[AMRadio] High Voltage Power Supplies


Ed Swynar gswynar at durham.net
Sun Oct 30 11:07:24 EST 2005


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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