[Amps] caps across rectifier diodes

[Carl]

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.

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.

Old technology diodes and mercury vapor rectifiers are well documented 
as hash producers.

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
> _______________________________________________
> Amps mailing list
> Amps at contesting.com
> http://lists.contesting.com/mailman/listinfo/amps