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Regulated DC power supply

2015-03-18 22:30  
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Regulated DC Power Supply using transistors.

A low ripple regulated DC power supply designed based on transistors is shown here. Such transistor voltage regulators are suitable for application where high output current is required. Conventional integrated series regulators like 7805 can only deliver up to 1A. Additional series pass transistors have to be added to the 7805 based regulator circuit for improving their current capacity.


The circuit shown below is a basic series voltage regulator based on transistors. Transistor Q1 (2N 3054) and Q2 (2N 3055) form a darlington pair. Resistor R1 provides the base current for Q1 and also keeps the zener diode D2 in the active region. The overall working of the circuit can be demonstrated by explaining two situations.

  • When the input voltage (output of the rectifier section) increases, the output voltage of the regulator (Vout) also increases. This increase in Vout decreases the base emitter voltage of Q2 because the zener diode D2 is operating in the breakdown region and the voltage across it is unchangeable. This decrease in VBEincreases the collector emitter resistance? of Q2 and so the output voltage (Vout) gets reduced accordingly.
  • When the output load increases, the output voltage (Vout) gets reduced. This decrease of output voltage (Vout) makes VBEof Q2 to decrease. This reduces the collector emitter resistance of Q2 and so the output voltage gets increased accordingly.

    Circuit diagram.

    zener regulated dc power supply

    Regulated dc power supply


    If 5A bridge is not available, then make one using 6A6 diodes.Transistor Q2 requires a heat sink.An optional 5A fuse can be added in series to the output.Breakdown voltage of the zener diode D2 must be chosen according to the output voltage you need, and it is according to the equation, Vout = Vz – 0.7.
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