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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. |
