Page 309 - Op Amps Design, Applications, and Troubleshooting

Troubleshooting Tips for Power Supply Circuits 287

3. Check for possible short circuits. Once the unregulated input voltage is
shown to be correct, we can concentrate on the regulator portion of the
supply. If the regulator was disconnected during step 2 and you have
reason to believe a short circuit exists in the regulator, DO NOT reconnect
the regulator and apply full power. If you do and a short does exist, the
newly repaired unregulated source will be damaged again. A better ap-
proach is to connect the regulator to the unregulated supply via a current
meter. Use a variable autotransformer to supply the AC power to the
unregulated power supply, and slowly increase the AC input voltage while
monitoring the current meter. If a short exists in the regulator, the current
meter will exceed normal values with a very low-input voltage. If this is the
case, you must rely on your theory of operation and an ohmmeter as your
major tools.
4. Open the regulator loop. If the full supply voltage can be applied safely,
but the regulator still doesn't work properly, then you can add a voltmeter
to your tool kit. Since the regulator is inherently a closed-loop system, it is
often difficult to distinguish between cause and effect. If the loop can be
easily broken (e.g., removing a wire from the wiper arm of a potentiometer,
removing a socket-mounted transistor, etc.), this can help isolate the
problem. After the loop has been opened, you can inject your own "good"
voltage at the open-loop point from an external DC supply. The system can
then be diagnosed using the split-half method, signal tracing, and so on,
like any other open-loop system.
5. Force the circuit to known extremes. If it is impractical to open the loop
of the regulator, you can sometimes force a condition at one point in the
loop and watch for a response at another point. Your understanding of the
operation of the components between the forced point and the monitored
point can lead you to the problem. A good example of forcing a condition is
to either short the emitter-base circuit of a transistor to force it to cut off, or
to short the emitter-collector circuit to simulate a saturated condition. Be
sure to examine the circuit carefully before shorting these elements, but
in most circuits neither of these shorts will cause damage (see step 6), al-
though they will force the circuit to go to one of two extremes. The extreme
change will be passed through the rest of the circuit if everything is normal.
A defect, however, will not respond to the change and thus reveal its
identity to an alert technician or engineer.
6. Use special care with switching regulators. Here the regulator transistor
has been selected on the assumption that it is switching from full on to full
off and therefore dissipating minimum power. If any portion of the regu-
lator circuit causes the switching to stop and the pass transistor is in the
ON state, then the pass transistor will almost certainly be damaged. This
has two important ramifications. First, if your diagnosis reveals that no
switching signal is being applied to the switching transistor, then suspect a
bad transistor after you correct the switching signal problem. Second, you
should never intentionally stop the oscillation in a switching supply by
shorting components as described in step 5.