______________________________________________________________ Solid State Power Supplies
results in the loss of voltage regulation. Studying the schematic in Figure 4-40, you should
remember that any increase in load resistance causes a drop in current. To maintain a
increases. This causes the total resistance to remain constant. An increase in the input
voltage must be compensated for by an increase in the resistance of RV, thereby
maintaining a constant current flow. The operation of a current regulator is similar to that
of a voltage regulator. The basic difference is that one regulates current and the other
regulates voltage.
Figure 4-40. Schematic of Current Voltage Regulator
4-117. Since use of a variable resistor is not a practical way to control current fluctuation,
or variation, a transistor and a Zener diode (together with necessary resistors) are used.
Remember that the Zener diode provides a constant reference voltage. Figure 4-41 shows
the schematic of a current regulator circuit. Except for the addition of R1, the circuit shown
is similar to that of a series voltage regulator. The resistor is connected in series with the
load and senses any current changes in the load. Notice the voltage drop across R1 and the
negative voltage polarity applied to the emitter of Q1. The voltage polarity is a result of
current flowing through R1 and this negative voltage opposes the forward bias for Q1.
However, since the regulated voltage across CR1 has an opposite polarity, the actual bias
of the transistor is the difference between the two voltages. Therefore, the purpose of R2 is
to function as a current-limiting resistor for the Zener diode.
Figure 4-41. Current Voltage Regulator Circuit
4-118. The purpose of a current regulator is to provide a constant current regardless of
changes in the input voltage or load current. Figure 4-42 shows the schematic of a circuit
designed to provide a constant current of 400 milliamperes. Voltmeters are shown in the
schematic to emphasize the voltage drops across specific components. These voltages will
help you understand how the current regulator operates. The voltage drop across the base
23 June 2005
TC 9-62
4-37
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