Download Integrated Circuit Voltage Regulators

Integrated Circuit Voltage Regulators
Power supply units are intended to provide a constant d.c. output voltage to supply
electronic circuits and electrical equipment. Any change in the supplied voltage can
cause erratic behaviour. The part of a power supply unit responsible for maintaining a
constant output voltage is called the voltage regulator. Voltage regulators maintain a
constant output voltage even when operating conditions tend to cause a change in
voltage. Two such conditions may occur:
1. The amplitude of the mains supply to the PSU may change within allowed
tolerances – e.g. +/- 6%.
2. The current drawn by the circuit that is receiving power may change – i.e. the
effective load resistance may change.
An effective voltage regulator will hold a constant supply voltage in the face of
either/both of these circumstances. Three terminal integrated circuit regulators are
commonly used for this purpose. In this laboratory, we will examine the behaviour of
one such regulator – the LM7805.
Mains
Unregulated
V Load
PSU
Load
VLoad can change if the unregulated PSU experiences a change in mains supply
voltage or if the load changes.
7805
Mains
In
Unregulated
PSU
Out
Gnd
V Load
Load
VLoad is held constant by the LM7805 regulator even if the unregulated PSU
experiences a change in mains supply voltage or if the load changes.
In this experiment, we will use a bench power supply to simulate the voltage from the
unregulated PSU and a resistor to simulate the load. By changing the bench supply
output voltage and the value of the load resistor, we will be able to see the extent of
the regulation provided by the 7805 regulator (the 7805 regulator produces a nominal
5 volts output).
Procedure:
1. Connect the LM7805 as shown in the diagram below. RLoad = 4.7 kΩ.
7805
In
Out
Gnd
Bench PSU
+
-
V
R Load
Load
2. Set the bench supply to the voltages specified below and for each value, record
the measured value of VLoad on a report sheet.
3.
4.
5.
6.
5V
6V
7V
8V
10 V
15 V
20 V
30 V
Compute the % Line Regulation from your results
Now, with the bench supply set to 8 volts, change the load resistor to 270 .
Measure the load voltage again and determine the difference between this and
the previous measurement at 8 volts.
Compute the % Load Regulation from your results.
Comment on the behaviour of the regulator.