Self switching Power Supply
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.
Title: Self switching Power Supply
electronic circuit
Source: www.electronic-circuits-diagrams.com
Published on: 2005-02-01
Reads: 1245
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