[ARC5] Diode Mystery

[J Mcvey]

 30 volts on a 12V DC line? That's a hell of a lot of noise on a steady state. No inrush, no nothing.

I've done countless measurements with all kinds of instruments over the decades i've been doing electronics.One thing I have noted that most anomalous readings can be attributed to either operator or equipment failure.
Always back up and check them.


   On Monday, November 30, 2020, 4:17:45 PM EST, Dennis Monticelli <dennis.monticelli at gmail.com> wrote:  
 
 I had a 40 yr career in the semiconductor industry and can share some knowledge.  Let's think about this diode noise aspect by examining how a junction diode behaves in general.
Under forward bias it produces a very small amount of broadband noise; Shot noise proportional to the square root of current plus some 1/f Excess noise that depends largely upon how the diode was made.  It is not sufficient to cause any problems, period.  There is only leakage under reverse bias.  Again, no noise.
A silicon diode is capable of relatively fast switching up through HF frequencies.  So if presented with an abrupt change in excitation it will follow it.  Hit it with an abrupt step in voltage drive and it will conduct current equally abruptly.  That sharp change in current can produce a spurt of RFI.  Turn off the voltage drive abruptly and the current will collapse abruptly (somewhat shaped by the stored charge in the junction).  This can also cause RFI.  A 60Hz sine wave is not abrupt excitation.  So basic rectifier action will not cause RFI.  
There is a related issue, though.  If the circuitry is exposed to strong RF, such as your transmitter coupling into the AC mains, the RF can induce mixer action within the diodes of an ON power supply as those diodes enter into and out of their conduction cycles.  The diodes are non-linear during those periods so harmonics can be produced, including BCB harmonics.  It can also happen if the supply is OFF should the coupled RF be large enough.  Placing 0.01uF caps across the diodes fixes that potential problem.  
I don't see how a 100% forward biased supply protection diode could produce the noise problem theorized. Sounds more like a measurement problem of some kind. DVMs can be fooled, particularly when in the presence of a strong RF field.
Hope this helps with the understanding.
Dennis AE6C
On Mon, Nov 30, 2020 at 12:42 PM Owner <kargo_cult at msn.com> wrote:

I think you want to take a good look at the output of the bench supply 
with a scope set to a low AC/RF voltage
setting. I suspect the output has harmonic components from clipped 
sinewave somewhere in it. The DVOM
that read high has for some reason an unusual susceptibility to this 
stray voltage component.
Hue Miller

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