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One of the main features of the
regulated power supply circuit being presented is that though
fixed-voltage regulator LM7805 is used in the circuit, its output
voltage is variable. This is achieved by connecting a
potentiometer between common terminal of regulator IC and ground.
For every 100-ohm increment in the in-circuit value of the
resistance of potentiometer VR1, the output voltage increases by 1
volt. Thus, the output varies from 3.7V to 8.7V (taking into
account 1.3-volt drop across diodes D1 and D2).
Another important feature of the supply is that it switches itself
off when no load is connected across its output terminals. This is
achieved with the help of transistors T1 and T2, diodes D1 and D2,
and capacitor C2. When a load is connected at the output,
potential drop across diodes D1 and D2 (approximately 1.3V) is
sufficient for transistors T2 and T1 to conduct. As a result, the
relay gets energised and remains in that state as long as the load
remains connected. At the same time, capacitor C2 gets charged to
around 7-8 volt potential through transistor T2. But when the load
is disconnected, transistor T2 is cut off. However, capacitor C2
is still charged and it starts discharging through base of
transistor T1. After some time (which is basically determined by
value of C2), relay RL1 is de-energised, which switches off the
mains input to primary of transformer X1. To resume the power
again, switch S1 should be pressed momentarily. Higher the value
of capacitor C2, more will be the delay in switching off the power
supply on disconnection of the load, and vice versa.
Though in the prototype a transformer with a secondary voltage of
12V-0V, 250mA was used, it can nevertheless be changed as per
user’s requirement (up to 30V maximum. and 1-ampere current
rating). For drawing more than 300mA current, the regulator IC
must be fitted with a small heat sink over a mica insulator. When
the transformer’s secondary voltage increases beyond 12 volts (RMS),
potentiometer VR1 must be redimensioned. Also, the relay voltage
rating should be redetermined.
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