Hi All I ran the engineering department at a semiconductor burn lab for some years and we had well over a thousand power supplies of analog, switchers and SCR pre-regulated designs. We even modified Hobart MIG welders into power supplies (0-60 volts up to 600 amps). It has been my experience that the switchers will fail in a no output mode most of the time. This makes sense because the usual failing part is the switching transistors. If one or more of them fail the primary fuse will usually blow. With the output fed from transformer secondary there will be no output voltage or low output even if the fuse does not blow because of the distorted waveform driving the transformer. Analog power supplies usually fail by going to the rail because the series pass transistor shorts and connects the raw supply to the output. This is why most high end analog power supplies have some kind of crowbar circuit on the output that shorts the output if it exceeds some set voltage. It is always possible that the regulator could fail or even the feedback circuit that sets the output voltage could fail and cause an overvoltage condition although this is very rare. A dirty pot or loose connection at the Kelvin jumpers can cause any supply to go to the rail. A crowbar circuit would be easy to add to any power supply. A power SCR, a few resistors, a zener and a fuse is all that is needed. Some of the power supply Mfg sold these as an outboard add on for their supplies.Don Brown KD5NDB
> Date: Tue, 6 Nov 2007 09:35:11 -0600> To: elecraft@mailman.qth.net> From: > [EMAIL PROTECTED]> Subject: [Elecraft] Anyone know this info about switching > supplies?> > Folks:> > A local discussion has 'erupted' regarding switching > poser supplies and whether> they can fail in an UNregulated (e.g. high) > output voltage condition, or if> when they fail will they (almost) always > fail in a NO OUTPUT mode?> > I think this discussion may have been started as > a result of a subject here on> the reflector about overvoltage protection > (OVP), but it's managed to take on> a life of its own with respect to the > MODES in which a switching P/S > can/will fail.> > Anyone have enough real > experience to express their thoughts on this subject.> > Must admit... I have > no clue myself. I've 'heard' that most switchers fail in> the NO OUTPUT mode, > but this is strictly hearsay.> > Let's try to get several responses and the > let the subject drop before IT takes> on a life of its own on the reflector > and Eric has to step in... he's > busy enough> right now. <G>> > 73,> > Tom > N0SS> > _______________________________________________> Elecraft mailing > list> Post to: Elecraft@mailman.qth.net> You must be a subscriber to post to > the list.> Subscriber Info (Addr. Change, sub, unsub etc.):> > http://mailman.qth.net/mailman/listinfo/elecraft > > Help: > http://mailman.qth.net/subscribers.htm> Elecraft web page: > http://www.elecraft.com_______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
