[ARC5] Diode Mystery

[Kenneth G. Gordon]

On 30 Nov 2020 at 10:32, Robert  Eleazer wrote:

> 
> The other day I installed a rectifier diode in the 12VDC B+ line of a radio for reverse current 
> protection.  When I used my trusty Simpson DVOM to check proper continuity I was shocked to 
> see that the voltage read something like 30VDC.  At first I thought that the output voltage meter on 
> the bench power supply I was using was wildly off, but checking it at the output terminals showed 
> that it was the correct 12V.  But on the other side of the diode it read about 30VDC.
>  
> I used a different DVOM and it read the proper voltage on both sides of the diode.
>  
> Anyone have any idea what could have happened here?  Is it possible the diode created some 
> noise that confused the Simpson DVOM?  If so, should I be concerned about that noise getting 
> into the radio?

I have experienced that same event, Wayne. At the time, I suspected that the diode was 
simply making noise, and the DVOM was reading that noise. Like you, using an analog meter 
or a different make of DVOM read the correct voltage.

Furthermore, in many cases, I have heard RFI noise in nearby receivers from diodes in 
power supplies. This is why I routinely used to add a disk-ceramic by-pass capacitor around 
diodes in power supplies thst I built.

I attributed the noise to "too sudden" shut-off of the diode when in reverse-bias mode, but 
that is only a WAG.

It doesn't always happen, especially with more modern diodes...

Ken W7EKB