[ARC5] Solid State --- diodes...and RFI

Tom Lee tomlee at ee.stanford.edu
Thu Feb 8 21:03:26 EST 2018


One minor typo in my previous message -- it should have read 
"surprisingly abrupt transitions from on to off" instead of the other 
way around. Aargh.

All junction diodes exhibit the reverse recovery phenomenon to one 
degree or another, so by all means feel free to keep using those bypass 
caps. Schottky devices do not exhibit reverse recovery "snaps" [an 
exception applies if the Schottky was excessively forward-biased; a 
protective junction diode that is in parallel with nearly all production 
Schottkys can then turn on, re-introducing the reverse recovery 
problem], and "fast recovery" diodes recover, well, fast. 1N400x-type 
diodes are "slow recovery" devices and need your caps.

Cheers
Tom

-- 
Prof. Thomas H. Lee
Allen Bldg., CIS-205
420 Via Palou Mall
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu
650-725-3383 (public fax; no confidential information, please)

On 2/8/2018 5:45 PM, Kenneth G. Gordon wrote:
> On 8 Feb 2018 at 15:38, Tom Lee wrote:
>
>> Your analysis is spot on. Even "slow" rectifiers can have surprisingly
>> abrupt transitions from off to on (it's a phenomenon called reverse
>> recovery -- the diode stays on well after the voltage across it has
>> reversed polarity; when it finally does turn off, it does so with very
>> fast edges). The spectrum of the diode current looks like a remarkably
>> flat comb well past MF. Shunting that RF with a cap does an effective
>> job of knocking down the RFI.
>>
>> Tom
> That is very interesting to know, Dr. Tom. At the time, I was only guessing at the cause, but I
> did know that my "fix" worked. No more noise.
>
> So, now I am wondering if paralleling modern diodes with a disk-ceramic cap is still a good
> idea?
>
> I can see no bad effects for doing this, but I am not a professional at this sort of thing either,
> so I may be missing something.
>
> Thanks again,
>
> BTW, I am really a "belt and suspenders" type of technician anyway.
>
> Ken W7EKB
>
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