> > I am going to read your answer more carefully later. But I wanted to check > one thing. I measured the base-emitter voltage as negative in both cases, > and yet the TIP121 appears to be conducting on the bad PSU. Surely that > means that the TIP121 is not working correctly? >
I'd be very reluctant to draw any conclusions from measurements made when the device is not biased correctly. As Brent says, there could be current flowing through the presumably forward biased base-collector junction. Also, this device is a not just a straighforward transistor. It's data sheet says it is a package containing two transistors and some other components, including a normally reverse biased diode between the collector and emitter terminals. A positive voltage of sufficient magnitude on it's emitter could be forward biasing this diode resulting in current flow through it and therefore also through the 20R resistors causing a voltage to be developed across them. I think a better way to determine if the TIP121 is causing excess current draw on the -12V line is to make the voltage at it's emitter approximately correct and see if it then draws enough current through itself to cause the excess current trip to operate. This current can then be measured by observing the voltage across the 20R resistors (or the smoke coming from them if they are not of sufficient power rating...). It should also be possible to observe the conditions around the zener diode and the MPSA55 and see if these components are behaving reasonably. Regards, Peter.
