>
Mr. Rauch wrote:
>I called Motorola, and no one there had any data to support this
>claim one way or another.
>
>It was all mostly rumor.
>
>When I finally reached a semiconductor engineer who actually was
>involved in diode design and testing, he said he'd think about the
>problem and call me back.
>
>About a week later, he called and said he had measured some
>diodes and he recommended using both capacitors and resistors
>across the diodes.
>
>The reason he gave was there is no guarantee the diodes are of the
>same characteristics for reverse recovery time, junction
>capacitance, or resistance when they go into cutoff. He said there
>was very large differences from batch to batch, and that the only
>test was a minimum acceptable rating for each characteristic. He
>said they were two cent parts, not 35 dollar parts and there was no
>guarantee they would be even close to the same specs.
>
>Since the proper components cause no harm, since they also aid
>in RFI protection, and since there was no professional engineering
>support available that said they were NOT necessary, I always use
>them.
>
>> Hi Vic, from 1996 ARRL Handbook, DIODES IN SERIES, page 11.9:
>>
>> "There used to be a general recommendation to place a resistor across each
>> diode in the string to equalize PIV drops. With modern diodes, this
>> practice is no longer necessary." It goes on to explain why then
>> concludes with: "In fact, shunt resistors can actually create problems.."
>> and why again.
>
>While it is true the WRONG type of resistor can be harmful, the
>correct type of components can do no harm.
? I contend that the statement "In fact, shunt resistors can actually
create problems..." has nothing to do with resistor problems. I have
seen situations where "equalizer resistors" caused potentially
destructive reverse current to flow in series diodes.
- - Please consider the following problem: Two diodes are connected in
series. One diode has a measured piv of 800v and the other has a
measured piv of 1001v. If 1800v is applied, zero reverse current flows.
// Now add a 100k-ohm "equalizer" resistor across each diode. Apply
1800v.
- Does any reverse current flow? If yes, in which diode and how much?
>........
- cheers, Tom.
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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