[ARC5] Diode Mystery

[Bob Groh]

Regarding RFI from diodes in a power supply, that is (in my experience as a
design engineer) due to current spike that occurs every cycle when the
diode turns ON and the capacitor in the power supply is wacked with the
input voltage. When doing a redesign on HP-1144() power supply at Heathkit,
we initially failed RFI tests so we dug into it and there it was - on every
1/2 cycle when the diode input swung up and the input voltage exceeded the
filter capacitor voltage by rectifier diode drop, there would be a jump as
current rushed into the filter cap to replace the energy lost during the
off time. That current 'bump' generated quite a batch of RFI!

Hope that makes sense.

73
Bob Groh, WA2CKY

On Mon, Nov 30, 2020 at 12:19 PM Kenneth G. Gordon <kgordon2006 at frontier.com>
wrote:

> 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
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