[Amps] caps across rectifier diodes

[Roger]

Carl wrote:
> That is only one persons OPINION and is in no way a complete 
> explanation.
>
> A capacitor across a single diode will snub the spikes if they actually 
> exist. A cap across each diode of a string will do nothing except hasten 
> a possible failure in a marginal string.
>
> Learned PS engineers suggest that with some diodes that a .001 to .01 be 
> placed across the complete string and with another to ground at their 
> output to the filter. Different diodes require a different cap value. 
> >From what Ive read the 1N5408 isnt a candidate unless it is defective. 
> For years cheap hams have been buying hamfest bargain diodes that are 
> often reject floor sweepings.  The statement offered earlier about using 
> diodes from the same batch is most valid.
>   
I had to laugh at this part.  In a former life (pre college) I worked in 
the semiconductor industry and in particular in the labs. I was given 
boxes of diodes.  *Some* were great with high voltage and relatively 
high current, but the majority were...well...worth what you'd seep off 
the floor.  I don't even have the equipment for testing that stuff any more.
> Later today I will test 6 different amps in standby and with keyed idle 
> current using a 300 mHz Tek scope with HV probe and report back.
>
> In the past I have looked at many amps and PS with that setup with the 
> goal of eliminating RF getting to the electrolytics and causing 
> premature failure. Something that Ameritrons and Dentrons have in 
> common. I have not noticed any diode noise but that doesnt mean it wasnt 
> there.
>   
I've never noticed diode noise in any of these amps or stuff I've built 
and that includes some mighty big mercury vapor rectifiers out of amps 
putting out a nominal 6 KW into a wide range of impedances.  The lowest 
I ever saw was around 5:1. OTOH when it comes to hearing noise,  for the 
stuff running on HF frequencies, I used to tune around on the trip into 
work looking for some specific signatures.  I rarely heard anything, but 
a few times I heard that signature well out. Once was almost from my 
driveway (Nearly 20 miles).  That ended up in a lot of door tightening 
in a hurry when I passed that on when I arrived at work.

Then once a month it was my duty to dig out the old "Singer Metrics" rig 
with the loop and do some serious measurements.  Man, was that thing heavy.
> Old technology diodes and mercury vapor rectifiers are well documented 
> as hash producers.
>   
I heard lots of stories and read lots of articles, but never ran into 
that problem, thank goodness.

73

Roger (K8RI)
> I suggest that anyone else with similar testing equipment do the same.
>
> Carl
> KM1H
>
>
> ----- Original Message ----- 
> From: "Vic K2VCO" <vic at rakefet.com>
> To: "Doug Renwick" <ve5ra at sasktel.net>
> Cc: "'Ian White GM3SEK'" <gm3sek at ifwtech.co.uk>; <amps at contesting.com>
> Sent: Tuesday, March 03, 2009 11:35 AM
> Subject: Re: [Amps] caps across rectifier diodes
>
>
>   
>> Doug Renwick wrote:
>>     
>>> Sorry? ... on the contrary.
>>> I believe the answer from the archives is "capacitors prevent RF from
>>> being rectified in the diodes, and generating harmonics."
>>>       
>> Here's the complete explanation in that thread from W8JI, who designed 
>> the AL80A and the
>> SB-1000 (which I have and which has capacitors across 1N5408's):
>>
>> --------------------------------------
>> "1.) The diode is a non-linear component with different recovery 
>> times. Let's picture one
>> diode in the string having much faster recovery time. This goes hand 
>> in hand with that
>> diode having less voltage breakdown. Slowing the rise time of voltage 
>> across that diode
>> (by having the capacitors reactance be much less than the diodes 
>> impedance near breakdown)
>> prevents one FASTER diode from taking a big hit in voltage cycle after 
>> cycle.
>>
>> 2.) When the diodes do switch, the waveform is not a sine wave. Being 
>> non-sinusoidal means
>> the waveform contains harmonics. These harmonics can be heard in 
>> receivers, and most ham
>> stations have receivers. Even if the PA does not have to pass CE tests 
>> for powerline
>> conducted EMI, why risk generating trash that can affect the receiver?
>>
>> 3.) Bypassing is almost always often less than perfect in high power 
>> PA's. Capacitors
>> prevent RF from being rectified in the diodes, and generating 
>> harmonics."
>> -----------------------------------------
>>
>> -- 
>> 73,
>> Vic, K2VCO
>> Fresno CA
>> http://www.qsl.net/k2vco
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>>     
>
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