______________________________________________________________ Solid State Power Supplies
4-111. Figure 4-37 is the schematic of a shunt voltage regulator. Notice that Q1 is in
parallel with the load. Components of this circuit are identical with those of the series
voltage regulator except for the addition of fixed resistor Rs. Notice in this schematic that
this resistor is connected in series with the output load resistance. The current limiting
resistor (R1) and Zener diode (CR1) provide a constant reference voltage for the base-
collector junction of Q1. The voltage drop across Rs and R1 determines the bias of Q1.
Remember, the amount of forward bias across a transistor affects its total resistance. In this
case, the voltage drop across Rs is the key to the total circuit operation.
Figure 4-37. Schematic of Shunt Voltage Regulator
4-112. Figure 4-38 is the schematic for a typical shunt-type regulator. Notice that the
schematic is identical to the schematic shown in Figure 4-37 except that voltages are
shown to help you understand the functions of the various components. In the circuit
shown, the voltage drop across the Zener diode (CR1) remains constant at 5.6 volts. This
means that with a 20-volt input voltage, the voltage drop across R1 is 14.4 volts. With a
base-emitter voltage of 0.7 volts, the output voltage is equal to the sum of the voltages
across CR1 and the voltage at the base-emitter junction of Q1. In this example, with an
output voltage of 6.3 volts and a 20-volt input voltage, the voltage drop across Rs equals
13.7 volts. Study the schematic to understand fully how these voltages are developed.
Figure 4-38. Shunt Voltage Regulator (With Voltages)
4-113. Figure 4-39, view (A) shows the same schematic diagram of the shunt voltage
regulator shown in Figure 4-38. The difference is that this view shows an increased input
voltage of 20.1 volts. This increases the forward bias on Q1 to 0.8 volts. Remember that
the voltage drop across CR1 remains constant at 5.6 volts. Since the output voltage is made
up of the Zener voltage and the base-emitter voltage, the output voltage momentarily
23 June 2005
TC 9-62
4-35
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