Analysis Example: A Zener Diode Regulator Circuit

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Analysis Example: A Zener Diode Regulator Circuit [Copy link]

OBJECTIVEA regulator is a circuit that maintains a constant output voltage across a load regardless of load current. For example, the load could be a microcontroller system that requires the supply voltage to remain constant even as its current varies with system activity. The Zener diode regulator in Figure 1 provides a very simple way to maintain the load voltage, VL, at the same value as the reverse breakdown voltage of the Zener diode, as long as the load resistance, RL, remains above a certain lower limit. The voltage source, VIN, and resistor, RS, simulate the input circuit characteristics through the Thevenin equivalent circuit, such as converting a high voltage (such as a 120 V AC source) to a lower DC voltage source that is not regulated and filtered.
MaterialsADALM2000
Active Learning Module
Solderless Breadboard1
1 kΩ Resistor (RS)
1 5 kΩ Variable Resistor, Potentiometer (RL)
1 Zener Diode (1N4735 or Similar)
InstructionsBuild the circuit shown in Figure 1 on a solderless breadboard using a 1N4735 6.2 V Zener diode. Use AWG1 (5 V constant) and a –5 V Vn user supply for the DC supply, VIN. RL uses a variety of fixed and variable resistors.

Figure 1. Zener diode regulator.

Figure 2. Zener Diode Regulator Breadboard Circuit Procedure Steps Step 1
For the following values of RL, measure VL using the Scopy voltmeter to monitor and report the load voltage, VL:
Open circuit (see Figure 3)
10 k (see Figure 4)
1 k (see Figure 5)
100 (see Figure 6)

Figure 3. RL = Open Zener diode regulator waveform

Figure 4. RL = 10 kΩ Zener diode regulator waveform

Figure 5. RL = 1 kΩ Zener diode regulator waveform

Figure 6. RL = 100 Ω Zener diode regulator waveformsStep 2Replace
the load RL with a 5 kΩ potentiometer and adjust the potentiometer to determine the minimum value of RL for which VL remains within 10% of the Zener voltage, VZ. Measure and report the resistance of the potentiometer setting. How does this resistance relate to the value of RS?
Further explorationUsing the procedures described in Step 2, investigate the current/voltage characteristic of the Zener diode by measuring the current in RS using oscilloscope channel 2 and plotting the voltage across the Zener diode versus the current in the oscilloscope's XY mode. Be sure to adjust the horizontal voltage range and offset to include the 6.2 V breakdown voltage. Discuss your results, especially discussing how the Zener diode is similar to and different from a regular diode.
Driving Higher Load CurrentsAs we saw in the simple Zener diode regulator of Figure 1, the maximum load current is determined by the resistor, RS. Also, the circuit is very inefficient at small load currents relative to the maximum current because additional current flows into the Zener diode when the current in the load is small. Compared with the voltage regulator circuit shown in Figure 2, adding an emitter follower or Darlington emitter follower current amplifier can greatly improve the efficiency of the voltage regulator circuit.
Additional Materials
Two NPN transistors (2N3904 and TIP31)
Two small signal diodes (1N914 or similar components)

Figure 7. Adding a Current Amplifier Stage Guide Construct any of the circuits shown in Figure 7 on a solderless breadboard using a 1N4735 6.2 V Zener diode for D1 and a 2N3904 or TIP31 power transistor for Q1. Q2 can be a 2N3904 and D2 and D3 can be 1N914.
Diode D2 is added in series with the Zener diode to partially offset the additional VBE drop caused by the emitter follower Q1. Similarly, two diodes (D2, D3) are added in a Darlington configuration to also partially offset the two VBE drops of the Darlington follower.