>
>On Wed, 28 May 1997 21:35:13 +0000 w8jitom@postoffice.worldnet.att.net
>writes:
>er.
>>
>>The design engineer laughed when I asked him about "modern diodes".
...snip...
>
>Which is 180 degrees away from any other source Tom. You certainly enjoy
>providing your own view of reality.
>
During the vhf parasitics debate, Rauchian reality consisted of denying
the validity of standard AC Circuit Analysis, promoting/demoting ex-Eimac
employees, and denying the validity of any piece of test equipment which
dared to disagree with Rauchian decrees.
The difference between 1960s Si rectifiers and modern Si rectifiers can
be observed with a high-potential breaksown tester. Apply reverse
voltage. Do not exceed the reverse-current rating (usu. about 5uA).
Observe the rate of change of reverse I as the reverse breakdown V region
is slowly approached. Modern Si rectifiers transistion smoothly from no
conduction to conduction. Old Si rectifiers transistion abruptly. It's
also interesting to observe what happens to the reverse breakdown V when
the rated reverse current is maintained, and junction temperature
increases.
>>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. As the owner of
>>several 1960's vintage muscle-cars, I'm not used to "modern"
>>being that old! ;-) To me that's antique!
>
>As an owner of vintage muscle cars also I fail to understand your
>comment. I suggest that you do a bit of research into the development of
>the 1N54xx series and "modern" process control before you start condeming
>it out of ignorance.
>
>>
>>> 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.
>
>
>Using roughly 2X the PIV goes back to the early days of SI diodes and can
>even be found in the ARRL Handbook along with the resistor and capacitor
>requirements.
>
>ABSOLUTELY nothing has changed in 1997; it is still prudent to use those
>values....just without all the unnecesary caps and resistors.
>
The new *Handbook* has changed. It says that shunt resistors can cause
reverse breakdown in series rectifiers.
...snip...
>I still have heard ( heard..not confirmed) suggestions that capacitors
>across diode strings can cause problems. From some early conversations a
>few years ago I get the impression that the problem is cumulative.....a
>few diodes/caps are OK but a long string with caps can be destructive. I
>will try and get some professional engineering advice on this before I
>saw off the limb!
>
My ancient, working-well, SB-220 still has 10 of the original dozen
ancient Si rectifiers. Two of the orginals had bad die-welds and opened.
They were replaced with modern rectifiers. No "equalizer" capacitors
are used.
Potted series-diode block manufacturers don't use "equalizer" capacitors
either, Carl. I don't believe you are out on a limb.
Considering that whenever a plus or minus line spike arrives, it gets
rectified and shunted to the filter C, why do we need spike suppression
caps across recifiers? . At the risk of being boring, I believe that Tom
Rauch is right-on about old habits being hard to break. It wasn't
really that long ago when the folks in Salem thought that burning witches
at the stake was a pretty good precaution for them to take.
Rich---
R. L. Measures, 805-386-3734, AG6K
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