680 Chapter 19
IL is the load current,
IZ is the zener current, (normally 10% of IL.)
The power dissipated in R 1 is I ²R. The dissipation is
only for a condition where the load current remains
constant at its design current. If the load current is
completely removed, the current through the diode
increases to the design load current plus the design
zener current.
Two additional voltage-regulating circuits are shown
in Fig. 19-10B and C. Zener diodes can be connected in
series across the output of a dc supply, provided the
power-handling capabilities and the current-operating
ranges are similar.
A cascade shunt regulator is given in Fig. 19-10D.
The zener diode controls the base potential of transistor
Q 1 , which functions as an emitter follower and circuit
amplifier. This circuit is used where large current varia-
tions are encountered.
If only a small voltage drop is required, i.e., 5–6 V,
the configuration in Fig. 19-10E might be employed. In
this instance, the entire load current plus the current
through R 1 must flow through the diode, and it could be
easily damaged.
A current-regulator circuit is shown in Fig. 19-10F.
The load current remains essentially constant until RL
increases to where the average voltage drop across RL is
as large as the voltage drop across R 3.
19.4.2 Complex Power Supplies
Complex supplies include a pass element, a sampling
element, and a comparator element, and they may
include a preregulator, current limiting, undervoltage
and overvoltage protection, and remote sensing.
Pass Elements. A transistor or group of transistors con-
nected in parallel and placed in series with the output of
a regulated power supply to control the flow of the out-
put current. A pass element is another name for control
element.
Reference Elements. The unit that forms the founda-
tion of all voltage regulators. The output of the regu-
lated power supply is equal to or a multiple of the
reference. Any variation in the reference voltage will
cause the output voltage to vary; therefore, the reference
voltage must be maintained as stable as possible.
Sampling Elements. The device that monitors the out-
put voltage and translates it into a level comparable to
the reference voltage. The variations in the sampling
voltage versus the reference voltage is the error voltage Figure 19-10. Various regulator circuits using zener diodes.
dcindcinVZ V
out = dc in −^ VZdcregVZE. Series connection for a zener diode
when only a small voltage drop is required.F. Current-regulator using a transistor and zener diode.++−−+−+−dcinRe Q 2Q 1dcregD. Cascade shunt zener-diode voltage-regulator circuit.+−+−VZVZCommonVZ
dcin0 − 10 V+10 VdcregB. Zener diodes connected in series for regulation
and voltage division circuits.C. Zener diode regulator circuit where voltages
lower than the zener diode are desired.+−− −dcin
IZ ILdcregA. Simple zener voltage regulator circuit.+−+−R 1
RLoaddcinVZVZV 4V 3V 2dcregdcregdcreg+ +−+++V 1VZR 1R 1R 1