core reaches saturation the current will rise sharply.
>
> > "Why"? - - With a full-wave
> >rectifier and filter C there is a low-ESR load - as he explained.
> >
> That wasn't how I read Eric's original posting, but I accept your and
> Tom's point that short, narrow spikes will be quite effectively clamped
> to the DC voltage on the capacitor.
>
> However, that argument cannot hold up forever as the spikes get longer,
> and turn into voltage "surges" that last for more than one mains cycle.
> It only takes a couple of half-cycles at a new, higher primary voltage
> to pump the capacitor up to the new rectified DC voltage (within a few
> percent). Such surges are at the fundamental mains frequency, so any HF
> roll-off through the transformer would not help.
>
> These longer surges can come from things like braking a large spinning
> motor while it's still connected to the mains, as Paul mentioned.
> Another common source is the Field Day generator, when all the stations
> just happen to stop transmitting at the same time; for a few moments the
> motor is left running at full throttle, so there is a significant
> frequency/voltage surge before an older-style motor governor can catch
> up with events.
>
> If one of those longer surges arrives during a RX period, when the HV
> supply is already at maximum output voltage and there is no significant
> load to pull the voltage down, then it will simultaneously pump up the
> output voltage and increase the voltage on the reverse-biased diodes. I
> suspect that's what kills 'em.
If you have surges that big MOV's aren't going to help either. The MOV will get
cooked too.
73
Gary K4FMX
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