[Amps] caps across rectifier diodes

[Gary Schafer]

Diodes today are built differently than they were years ago. The old diodes
had no reverse current protection in them and external resistors were used
to equalize reverse voltage across the string so that the reverse voltage
limit was not exceeded on any one diode.
The problem with the resistor string comes from resistors not all being the
same exact value. The resistors could actually force reverse current thru
some diodes.

The new diodes have a reverse current avalanche area in them that acts like
a zener diode. When the reverse voltage across them exceeds the limit the
zener starts to conduct to protect the diode from reverse current. The zener
is small and can not handle a lot of current but it is enough to keep from
zapping the diode. 
When you have a string of diodes there may be some small leakage current in
the reverse direction that can cause the zener to conduct. But if one does
conduct, all of the diodes in the string would also have to conduct in order
to do any damage. Since it is a series string the current will be the same
in all elements. With resistors across each diode, a diode could conduct in
the reverse direction without having the voltage margin of the whole string.
It can force enough reverse current in the diode to destroy it that would
not happen without the resistor.

Same thing with capacitors across the diodes. Unless you have close
tolerance capacitors they can unbalance the string and actually force
reverse current thru some diodes.

This is why resistors and capacitors are not recommended across modern
diodes.

All this is spelled out in the latter ARRL handbooks in the power supply
section.

73
Gary  K4FMX  

> -----Original Message-----
> From: Paul Kraemer [mailto:elespe at lisco.com]
> Sent: Monday, March 02, 2009 1:02 PM
> To: Doug Renwick; garyschafer at comcast.net; 'Steve Flood';
> amps at contesting.com; rfamplifiers at yahoogroups.com
> Subject: Re: [Amps] caps across rectifier diodes
> 
> My question too, what does it hurt to put the resistors and capacitors
> across modern diodes?
> Don't tell me it isn't needed. Tell me what is it going to hurt if I want
> the belt and suspenders.
> I've always put the resistors and capacitors across my diode string
> designs
> and have never had a problem doing that, so please enlighten me, what is
> it
> going to harm.
> Paul K0UYA
> ----- Original Message -----
> From: "Doug Renwick" <ve5ra at sasktel.net>
> To: <garyschafer at comcast.net>; "'Steve Flood'" <kk7uv at bresnan.net>;
> <amps at contesting.com>; <rfamplifiers at yahoogroups.com>
> Sent: Monday, March 02, 2009 11:51 AM
> Subject: Re: [Amps] caps across rectifier diodes
> 
> 
> > My Ameritron amp diode board with 1N5408 diodes has .01M 1KV caps across
> > them.
> >
> > Doug
> >
> > The train doesn't stop here anymore.
> >
> > -----Original Message-----
> > From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com]
> > On Behalf Of Gary Schafer
> > Sent: March 1, 2009 2:25 PM
> > To: 'Steve Flood'; amps at contesting.com; rfamplifiers at yahoogroups.com
> > Subject: Re: [Amps] caps across rectifier diodes
> >
> > No, modern diodes don't need or want resistors or capacitors across
> > them.
> >
> > 73
> > Gary  K4FMX
> >
> >> -----Original Message-----
> >> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com]
> > On
> >> Behalf Of Steve Flood
> >> Sent: Sunday, March 01, 2009 11:53 AM
> >> To: amps at contesting.com; rfamplifiers at yahoogroups.com
> >> Subject: [Amps] caps across rectifier diodes
> >>
> >> I am using a string of 4 6A10 diodes in each leg of my HV power
> > supply.
> >> Should these each have a .01uF 1kv cap across them?  I see some
> > designs
> >> that do this and some do not.
> >>
> >> Tnx,
> >> Steve KK7UV
> >
> >
> > _______________________________________________
> > Amps mailing list
> > Amps at contesting.com
> > http://lists.contesting.com/mailman/listinfo/amps