[cctalk] Re: Rainbow H7842 PSU Fault

I don't think we can draw any conclusions from voltage measurements around the TIP121 without the -12V line being approximately correct or at the very least being negative. It's collector voltage being 0V on the good PSU suggests there is no current flowing through the parallel 20R resistors and therefore no emitter-collector current through the TIP121 so it's not being switched on meaning that it's driver doesn't feel the need to pull the -12V line down nearer 0V because it's already low enough. (Maybe having a load is enough to keep it at the correct level and the shunt regulator only starts doing stuff if the load is less than expected or removed altogether? Or maybe the -12V line is not present when the test was done?) The collector voltage of the TIP121 on the bad PSU being slightly positive is probably due to the voltage on it's emitter (ie the -12V line) being positive instead of negative which makes it's bias conditions all wrong and any measurements around it mostly meaningless. How about applying -12V or slightly less from a bench supply current limited to about 150mA to the -12V line and seeing what happens? If the PSU and/or -12V load tries to draw more than that there would seem to be a real overload and further investigation is needed to find out what is drawing the current. Measure the voltage across the zener diode in the -12V regulator and compare with the good PSU. Also compare the voltage across the parallel 20R resistors. A larger voltage here would indicate more current being drawn through the TIP121. Under these conditions, there shouldn't be any voltage across the 51R current sensing resistor for the -12V line. If there is, it suggests there could be reverse leakage through the -12V rectifier diode. However, Brent's calculations show that the current trip value for the -12V line is as low as 1.3mA and I can't see any reason to disagree with his calculations or his conclusion that this seems very low (except that another tiny smidgen of current is available from the negative startup supply but this won't really have any bearing on things). If this is really the case, then placing something like a 5k6 resistor across the -12V line on the good PSU should cause enough current to flow for the trip to operate. Finding this level of leakage in the failed PSU is not going to be easy. On the other hand, if this test doesn't trip it, then please look very closely at the resistors and connections to the inputs of E3d and verify that they are as described on the circuit diagram. It seems very strange indeed to have a trip value as low as 1.3mA combined with a shunt regulator whose method of regulation is to pull the voltage down by drawing current from the supply line. Perhaps the shunt regulator might be able to pull enough current to cause the trip to operate if the -12V line was too high (in the negative sense) or if the shunt regulator was under the mistaken impression that the -12V line was too high? (This is a bit unlikely but the 115V/230V switch is set correctly, isn't it?) On the other other hand, if the manual says that the -12V line is supposed to be able to supply 150mA, then it doesn't make sense for the current trip to operate at 1.3mA and we must be going wrong somewhere. Regards, Peter.