increases to 6.4 volts. At this time, the increase in the forward bias of Q1 lowers the
resistance of the transistor allowing more current to flow through it. Since this current must
also pass through Rs, there is also an increase in the voltage drop across this resistor. The
voltage drop across Rs is now 13.8 volts and therefore the output voltage is reduced to 6.3
volts. Remember, this change takes place in a fraction of a second.
Figure 4-39. Shunt Voltage Regulator (Increased Input Voltage)
4-114. Figure 4-39, view (B) shows the same schematic of the shunt voltage regulator
shown in Figure 4-38. The difference is that this view shows a different output voltage.
The load current has increased causing a momentary drop in voltage output to 6.2 volts.
Remember that the circuit was designed to ensure a constant output voltage of 6.3 volts.
Since the output voltage is less than that required, changes occur in the regulator to restore
the output to 6.3 volts. Because of the 0.1-volt drop in the output voltage, the forward bias
of Q1 is now 0.6 volts. This decrease in the forward bias increases the resistance of the
transistor, thereby reducing the current flow through Q1 by the same amount that the load
current increased. The current flow through Rs returns to its normal value and restores the
output voltage to 6.3 volts.
Current Voltage Regulators
4-115. Remember that voltage regulators work to provide constant output voltages. In
some circuits it may be necessary to regulate the current output. The circuitry that provides
a constant current output is called a constant current regulator or just CURRENT
REGULATOR. Figure 4-40 shows a simplified schematic for a current regulator. The
variable resistor shown on the schematic is used to show the concept of current regulation.
4-116. Remember that a variable resistor does not respond quickly enough to compensate
for the changes. Notice that an ammeter has been included in this circuit to indicate that the
circuit shown is that of a current regulator. When the circuit functions properly, the current
reading of the ammeter remains constant. In this case, the variable resistor (RV)
compensates for changes in the load or DC input voltage. Adequate current regulation
23 June 2005