Page 308 - Op Amps Design, Applications, and Troubleshooting

286 POWER SUPPLY CIRCUITS

a point, the voltage on the (-) input to the comparator decays while the voltage on
the (+) input remains constant. When the voltage on the (-) input passes the lower
threshold of the voltage comparator, the output quickly changes states, signaling a
coming power loss. A computer monitoring this signal can then take the necessary
action to protect critical data. Resistors R 3 and K 4 provide hysteresis for the com-
parator.
The amount of time between primary power interruption and the point
where the regulated output begins to drop is called hold-up time and is generally
tens or even hundreds of milliseconds. Since a computer executes in the micro-
second range, there is plenty of time to save the critical information after me unreg-
ulated input has started to decay but before the regulated output begins to drop.

6.8 TROUBLESHOOTING TIPS FOR POWER SUPPLY CIRCUITS

Power supply circuits are considered by some technicians and engineers to be
simple, fundamental, nonglamorous, and even boring. However fundamental or
boring the purpose of power supplies may be, the troubleshooting of a defective
supply is not always a simple task. What complicates the troubleshooting of a reg-
ulator circuit is the closed-loop nature of the system. A defect in any part of the
loop can upset the voltages at all other points in it, thus making it difficult to dis-
tinguish between cause and effect.
Nevertheless, armed with a thorough understanding of circuit operation and
guided by systematic troubleshooting procedures, a defective regulator circuit can
be quickly and effectively diagnosed. The following sequential steps will provide
the basis for a logical, systematic troubleshooting procedure applicable to voltage
regulator circuits:

1. Observe the symptoms. Because of the potentially high-power levels
available in a supply, visible signs of damage are common. DO NOT, how-
ever, simply replace a burned component and reapply power—in many
cases, the burned component is the result of a malfunction elsewhere in the
supply. Nevertheless, detecting the burned component will help you
narrow the range of possibilities.
Symptom observation also includes taking careful note of the output
symptoms. Is the output voltage too high, too low, 0, unregulated? Did the
user of the equipment say how the problem was caused (e.g., an accidental
short on the output)?
2. Verify that the input to the regulator is correct. If it is not correct, the
regulator may not be the cause. On the other hand, if the problem is no
input and the unregulated supply shows signs of damage, then suspect a
short in the regulator circuit. In these cases, it is often helpful to disconnect
the regulator circuit and get the unregulated supply back to normal as a
first step. A simple way to disconnect series regulator circuits is to remove
the pass transistor. This is a particularly simple task for socket-mounted
power transistors.