W8JI wrote:
>> W3VVN explained that modern rectifier diodes are designed not to fail by
>> the destructive arc mechanism.
>
>
>The design engineer laughed when I asked him about "modern diodes".
>He said the manufacturing process hasn't changed since the early days
>of the 1N400x series of diodes. As a matter of fact, he said there
>was LESS control today that years ago. Now the test is one of equal
>or better, rather than a curve trace method.
>
>But the conclusion was the same as you have below, with the
>exception that the arcing stopped way back in the 60's...... so
>"modern diodes" must mean less than 30 years old, hi.
Yes, and it's taken us almost that long to wake up to the change, and
re-think how to use the diodes.
>As the owner of
>several 1960's vintage muscle-cars, I'm not used to "modern"
>being that old! ;-) To me that's antique!
>
This is where it gets really confusing. I've seen those cars, and they
look like they were built yesterday!
>> The economics have also changed. PIV is now cheap, but high-voltage Rs
>> and Cs probably cost as much as the diodes. Thus it's possible to build-
>> in plenty of PIV (at least 2x the nominal) and not use the Rs and
>Cs.
>
>Bingo. That's what the engineer said. Use more diodes (at least two
>times expected PIV) and the caps and resistors can be removed.
> Of
>course I use the caps, because I like some RF and transient
>protection. Now with the darned CE testing, amplifier have to have
>bypass caps or diode-generated HARMONICS of the power line
>frequency can cause a PA to fail conducted EMI tests.
>
>Caps are cheaper than the
>mains filter.
>
Component costs are very different for manufacturers, than for
individuals. For me, .01uF close-tolerance 1-2kV caps would cost more
each than 1N5408s from a quality-assured supplier. A whole bridge of
20-28 caps would cost more than a surplus 16A mains filter! For a
manufacturer, the balance goes the other way, though that may change in
the future because prices of mains filters are still falling.
Given these differences in economics, I'm sure there is room for more
than one adequate technical solution.
The diode-generated harmonics are a problem - though again mostly for
manufacturers, because the European EMC regs don't require testing for
home-constructed amateur radio equipment. In professional power
supplies, harmonic considerations are pushing designers towards fast-
recovery rectifiers, even in 50/60Hz supplies. Unfortunately that means
more cost, especially for HV supplies because fast-recovery high-PIV
diodes are not cheap.
>> No individual builds enough power supplies to collect reliable
>> statistics on failures. The only reliable sources are commercial PSU
>> designers such as W3VVN, which is why I took good care to check the
>> background before going into print.
>
>Ameritron has sold ten's of thousands of HF amplifiers using 1N5408's
>with matched value .01 uF caps across the diodes. Out of all those
>PA's, diode failures are almost unheard of.
>The early AL-80 (Denny
>Had vintage) used 1/4 watt 470k resistors, the resistors would
>commonly arc and wipe out the 5408's. Diode failures were common
>(along with toasted diodes).
>
>Case history seems to indicate resistors are a problem when under
>rated for voltage, and high voltage rated resistors cost more than
>the diodes. The capacitors do, however, provide some benefits many
>people seem to agree about.
>
Thanks, Tom, that's very useful information.
>Thank you for the interesting post, I'm pleased the basic information
>agrees with what I've learned through other channels. "Classic"
>silicon power rectifier diodes apparently do not need equalizing, if
>you use enough safety margin and watch the temperature.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
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