9 May
2023
9 May
'23
4:57 p.m.
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.