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" <[EMAIL PROTECTED]> To: "Discussion of AM Radio" <amradio@mailman.qth.net>; <[EMAIL PROTECTED]> 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 <[EMAIL PROTECTED]> 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 > > > > > ______________________________________________________________ > > AMRadio mailing list > > Home: > > http://mailman.qth.net/mailman/listinfo/amradio > > Help: http://mailman.qth.net/mmfaq.html > > Post: mailto:AMRadio@mailman.qth.net > > AMfone Website: http://www.amfone.net > > AM List Admin: Brian Sherrod/w5ami > > > > ______________________________________________________________ > AMRadio mailing list > Home: http://mailman.qth.net/mailman/listinfo/amradio > Help: http://mailman.qth.net/mmfaq.html > Post: mailto:AMRadio@mailman.qth.net > AMfone Website: http://www.amfone.net > AM List Admin: Brian Sherrod/w5ami >
