ohms, then you must have 10 amperes of current across RV and RL. Remember that E = IR.
If the values of the resistance of RV and RL are equal, then 5 amperes of current will flow
through each resistance (RV and RL).
Figure 4-33. Shunt Voltage Regulator
(Determining Output Voltage Regulation)
4-100. If the load resistance (RL) increases, the current through RL will decrease. For
example, assume that the current through RL is now 4 amperes and that the total current
across Rs is 9 amperes. With this drop in current, the voltage drop across Rs is 18 volts.
Consequently, the output of the regulator has increased to 102 volts. At this time, the
regulating device (RV) decreases in resistance and 6 amperes of current flows through this
resistance (RV). The total current Rs is once again 10 amperes (6 amperes across RV and
4 amperes across RL). Therefore, 20 volts is dropped across Rs causing the output to
decrease back to 100 volts. You should know by now that if the load resistance (RL)
increases, the regulating device (RV) decreases its resistance to compensate for the change.
If RL decreases, the opposite effect occurs and RV increases.
4-101. Now consider the circuit when a decrease in load resistance takes place. When RL
decreases, the current through RL subsequently increases to 6 amperes. This action causes a
total of 11 amperes to flow through Rs, which then drops 22 volts. As a result, the output is
98 volts. However, the regulating device (RV) senses this change and increases its
resistance so that less current (4 amperes) flows through RV. The total current again
becomes 10 amperes and the output is again 100 volts.
4-102. From these examples, you should know that the shunt regulator maintains the
desired output voltage by doing the following:
Sensing the current change in the parallel resistance of the circuit.
Compensating for the change.
4-103. Refer to Figure 4-33 to consider how the voltage regulator operates to compensate
for changes in input voltages. Remember that the input voltage may vary. Any variation
must be compensated for by the regulating device. If an increase in input voltage occurs,
the resistance of RV automatically decreases to maintain the correct voltage division
between RV and Rs. Therefore, notice that the regulator operates in the opposite way to
compensate for a decrease in input voltage.
23 June 2005