Triple Power Supply
This low-cost, multipurpose
power supply fulfils the requirements
of almost all laboratory
experiments. Nonetheless, it can
be easily fabricated by hobbyists.
A single transformer is used to
build this triple power supply. Regulator
IC LM317 generates variable power
supply of 1.25 to 20V, 1A. The dual
±12V, 1A power supply is generated
by regulators 7812 and 7912. Similarly,
dual ±5V, 1A power supply is generated
by regulators 7805 and 7905.
‘On’/‘off’ switches (S2 through S4)
select the required power supply. Variable
power supply is used to study the
characteristics of devices. Fixed +5V
power supply is used for all digital,
microprocessor and microcontroller
experiments. Dual ±12V power supply
is used for op-amp-based analogue
circuit experiments.
Fig. 1 shows the circuit of the triple
power supply, while Fig. 2 shows the
pin configuration of the regulators
used in the circuit. Transformer X1
steps down the mains power to deliver
the secondary output of 18V-0-18V.
The transformer output is rectified by
full-wave bridge rectifier BR1, filtered
by capacitors C1, C2, C3, C7 and C8,
and regulated by IC1 through IC5.
Regulator IC1 (LM317) provides variable
voltages (1.25 to 20V), while IC2
and IC4 provide regulated +12V and
–12V, respectively. The output of IC2 is
fed to regulator IC3 (7805), which provides
fixed +5V. Similarly, the output
of IC4 is fed to regulator IC5 (7905),
which provides fixed –5V. Capacitors
C4 through C6, and C9 through C11,
are used for further filtering of ripples
in positive and negative regulated
power supplies. LED1 glows to indicate
that +5V is available, while LED2
indicates that –5V is available.
Switch S1 is used for mains ‘on’/
‘off’. Using switches S2 through
S4, any of the three supplies can be
independently turned off when not
required in a particular experiment.
This reduces unnecessary power dissipation
and increases the life and
reliability of the power supply. Since
the circuit uses three terminal regulators,
only capacitors are required at
the input and output. The use of few
components makes the circuit very
simple. The three terminal regulators
have heat-sink provision to directly
deliver 1A output current. To ensure
the maximum output, do not forget to