## Simple Zener-regulated power supply

This is a simulation of a simple Zener-regulated power supply, also called a shunt regulator.

The voltage from a half-wave power supply is applied to a resistor R and a Zener diode connected in series. In normal operation, the Zener diode operates in the "reverse- breakdown" mode, in which the reverse voltage is equal to the nominal Zener voltage and is independent of the current (as long as the current in non-zero). The resistor R helps regulate the current through the Zener diode. As the load current is increased, some of the current that was flowing through the Zener diode is bypassed into the load device, reducing the Zener current, but the voltage remains constant.

If the input voltage is too low, or if the load current is too large, the Zener will not conduct and voltage regulation fails.

Use the left slider to adjust the AC line voltage and the right slider to adjust the DC current drawn from the power supply. You can also change the transformer turns ratio, the capacitor value (µF), the resistor value (ohms), and the nominal Zener voltage. Just click on the value with the mouse pointer and edit like any text field.

### Student Questions

1. Launch (double-click on) "Zener regulator" and inspect the screen layout.

2. This circuit consists of a half-wave power supply (as in the first simulation) with a zener shunt regulator added. Begin by setting the primary voltage to 115 vac (left-hand slider), the transformer turns ratio to 0.11, the filter capacitor to 1000 µF, the resistor to 20 ohms, the nominal Zener voltage to 10 volts, and the load current to zero (right-hand slider). Note that the output voltage is equal to the Zener voltage (10 volts). Make a note of the current drawn by the Zener diode.

3. Change the primary voltage to simulate fluctuations in the line voltage (brown-outs, etc). Is there any evidence that the DC output voltage is stabilized against such fluctuations? Return the primary voltage to 115 vac.

4. Now increase the load current gradually. Note that as the load current increases, the output voltage remains the same, the Zener current decreases, and the current through the resistor remains the same. What is the largest output current that can be drawn before the output voltage decreases?

How does this compare to the current that flows through the Zener diode when the load current is zero?

In this circuit, the power rating of the Zener diode and the resistor would have to be __________ and _________, respectively, for safe operation.

5. What changes in circuit values and power ratings would you have to make in order to be able to supply a regulated 5 volts at currents up to 1.0 amps?

Zener voltage___________ Power rating ____________

Resistor ohms ____________ Power rating____________