Regulated DC power supply

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.

Description.

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 VBE increases 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 VBE of 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

Notes.

  • 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.
Show Comments

17 thoughts on “Regulated DC power supply

  1. Seetharaman

    Hi friends The capacitor shown are rated for 50volts hence the cannot be more than 35volts. hence the transformer has to be rated for 24volts AC. The output cannot be greater than 30volts. The current can be around 2 to 3 amps.

    Reply
  2. Nabeel Al-hossiny

    this is the requlated crt can be used from 18 Ac up to 240 vac denpened on the diode and capcitor volrage rate

    so ransfromer can be used for step down the repuired ac voltage

    Reply
  3. seetharaman

    Hi KE730 in series regulators when you use it at very low voltage at max current, with high voltage input the series pass transistor will get heated up, hence you have to use a good heat sink.
    D2 should have been shown as 500mw zener of voltage rating equal to, output voltage required plus 1.4 volts. Vout = Vz – 1.4 volts, since two base emitter junctions are involved. By connecting 2.2k 3watt wire wound potentiometer in parallel with the zener diode and connecting the slider of the potentiometer to base of Q1 base will make it a simple variable bench supply, which is still in service with me.

    Reply
  4. Ed

    Then why you not show Transformer,? ,, most who come here, not here because they already know things, they here to learn, don’t be halfwit,, do complete schematic so not to confuse us. YOU the one who show way, you leader, teacher, do good job or quit

    Reply
  5. admin Post author

    reply to nireekshith

    this is not a transformer less power supply.

    transformers secondary has to be connected to the input of the bridge.
    ratings of the transformer depends on what voltage/current you need at the regulator output.

    Reply
  6. nireekshith

    Is this regulated dc power supply circuit with out a transformer……….?, why transformer had not been used in this…….?

    Reply
    • admin Post author

      reply to nireekshith

      this is not a transformer less power supply.

      transformers secondary has to be connected to the input of the bridge.
      ratings of the transformer depends on what voltage/current you need at the regulator output.

      Reply

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