Jan 272021

12V Power Supply using Zener and 741

This 12V power supply has the characteristic of having a very low output impedance. This is a highly desired characteristic on power supplies and is achieved thanks to the large open loop gain of the op. amp.

The Zener diode D1 is used as a voltage reference and a PNP transistor (Q1) is used to regulate the output voltage.

To achieve stability of the output voltage, it is fed back to the non-inverting input of the operational amplifier, through resistor R4. Potentiometer P1 is used for fine adjustment of the source output to 12VDC.

12V Power Supply using Zener and 741 Op. Amp.

12V Voltage Source Circuit Operation

The inverting input of the operational amplifier is maintained at 5.1 V by means of the zener diode (D1). If the output voltage of the power supply drops, the voltage at the op-amp’s non-inverting terminal also drops, as well as the op-amp output.

This causes transistor Q1 to conduct and raise the regulator’s voltage output, so the voltage output is stabilized.

Unregulated Power Supply - 12V Power Supply using Zener and 741 Op. Amp.

The zener diode D2 has also been included so that the regulated output of the operational amplifier can work approximately 7 V below the unregulated input.

In this way, the regulated output is maintained until the PNP transistor saturates. This means that the unregulated input can drop below (within approximately 200 mV) the regulated output.

The output of the unregulated source (A) can be between 30 and 12.2 V. In this design, the output is at approximately 18 V.

741 Op Amp Pin Out

741 Operational Amplifier Pinout

Note: The unregulated output (see point A in the second diagram) connects to the unregulated input in the first diagram.

List of components of the 12 Volt Power Supply

  • 1 741 operational amplifier (U1)
  • 1 PNP transistor (Q1)
  • 2 5.1V zener diode (D1, D2)
  • 3 4.7k resistors (R1, R2, R3)
  • 1 6.8k resistor (R4)
  • 1 1k potentiometer (P1)
  • 1 10uF (microfarad), electrolytic capacitor (C1)
  • 1 240/120 VAC to 24 VAC transformer with central tap and 1.5 A (T)
  • 1 2200uF electrolytic capacitor, 50 V or more (C)
  • 2 rectifier diodes 1N5400 or similar (D)


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