circuit (fixed output)
In this project, you're going to make a simple constant-voltage circuit using a zener diode and a transistor. You'll change the load current of the constant-voltage circuit to verify that the circuit is properly working as a constant-voltage circuit by checking the brightness of an LED.
When you finish wiring, turn power ON and make sure LED 1 is shining bright. Here, press S1 to increase the load current. What happened to LED 1?
Did it change in brightness?
You've seen that the constant-voltage circuit has the output voltage unchanged with change in load current. The zener diode D1 carries bias current coming through R1 and develops zener voltage across it.
The emitter of the transistor Q1, or the output voltage Vo of this constant-voltage circuit, is given as
VO = Vz (Zener Voltage) - Vbe (Voltage drop between base and emitter) = 4 - 0.6 = 3.4 [V]
You've realized that you can build a constant-voltage circuit you want by selecting a zener diode.