Zener Controlled Transistor Voltage Regulator
Zener Controlled Transistor Voltage Regulator
Zener Controlled Transistor Voltage Regulator
Evangelista IV
BET – MecT 3A
Voltage Regulators
The main principle behind the working of such a regulator is that a large
proportion of the change in supply or input voltage appears across the transistor
and thus the utput voltage tends to remain constant.
Operation
When the input supply voltage Vin increases the output voltage Vload also
increases. This increase in Vload will cause a reduced voltage of the transistor
base emitter voltage Vbe as the zener voltage Vzener is constant. This reduction
in Vbe causes a decrease in the level of conduction which will further increase the
collector-emitter resistance of the transistor and thus causing an increase in the
transistor collector-emitter voltage and all of this causes the output voltage Vout
to reduce. Thus, the output voltage remains constant. The operation is similar
when the input supply voltage decreases.
Limitations
The limitations listed below has proved the use of this series voltage regulator
only suitable for low output voltages.
1. With the increase in room temperature, the values of Vbe and Vzener tend to
decrease. Thus the output voltage cannot be maintained a constant. This will further
increase the transistor base emitter voltage and thus the load.
2. There is no option to change the output voltage in the circuit.
3. Due to the small amplification process provided by only one transistor, the circuit
cannot provide good regulation at high currents.
4. When compared to other regulators, this regulator has poor regulation and ripple
suppression with respect to input variations.
5. The power dissipation of a pass transistor is large because it is equal to Vcc Ic and
almost all variation appears at Vce and the load current is approximately equal to
collector current. Thus for heavy load currents pass transistor has to dissipate a lot of
power and, therefore, becoming hot.
Zener Controlled Transistor Shunt Voltage Regulator
The image below shows the circuit diagram of a shunt voltage regulator. The
circuit consists of an NPN transistor and a zener diode along with a series resistor
Rseries that is connected in series with the input supply. The zener diode is
connected across the base and the collector of the transistor which is connected
across the output.
Operation
As there is a voltage drop in the series resistance Rseries the unregulated voltage
is also decreased along with it. The amount of voltage drop depends on the
current supplied t the load Rload. The value of the voltage across the load
depends on the zener diode and the transistor base emitter voltage Vbe.
Limitations
The series resistor causes a huge amount of power loss.
1. The supply current flow will be more through the transistor than it is to be
through the load.
The block diagram of a discrete transistor type voltage regulator is given below.
A control element is placed to collect the unregulated input which controls the
magnitude of the input voltage and passes it to the output. The output voltage is
then fedback to a sampling circuit and then compared with a reference voltage
and sent back to the output.
Thus, if the output voltage tends to increase the comparator circuit provides a
control signal to cause the control element to reduce the magnitude of the
output voltage by passing it through the sampling circuit and comparing it,
thereby maintaining a constant and steady output voltage.
Suppose the output voltage tends to decrease, the comparator circuit provides
provides a control signal that causes the series control element to increase the
magnitude of output voltage, thus maintaining the steadiness.
Discrete Transistor Shunt Voltage Regulator
The block diagram of a discrete transistor shunt voltage regulator is given below.
As the name says the voltage regulation is provided by shunting the current away
from the load. The control element shunts a part of the current that is produced
as a result of the input unregulated voltage that is given to the load. Thus the
voltage is regulated across the load. Due to the change in load, if there is a
change in the output voltage, it will be corrected by giving a feedback signal to
the comparator circuit which compares with a reference voltage and gives the
output control signal to the control element to correct the magnitude of the
signal required to shunt the current away from the load.
If the output voltage increases, the shunt current increases and thus produces
less load current and maintains a regulated output voltage. If the output voltage
reduces, the shunt current reduces and thus produces more load current and
maintains a regulated constant output voltage. In both cases, the sampling
circuit, comparator circuit and control element plays an important role.
Limitations of Transistor Voltage Regulators
The steady and stabilized output voltage that is obtained from the regulator is
limited to a voltage range of (30-40) Volts. This is because of the small value of
the maximum collector emitter voltage of transistor (50 Volts). This puts a limit to
the use of transistorized power supplies.