[AMRadio] High Voltage Power Supplies


Gary Schafer garyschafer at comcast.net
Sun Oct 30 21:09:59 EST 2005


Hi Eddy,

The first thing I would do is get rid of those resistors and capacitors
across the diodes! They can cause more problems than they cure. Especially
if the resistors are carbon type. They change values tremendously. But if
you want to still use old vintage diodes you should have resistors and
capacitors across them. But use film resistors rather than carbon.

I would put in all new diodes like 1n4007 or 1n5408, s. 
With modern diodes you should not put any resistors or capacitors across
them. They can upset the reverse current balance in the diode string and
actually cause failure.

If you can use a bridge instead of a full wave circuit you will also have
fewer problems with transients. With a bridge circuit there can be no
reverse transients across the diodes as the large filter capacitors are
always across the diodes.

73
Gary  K4FMX

> -----Original Message-----
> From: amradio-bounces at mailman.qth.net [mailto:amradio-
> bounces at mailman.qth.net] On Behalf Of Ed Swynar
> Sent: Sunday, October 30, 2005 10:07 AM
> To: Discussion of AM Radio
> Subject: Re: [AMRadio] High Voltage Power Supplies
> 
> Hi Jim,
> 
> Many thanks for all the info!
> 
> Well, mine is the "classic" late 70's era string of 7 diodes per leg, each
> paralleled with a resistor / disk capacitor combo. The transformer is,
> indeed, centre-tapped, & the design is full-wave rectification, the output
> of which goes directly to a series of "parallel resistor-equalized"
> high-voltage electrolytics.
> 
> The transformer itself is a classic --- an old Fred Hammond job, with a
> cast
> iron(!) frame! The thing weighs-in at a "mere" 90 pounds! I got it surplus
> NOS from an old surplus house in Montreal some 30 years ago now...
> 
> I took a suggestion of Bry's, Jiim, & placed an RCA plug-in type surge /
> transient suppressor that I happened to have available between the p.s. &
> the AC outlet --- that is one thing that I'd never done before. If / when
> the supply fails again in future, I'll most likely "upgrade" the silicone
> string with some of the newer, more robust chunks of silicone that were
> simply unavailable 30-odd years ago...
> 
> ~73!~ Eddy VE3CUI - VE3XZ
> 
> 
> 
> 
> ----- Original Message -----
> From: "Jim Candela" <jcandela at prodigy.net>
> To: "Discussion of AM Radio" <amradio at mailman.qth.net>;
> <glowbugs-list at piobaire.mines.uidaho.edu>
> Sent: Sunday, October 30, 2005 9:42 AM
> Subject: Re: [AMRadio] High Voltage Power Supplies
> 
> 
> > Eddy,
> >
> >     It might be helpful if you describe that power
> > supply topology. Is it full wave with transformer
> > center tap, full wave bridge, and is the filter a pi
> > type, choke input filter, etc. If this is an option
> > for you, make a sketch, and scan it into a JPEG file,
> > and upload it to the net soemwhere. I have net space
> > if you email it to me. This way we can all look at
> > your schematic, and comment better without guessing.
> >
> >    In general diodes fail from two transient factors.
> > The first is current surge, and the second is
> > avalanche reverse voltage breakdown. Since you added a
> > step/start circuit already, the current surge issue
> > should be contained. That leaves reverse breakdown. As
> > Brian mentioned, a transformer primary varistor (like
> > V130LA10A, for 115 volts, or V250LA10A, for 220 volts)
> > might help from power line transients.
> >
> >     If you look at the Bill Orr handbooks around 1970,
> > Bill goes into detail describing diode failure modes,
> > and ways to protect them. This includes a custom
> > series R-C across the transformer secondary, and
> > across the filter choke (if choke input). Today's
> > diodes are tougher, and this precaution is often
> > unnecessary so long as good diodes are used, and the
> > diode PIV rating is at least 2X what the formulas
> > state you need. However diodes like the 1N4007 (1A 100
> > PIV) need protection, whereas diodes like the 1N5408
> > (3A 100 PIV) are a lot more rugged. In researching
> > diodes, look for the term "controlled avalanche".
> > These are the best because they can take repetitive
> > PIV spikes beyond rating without turning into a piece
> > of wire. The 1N4001 to 1N4007 series are not in this
> > catagory. Unfortunately the data sheets sometimes omit
> > the "controlled avalanche" term.
> >
> > Regards,
> > Jim Candela
> > WD5JKO
> >
> > --- Ed Swynar <gswynar at durham.net> wrote:
> >
> > > 'Morning All,
> > >
> > > I'm curious as to why I have to replace the silicon
> > > diode strings in my 2500 VDC power supply about
> > > every 8-10 years, or so...I just went through the
> > > exercise again early this morning --- turned on the
> > > B+ to my 2 x 813 linear, & got nothing back for my
> > > trouble but smoke from inside the enclosed p.s.
> > > unit...
> > >
> > > Opened it up, & the resistor across the relay in my
> > > time delay circuit --- transformer primary side ---
> > > was fried. I checked the conductivity of both legs
> > > of my diode string (it's a full-wave set-up), & sure
> > > enough, one leg was A-OK, but the other displayed
> > > conductivity on BOTH sides of each diode...not good.
> > >
> > > Is this the reult of transients / voltage spikes
> > > somehow "overwhelming" the diodes...?
> > >
> > > It used to be more frequent prior to the
> > > incorporation of the delay circuitry --- but,
> > > despite its presence, this HAS to be at least the
> > > second time that a re-build was in order.
> > >
> > > Any thoughts re. a dose(s) of possible "preventative
> > > maintenance" for the future...? I'm at a loss as to
> > > anything else here that I might try...
> > >
> > > Thanks in advance...
> > >
> > > ~73!~ Eddy VE3CUI - VE3XZ
> > >
> > >
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