Constant-voltage circuit (variable output)
In this project, you're going to make a constant-voltage circuit that can change output voltage. You'll be able to change the output voltage to a desired level within a certain range by rotating the control volume. You'll check that your constant-voltage circuit properly works by looking at an LED.
When you finish wiring, turn power ON and rotate the control volume full counterclockwise. LED 1 should be shining bright. Try to press S1 to increase the load current. The brightness of LED 1 does not change. From this you know that the voltage to LED 1 is constant.
Next, rotate the control clockwise. LED 1 becomes less bright. Press S1 again to see how the brightness changes. (If you rotate the control far too clockwise, LED 1 will go out.)
The constant-voltage circuit compares two voltages:
(1) the zener voltage of zener diode D1, used as the reference voltage, and
(2) the output voltage Vo of the constant-voltage circuit, which is divided by R3, the control volume, and R4.
The difference of the two voltages are amplified so that Vo is kept constant. If the load current changes and the output voltage Vo changes accordingly, the resultant difference of the voltages is instantly fed back to provide a relatively stable output.