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Self-switching
Power Supply
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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).
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| 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.
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| 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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