Rich wrote:
>
>
>>
>>>Well, so far the only significant reason that has been offered, by Steve K.,
>>>is that for fast transients, the rectifier string "appears to be a string of
>>>small capacitors." True enough, and indeed small they are. Down in the 10pf
>>>range.
>>>
>>>Now. lets consider this in context. These rectifiers will typically appear on
>>>the secondary side of a large transformer that is not exactly a shining
>>>example of high-frequency transformer design. Although I have never gone
>>>looking for it, I would not expect to see much high frequency energy on the
>>>secondary side. Especially peaks in the range of many kilovolts.
>>>
>>Why not?
>>
>>It is quite common to see mains spikes of 1kV and more on the *primary*
>>side of the transformer. According to ANSI/IEEE statistics, "medium
>>exposure" sites in the USA can expect several hundred 1kV-peak mains
>>spikes per year, and even 5kV-peak spikes at about 1 per year.
>>
>â?¬ Only with no load on the transformer.
These are incoming spikes on the 115/230V mains.
The issue is how much they are stepped up by the transformer, and then
damped down by the rectifier/capacitor.
I accept Tom's statement that a power transformer does not have the same
high-frequency response as a purpose-designed audio transformer - but a
typical HV transformer has a step-up turns ratio of 10:1 or more, and it
takes a lot of higher-frequency roll-off to compete with that.
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?
> "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.
--
73 from Ian G3SEK
New e-mail: g3sek@ifwtech.co.uk
New website: http://www.ifwtech.co.uk/g3sek
|