--- In Repeater-Builder@yahoogroups.com, "skipp025" <[EMAIL PROTECTED]> wrote: > > Bob, > > Were you using the standard generic 3AG glass fuse..? Was it a fast > or slow blow..? If you still have the dead fuse... who is the mfgr? > > It takes quite a bit to kill the diodes or a bridge. Replacing > the mentioned with a higher capacity type is probably a good idea > but you should also toss the generic fuse and get a high spec > quality brand proper-value replacement. > > Something is probably causing the crowbar to fire... the fuse > doesn't open fast enough. Are both diodes shorted or just one > side? > > cheers, > s. > > > > > "Bob M." <msf5kguru@> wrote: > > > > Some of the regular readers may recall I had an RS35M > > (bought new in 2002) blow its diodes in July 2006. > > These were two 35A, 50V bridges made by Diotec. Astron > > only uses the positive half and wires them in > > parallel. I replaced these with Vishay GBPC35 > > rectifiers, tested the supply, and put it aside. > > > > A friend loaned me his old RS35M which got the > > repeater back on the air in 90 minutes. > > > > I bought a brand new RS35M supply in July 2006 and > > finally got around to installing it in October 2006. > > > > The other day the same thing happened. The repeater > > had been quiet all morning, someone called me on it, > > and mid-way through the CW ID (after transmitting for > > about 10 seconds), it just went off the air. I grabbed > > the repaired RS35M, went to the site, and put it into > > the repeater, pulling the other one back down to my > > shack. The repeater was back on the air after 3 hours > > (hey, it was cold outside and I didn't feel like > > driving up there right away). > > > > Back on the bench, I tried a new 8A fuse; it blew > > immediately. I measured the resistance across the > > diodes; I read 0 ohms, but without disconnecting them > > from the transformer, this is not an accurate reading. > > I'd surmise that one or both bridges have a shorted > > diode in them. Seems like the same failure as the 2002 > > supply had. This one, however, is still under > > warranty. I'm not sure it will be economically > > feasible to ship it to Astron where they'll put in > > exactly the same diodes, in the same configuration, > > where they'll just blow again. Until I hear back from > > them, I'm not going in there to disconnect the diodes > > to actually measure them. > > > > I've purchased some new 50A 1000V bridges which I will > > put into these supplies from now on. No more "diodes > > in parallel". > > > > Incase you ask, the load on the supply is 25 amps at > > 14.0 volts, it has plenty of forced air cooling on it, > > the environment is 70F, and the repeater is low usage: > > a couple of hours per day with a sustained usage at > > drive time of about 45 minutes on a busy day. > > > > I was taught that running semiconductors (diodes and > > transistors) in parallel without some kind of load > > balancing components is a bad thing. I'd rather have a > > single pair of diodes in these supplies instead of the > > pairs of bridges. Anyone else care to comment pro or > > con? > > > > Needless-to-say, my next power supply will be a > > different brand. > > > > Bob M. > > > > >____________________________________________________________________ _________ > > Do you Yahoo!? > > Everyone is raving about the all-new Yahoo! Mail beta. > > http://new.mail.yahoo.com > > > Hi, your friend was quite correct in what he said about doides/transistors in paralell without current sharing resistors (say 0.1R) is VERY bad practice.Without you get "current hogging) where one device takes the lions share of the current is overloaded goes short and takes down the other device.Many power supplies sold kere in the U.K suffer this design fault.A supply may be quoted as 20a continuous and 25a surge with the bridge rectifier quoted by the manufacturer quoting 25a peak current!.Needless to say over the years I have repaired quite a few!.The other failure is the smoothing capacitors take quite a hammering too.Good practice also says 2200uF per amp of output current,rarely fitted,or of adequate ripple-current so they heat up and commonly explode.Power supply design is NOT a "Black Art" if the basic rules are followed and the supply is built a standard and not seen as a cost-cutting exercise then reliability is then not an issue.
G8UMX(30 years in military and defence electronics)
