[Heathkit] SB-610 Selenium Diodes - caps across silicon diodes.

[Garey Barrell]

The only instance where I have seen improvement by using caps to bypass 
individual diodes is in a VERY high gain audio amplifier power supply 
(such as in a "direct conversion" receiver).   They help with what was 
called "tunable hum", but required more on the order of 0.1 uF across 
each diode.  I use 1N4007 diodes for everything except transmitter power 
supplies where I use 1N5408s.   The 1N4007 is a PIN structure, possibly 
that accounts for the difference in noise generation?

Fast recovery diodes are preferred for DC/DC converters, but they are so 
noisy they are normally well shielded and bypassed enough to take care 
of any RF noise anyway.

None of my design references for power supplies recommends bypassing of 
individual diodes, and only a few suggest small (0.01 uF) caps across 
electrolytic capacitors.   Most designs for RF purposes have 0.01 - 0.1 
uF caps across output terminals and of course on transformer primaries 
with "AC Line rated" bypasses.  Small caps directly across transformer 
secondaries are typically discouraged since a failure will usually 
destroy the transformer.

73, Garey - K4OAH
Atlanta

Drake C-Line Service Manual
<http://www.k4oah.com> 



Kenneth G. Gordon wrote:

>Concerning capacitors across silicon diodes: 
>
>in my experience, this is a HIGHLY desireable thing to do, for the 
>following reason: silicon diodes switch so quickly that often, depending 
>on many factors including transformer impedance, load, and capacitor 
>values, among others, the diodes can generate copious quantities of 
>RF noise.
>
>Installing capacitors across each diode and from each HV transformer 
>lead to ground often eliminates this type of noise.
>
>I have had much experience with this and remember that the first time I 
>finally tracked down the source of the noise plaguing my receivers, I 
>was utterly amazed. 
>
>Since that time I routinely shunt each diode in ANY power supply I build 
>which uses silicon diodes with a PROPERLY SIZED disk-ceramic cap. I 
>don't always use the disk-ceramics from each HV transformer lead to 
>ground, installing those only when necessary. 
>
>Often, the center-tap of the transformer, or the connection to the power 
>line through the primary, is close enough to ground to bypass RF noise 
>from the bypassed diodes to ground through that connection.
>
>Sometimes, it is not enough though, especially when the transformer in 
>question does not have a centertap connection to ground.
>
>Just my $0.02.
>
>BTW, seleniums, for all their OTHER faults, are not as prone to this 
>since they switch much more "softly".
>
>Ken W7EKB
>
>  
>