Voltage Regulator Circuit

The purpose of this project is to consider and demonstrate the operation of an electronically controlled power supply. This circuit allows easy adjustment of the output voltage from a DC power supply. It also provides effective filtering action which amounts to capacitance multiplication.

The regulator circuit only uses four parts: a 2.2K Resistor, a 10uF Capacitor, a Transistor and a 50K potentiometer (the Control).

The Meter and 47K Resistor are included as a DC voltmeter. We've described the operation of this voltmeter previously, so won't repeat it. This circuit uses three basic characteristics of the Transistor.

They are:

1. Base current is always much less than collector or emitter current, and is the controlling factor over how much collector current can flow. The gain factor (Beta) is the ratio of collector current IC to base current IB and is typically greater than 80. (See project 160 for more about the Beta factor.)

2. Base-emitter (B-E) voltage is relatively constant and low in value. This is typically about 0.7V for silicon Transistors.

3. Any capacitance connected to the base appears at the emitter as if it were Beta-times higher in value.

For this circuit, operation may be described in terms of the above factors as follows. Output voltage may be adjusted from almost zero to almost the full 12V of the batteries as the emitter follows the base voltage obtained from the potentiometer (2). This is obtained by using a low-powered control because the base current requirements are low (1).

The output at the emitter is not only controlled in value but also is well filtered by the effective emitter capacitance (3). The Meter registers about 12V full scale with this circuit. Use this Meter to observe the control of output voltage as the Control is adjusted throughout its range.

Connect the output to one or more segments of the LED and check operation over the range of voltage available. The current which may be drawn from the circuit is limited to about 50 mA, so be careful what you try to power with this circuit. Actual voltage regulators, similar to this, use power transistors in order to handle higher load currents and power.