# Transistor – Zener diode Voltage regulator circuit

## The unregulated power supply

Recomended reading: the Zener diode voltage regulator.

If we use a half-wave rectifier or full-wave rectifier, we convert an alternating current voltage (ACV) into a direct current voltage (DCV). The following diagram shows a typical unregulated power supply.

The output voltage of the rectifier is an unregulated voltage, because its amplitude is not constant. The output voltage of a unregulated power source, may change due to load variations (variations of the load current).

To reduce these voltage changes, we can connect a regulator circuit in series with the unregulated power supply. Such circuit reduces the ripple voltage at the output of unregulated voltage sources and maintains the regulated voltage at a constant value, even when the load varies with time.

## Transistor – Zener diode Voltage regulator

This circuit delivers an output voltage established by the voltage of the zener diode – 0.7 volts. The 0.7 volts is the average voltage drop between the base and the emitter of a transistor. (Vbe). So, Vout = Vz – Vbe = Vz – 0.7 V.

• When the unregulated voltage goes up, the zener diode current will also goes up. The voltage across the zener diode has no or very little variation, so that the output voltage of the voltage regulator is stable
• When the unregulated voltage goes down, the zener diode current will also goes down. The voltage across the zener diode has no or very little variation, so that the output voltage of the voltage regulator is stable.

The regulated output voltage do not change if RL (the load) changes. Changes in the load current is no a problem, since it pass through the transistor

## Transistor – Zener diode Voltage regulator circuit Components list

• 1 NPN transistor: 2SC1061 (Q1)
• 1 12V or 13V / 1W Zener diode (Z1). A 12V zener diode for a 11.3V output voltage and 13V zener diode for a 12.3V output voltage.
• 1 Electrolytic capacitor 470 uF (C1)
• 2 Electrolytic capacitors 100uF (C2, C3)
• 4 rectifier diodes 1N4002 (D1, D2, D3, D4)
• 1 560 ohms resistor (R1)

A 12 ACV, 500mA transformer is connected to the input of the unregulated power supply.

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