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".
>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.
Which is 180 degrees away from any other source Tom. You certainly enjoy
providing your own view of reality.
>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.
>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.
Correct if no AC line filtering is employed. That DOES NOT happen in
anything except ham equipment which has managed to survive outside of US
regs pertaining to other RF devices. Dont knock it...just enjoy it while
we can.
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!
>
>> There are still problems with mixed-lot diodes having unequal
>reverse
>> leakage at high temperatures. In a 1-2A HV supply it's obviously a
>high-
>> risk strategy to use mixed lots of 1N4007s of unknown origin - they
>are
>> being pushed to the limits of their current rating, and you can bet
>that
>> a cheapskate manufacturer won't use enough of them either. The ideal
>> solution is to use strings of diodes that are matched in a high-
>> temperature-reverse-bias test. On the other hand, it seems pretty
>safe
>> to use branded 1N5408s (3A rated) from a reputable supplier, all off
>the
>> same reel, and with enough diodes to give a good margin on PIV
>without
>> help from any Rs and Cs.
Good point since the 1N5408 is becoming the standard that the 1N4007 had
for decades. With everyone getting into the act, overall quality will
suffer. I was fortunate to obtain two full reels at an auction a few
yewrs ago at far less $ than a tank of gas (petrol for Ian, etc!).
>He also warned me about temperature and the mixed lots. In the same
>box, you might get diodes from different countries as well batches,
>hi.
>
>> Also, we now have better ways of dealing with mains spikes than
>> connecting a C across each diode. Mains filters have never been
>cheaper
>> than they are today. In Europe it's practically impossible to market
>any
>> large power supply without a mains filter to ensure compliance with
>> interference regulations - that is, to prevent interference FROM the
>> power supply going back into the mains - but the filter also
>protects
>> the PSU components from mains-borne transients.
>
>See, you brought up that nasty CE subject. Caps are cheaper than the
>mains filter.
>
>> 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).
Tens of thousands Tom.???? A bit of an exaggeration IMO unless you or
Martin are willing to release statistics.
>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.
Keep an open mind Tom, the capacitor benefit with 1997 components is
still in question. I would like to here more on this subject.
>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.
Which goes back to Day 1 on their use.....absolutely nothing new there.
73....Carl KM1H
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