______________________________________________________________ Solid State Power Supplies
4-132. When load current is below 600 milliamperes, the base-to-emitter voltage on Q2 is
not high enough to allow Q2 to conduct. With Q2 cut off, the circuit acts like a series
regulator. When the load current increases above 600 milliamperes, the voltage drop across
R4 increases to more than 0.6 volts. This causes Q2 to conduct through resistor R2, thereby
decreasing the voltage on the base of pass transistor Q1. This action causes Q1 to conduct
less. Therefore, the current cannot increase above 600 to 700 milliamperes.
4-133. By increasing the value of R4, you can limit the current to almost any value. For
example, a 100-ohm resistor develops a voltage drop of 0.6 volts at 6 milliamperes of
current. You may encounter current-limiting circuits that are more sophisticated, but the
theory of operation is always the same. So, if you understand this circuit, you should have
no problems with the others.
Figure 4-50. Series Regulator With Current Limiting
TROUBLESHOOTING POWER SUPPLIES
4-134. There are safety precautions that are very important to remember whenever you are
working with electricity. In the front of all ETMs, you will find a section on safety
precautions. There should also be a sign posted on each piece of equipment listing the
specific precautions for that equipment. One area that is sometimes overlooked and is a
hazard is the method in which equipment is grounded. By grounding the return side of the
power transformer to the metal chassis, the load being supplied by the power supply can be
wired directly to the metal chassis. Therefore, the necessity of wiring directly to the return
side of the transformer is eliminated. This method saves wire and reduces the cost of
building the equipment. While it solves one of the problems of the manufacturer, it also
creates a problem for you. Observe the following precautions before starting to work on
any electronic or electrical equipment.
Always ensure that the equipment and any test equipment you are using are
Inspect the rubber mat you will be standing on to ensure it is in good condition.
As long as you follow these precautions, you should be able to avoid the possibility of
becoming an electrical conductor.
23 June 2005