200 Electrical Power Systems Technology
In an ideally regulated circuit, VNL would equal VFL.
In all types of power supplies that convert AC to DC, the DC output
levels are affected by variations in the load. The lower the percentage of
regulation (approaching 0 percent), the better regulated the circuit is. For
instance, power supplies are capable of having a voltage regulation of less
than 0.01 percent, which means that the value of the load has little effect
on the DC output voltage produced. A well-regulated DC power supply is
necessary for many industrial applications.
Voltage Regulators
A simple voltage regulation circuit that uses a zener diode is shown in
Figure 7-31. This circuit consists of a series resistor (Rs) and a zener diode
(D1) connected to the output of a rectifier circuit. However, we should re-
view zener diode operation before discussing the circuit operation.
Zener diodes are similar to conventional diodes when they are forward
biased. When they are reverse biased, no conduction takes place until a spe-
cific value of reverse breakdown voltage (or “zener” voltage) is reached. The
zener is designed so that it will operate in the reverse breakdown region of
its characteristic curve (see Figure 7-31B). The reverse breakdown voltage
is predetermined by the manufacturer. When used as a voltage regulator,
the zener diode is reverse biased so that it will operate in the breakdown
region. In this region, changes in current through the diode have little ef-
fect on the voltage across it. The constant-voltage characteristic of a zener
diode makes it desirable for use as a regulating device.
The circuit of Figure 7-31A is a zener diode shunt regulator. The zener
establishes a constant voltage across the load resistance within a range of
rectified DC voltages and output load currents. Over this range, the volt-
age drop across the zener remains constant. The current through the zener
(IZ) will vary to compensate for changes in load resistance, since IZ = IT
- IL. Thus, the output voltage will remain constant.
Transistor Voltage Regulators
An improvement over the zener voltage regulator is a transistorized
regulator, as shown in the circuit in Figure 7-32. This regulator has tran-
sistor Q1 placed in series with the load device (RL). Transistor Q1, then,
acts to produce variable resistance to compensate for changes in the input
voltage. The collector-emitter resistance of Q1 varies automatically with
changes in the circuit conditions. The zener diode establishes the DC bias
placed on the base of transistor Q1. When this circuit is operating proper-