Identifying the Surge Suppression Resistor in PDP11 H7140 PSU

Actually I am not sure it does make sense, but then I am not an electronics expert, so I may have this wrong. This resistor is not in parallel with the coil, but connected across the terminals of the relay, ie the terminals the relay is designed to short when it is switched on. The resistor is supposed to limit the current going to the big electrolytic smoothing capacitors that come right after the bridge rectifier and provide enough power for the circuitry to check that the mains is "OK" and then switch on the relay when the capacitors have charged up and connect them directly to the bridge rectifier instead of via the inrush limiter. I hope that makes sense. I still need to know what the spec of these components is so I can find replacements. Regards Rob On 12 July 2011 15:03, dwight elvey <dkelvey at hotmail.com> wrote: > > Hi > ?I think you are talking about a dampenning resistor. This is > placed in parallel with the coil. > ?The resistance is usually equal to or somewhat larger than > the coils resistance. > ?Coils like to keep current flowing and don't like having the > current source broken quickly. The voltage would go up > until something arced or broke down. > ?The choice of resistor would be determined by how quickly > one wanted the relay to release. > ?Although, counter intuitive, a smaller value of resistance > would prolong the time for the relay to release. A higher > value would cause the release to be sooner. > ?The higher value resistor would also create a higher voltage > across the resistor. > ?At the instance the drive is removed, the same current that > was running through the coil is now running through the > resistor at that instant. > ?This voltage adds to the source voltage and is the voltage > that the transistor ( if from a transistor ) has to hold off. > If the source voltage was 12V and it was an equal value > resistor, the transistor would have to take 24V when the > relay was turned off. > ?Make sense? > Dwight > > > > > From: robert.jarratt at ntlworld.com > > > > I used the wrong term I think, inrush limiter is the right one. > > > > Rob > > > > > -----Original Message----- > > > From: cctalk-bounces at classiccmp.org [mailto:cctalk- > > > bounces at classiccmp.org] On Behalf Of Rob Jarratt > > > Sent: 12 July 2011 04:10 > > > To: General Discussion: On-Topic and Off-Topic Posts > > > Subject: Identifying the Surge Suppression Resistor in PDP11 H7140 > > > PSU > > > > > > Work is continuing on repairing the H7140 PSU from my PDP11/24. > > > > > > Now it seems that the surge suppression resistor across the relay has > > failed. > > > This resistor is marked as R1 on the printset (p66 and p69) but > actually > > is two > > > components connected in series. The parts list on p71 of the printset > > lists a > > > 3ohm 7W 5% resistor, but it is not 100% clear if this is supposed to > > > be > > the > > > surge suppression resistor, although it does seem likely. > > > The thing is I measured the resistance of the two resistors > (in-circuit), > > one was > > > open circuit and clearly the failed component, the other measured > 10ohms, > > it > > > might be failing too I suppose. > > > > > > The two resistors are marked KCC 13-17198-00 8234. I have not been > > > able > to > > > find any information on them, so I can't verify the spec and I don't > know > > what > > > to replace them with. Does anyone have any information on these > resistors? > > > > > > Thanks > > > > > > Rob > > > >