It is often possible to infer the component ratings needed from the other components around them. A component in the base circuit of a transistor is likely to experience lower currents and voltages than one in the collector circuit.
In this case, we can see from Tony's diagram that there is a 2.7 Ohm resistor in parallel with the diode. Suppose it is a 1W resistor. This means that the from P = I squared R, the average current the resistor is likely to pass is less than 1A. From Ohm's law, V/I = R, this means the average voltage across the resistor is likely to be no more than 2.7 Volts. It is possible that the peak current / voltage involved could be higher than the average for short periods of time but we have plenty of margin for error here so we don't need to think about that too much. A diode with a PIV of 200V should be fine here. Regards, Peter Coghlan > > Tony Duell has reverse engineered the following schematic. > > > > http://www.bitsavers.org/pdf/dec/rainbow/duell_schematics/psu.pdf > > > > I will go with the 1000V as you suggest anyway. > > > > Thanks > > > > Rob > > > > From: Mattis Lind <mattisl...@gmail.com> > Sent: 25 November 2022 07:12 > To: r...@jarratt.me.uk > Cc: General Discussion: On-Topic and Off-Topic Posts <cctalk@classiccmp.org> > Subject: Re: [cctalk] Identifying a Failed Diode in a Rainbow H7842 Power > Supply > > > > > > > > > > On 24 Nov 2022, at 22:45, Rob Jarratt <robert.jarr...@ntlworld.com > <mailto:robert.jarr...@ntlworld.com> > wrote: > > > > Thanks for the suggestion Mattis. The UF4007 has a PIV of 1000V, I had a > suggestion that the PIV should be 200V. Not sure what rating I should be > going for here? > > > > Given that I didn’t have a schematic and this is on the primary side I went > for the recommendation of 1000V. 200V may a bit low on the primary side > depending on the application of the diode. On the primary there can be > sustained voltages up to 400V and peaks that go even higher. Using a diode > with higher PIV almost never affects the operation as long as other > parameters stay the same. In this case the most important parameter is the > trr. It has to be a fast recovery diode. In this case the UF4007 is slightly > slower than the UF4004. But I doubt it has a big significance. Actually the > A114 is much slower. 200 ns. > > > > :Mattis > > > > > > Regards > > > > Rob > > > > From: Mattis Lind <mattisl...@gmail.com <mailto:mattisl...@gmail.com> > > Sent: 22 November 2022 07:54 > To: r...@jarratt.me.uk <mailto:r...@jarratt.me.uk> ; > General Discussion: On-Topic and Off-Topic Posts <cctalk@classiccmp.org > <mailto:cctalk@classiccmp.org> > > Subject: Re: [cctalk] Identifying a Failed Diode in a Rainbow H7842 Power > Supply > > > > > > Hello Rob! > > > > > Given that before the transistor blew up there had clearly been another > failure somewhere else, I tried to find the original failure. There were no > obviously damaged parts, so I just probed around near the transistor for any > parts that were open circuit or short circuit. I found a diode connected to > the base of the transistor that appeared to be short circuit. So, I decided > to lift one end to check it. As I de-soldered one of the leads, the diode > broke in two. So clearly the diode was either damaged by the failure of the > transistor, or it was the cause of the failure. This is the diode: > https://rjarratt.files.wordpress.com/2022/11/img_20221120_165913.jpg. > > > > > > > > > DEC used a lot of A114x diodes in their PSUs. They looked exactly like that > one. Those are fast recovery diodes. > https://pdf2.alldatasheet.com/datasheet-pdf/view/7563180/2074/A114F.html > > > > I would replace it with a UF4007 or something similar. > https://www.mouser.se/datasheet/2/849/uf4001-2578577.pdf > > > > > > > > > I can't quite make out the markings on the diode to know what to replace it > with. I think it says "D610". Would that be the right designation? If so, > can anyone suggest a suitable replacement please? > > > > The diode seems to connect an inductor to the base of the switching > transistor and the collector of the transistor is connected to a > transformer. Should I be looking for other failed parts? Not sure if the > diode failed first and then caused the transistor to fail? Or if something > else has failed which caused these parts to fail? > > > > > > Also check all other semiconductors. Also on the outputs. If there is a 1 ohm > fusible resistor in the base drive circuit check that one as well. In the > VT100 PSUs it happens that it blows. > > > > > > > > > I do know that there are no shorts in the Rainbow itself, because I have a > spare PSU that still works fine in the same machine. > > > > I blogged this here (it repeats most of that I have said above): > https://robs-old-computers.com/2022/11/20/dec-rainbow-h7842-power-supply-failure/ > <https://robs-old-computers.com/2022/11/20/dec-rainbow-h7842-power-supply-failure/> > > > > > > > /Mattis > > > Thanks > > > > Rob >
