[ARC5] Diode Mystery

[J Mcvey]

 He's talking about a "protection diode" not a rectifier scenario....

    On Monday, November 30, 2020, 1:57:23 PM EST, <frledda at att.net> wrote:  
 
 #yiv8801669763 #yiv8801669763 -- _filtered {} _filtered {}#yiv8801669763 #yiv8801669763 p.yiv8801669763MsoNormal, #yiv8801669763 li.yiv8801669763MsoNormal, #yiv8801669763 div.yiv8801669763MsoNormal {margin:0in;font-size:11.0pt;font-family:sans-serif;}#yiv8801669763 a:link, #yiv8801669763 span.yiv8801669763MsoHyperlink {color:blue;text-decoration:underline;}#yiv8801669763 span.yiv8801669763EmailStyle18 {font-family:sans-serif;color:windowtext;}#yiv8801669763 .yiv8801669763MsoChpDefault {font-family:sans-serif;} _filtered {}#yiv8801669763 div.yiv8801669763WordSection1 {}#yiv8801669763 
As the filtering capacitance is increased, the spike of current in the diodes gets narrower in time and higher in intensity. As the Fourier analysis shows the EMI gets wider in frequency and stronger in amplitude, and the diodes have to work harder!

  

Best, Francesco K5URG

  

From: arc5-bounces at mailman.qth.net <arc5-bounces at mailman.qth.net> On Behalf Of Bob Groh
Sent: Monday, November 30, 2020 12:31 PM
To: Kenneth G. Gordon <kgordon2006 at frontier.com>
Cc: ARC-5 Maillist <Arc5 at mailman.qth.net>; Robert Eleazer <releazer at earthlink.net>
Subject: Re: [ARC5] Diode Mystery

  

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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