Sep 302020

9VDC Power Supply using Zener and Transistor

This 9VDC power supply uses a zener diode and a pass-trough transistor, can be used to power small circuits or electronic devices that would normally use a 9 volt battery, saving us the cost of a 9 volts battery while being used at home.

To better understand the operation of this 9VDC voltage source, it is recommended to first read the following tutorial: Zener diode voltage regulator, which explains the operation and design of a regulated voltage source that only uses a zener diode.

This 9VDC voltage source is very simple and can be assembled in very little space.

How the 9VDC Power supply works?

The main components of this circuit are a zener diode, which is used as a voltage reference, and a bipolar transistor to amplify the current that is delivered to the load. The transformer and rectifier diodes allow an unregulated voltage to be obtained at the electrolytic capacitor terminals of approximately 16.5 volts.

9VDC Power Supply using Zener and Transistor

9VDC Power Supply using Zener and Transistor

With a 10 volt zener diode (Z) connected to the base of the transistor, an output voltage of 9.4 / 9.3 volts is obtained at the emitter of the transistor: Vout = Vz – Vbe = 10V – 0.6V = 9.4 VDC, where Vbe is the base – emiter voltage of the bipolar transistor. (0.6 to 0.7 volts)

NTE128 NPN transistor pinout

A LED (D3) is included as indication that the power is ON. Although the transistor used in this circuit allows a maximum current of 1 ampere, it is recommended that the current is not greater than 500 mA.

List of the 9VDC Power Supply components

  • 1 NTE128 NPN or similar transistor (Q1)
  • 1 10V, 1/2 watt zener diode (Z)
  • 2 1N4001 or similar diodes (D1, D2)
  • 1 red or green LED (D3)
  • 1 150 ohm resistor, 1/2W (R1)
  • 1 680 ohm resistor, 1/4W (R2)
  • 1 2200uF / 25V electrolytic capacitor (C)
  • 1 120/240VAC to 24VAC center tapped transformer, 500mA (18V transformer can be used with no problems) (T1)

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