>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.
>>>
>>?Ǩ Only with no load on the transformer.
>
>These are incoming spikes on the 115/230V mains.
>
Incoming spikes get rectified and stored in the filter-C.
>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?
? no
>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
>_______________________________________________
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>
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
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