Voltage
Regulator
Introduction
There are many types of power supply.
Most are designed to convert high voltage AC mains
electricity to a suitable low voltage supply for
electronics circuits and other devices.
A more reliable method of obtaining DC power is to
transform, rectify, filter and regulate an AC line
voltage.
A power supply can by broken down into a series of
blocks, each of which performs a particular function.
Regulated power supply
Regulated power supply
Power supply: a group of circuits that convert the standard
ac voltage (220 V, 50 Hz) to constant dc voltage
Transformer : a device that step up or step down the ac
voltage to a desired amplitude through the action of a
magnetic field
Rectifier: A diode circuits that converts the ac input
voltage to a pulsating dc voltage
The pulsating dc voltage is only suitable to be used as a
battery charger, but not good enough to be used as a dc
power supply in a radio, stereo system, computer and so
on.
Rectifier
There are two basic types of rectifier circuits:
Half-wave rectifier
Full-wave rectifier - Center-tapped & Bridge full-wave
rectifier
In summary, a full-wave rectified signal has less
ripple than a half-wave rectified signal and is thus
better to apply to a filter.
Regulated power supply
Filter: a circuit used to reduce the fluctuation in
the rectified output voltage or ripple.
This provides a steadier dc voltage.
Regulator: a circuit used to produces a constant dc
output voltage by reducing the ripple to negligible
amount. One part of power supply.
Regulated power supplies
Regulator - Zener diode regulator
For low current power supplies - a simple voltage
regulator can be made with a resistor and a zener
diode connected in reverse biased condition.
Zener diodes are rated by their breakdown voltage
Vz and maximum power Pz (typically 400mW or
1.3W)
Types of regulators
Fundamental classes of voltage regulators are
linear regulators and switching regulators.
Two basic types of linear regulator are the series
regulator and the shunt regulator .
The series regulator is connected in series with the
load and the shunt regulator is connected in
parallel with the load.
Series voltage regulator
Control element in series with load between input
and output.
Output sample circuit senses a change in output
voltage.
Error detector compares sample voltage with
reference voltage → causes control element to
compensate in order to maintain a constant output
voltage.
Series voltage regulator
Control
Element
VREF
Sample
Error Detector Circuit
Series voltage regulator
The resistor R1 and R2 sense a change in the output voltage
and provide a feedback voltage.
The error detector compares the feedback voltage with a
Zener diode reference voltage.
The resulting difference voltage applied to transistor Q
1
controls the conduction to compensate for the variation of
the output voltage.
The output voltage will be maintained at a constant value of:
R1
Vo 1 VZ
R2
Series voltage regulator
When Vo decreases then the error voltage decreases
The comparator output increases driving the transistor
more towards saturation
Then Vce decreases and Vo increases
When Vo increases then the error voltage increases
The comparator output decreases driving the transistor
more towards cutoff
Then Vce increases to decrease Vo
Determine the output voltage for the regulator below.
(Solution: 10.2 V)
R
Vo 1 1 VZ
R2
Shunt voltage regulator
The unregulated input voltage provides current to the
load.
Some of the current is pulled away by the control
element.
If the load voltage tries to change due to a change in the
load resistance, the sampling circuit provides a feedback
signal to a comparator.
The resulting difference voltage then provides a control
signal to vary the amount of the current shunted away
from the load to maintain the regulated output voltage
across the load.
Shunt voltage regulator
Shunt voltage regulator
Shunt voltage regulator
When the output voltage tries to decrease due to a change in
input voltage or load current caused by a change in load
resistance, the decrease is sensed by R 1 and R2.
A feedback voltage obtained from voltage divider R 1 and R2 is
applied to the op-amp’s non-inverting input and compared to the
Zener voltage to control the drive current to the transistor. The
transistor moves towards cutoff and Ish decreases. Drop across
RS decreases Vo increases
The current through resistor RS is thus controlled to drop a
voltage across RS so that the output voltage is maintained.
If V0 increases, drive to transistor increases Ish and drop across
Rs increases and Vo decreases
