Practical case: Zener Diode as a Voltage Regulator

Zener Diode as a Voltage Regulator prototype (Maker Style)

Level: Medium. Design and verify a voltage stabilizer circuit using a Zener diode under load variations.

Objective and use case

In this session, you will build a shunt voltage regulator using a Zener diode and a series limiting resistor to maintain a fixed 5.1 V output from a 9 V source.

  • Why it is useful:
    • Provides a stable reference voltage for Analog-to-Digital Converters (ADCs).
    • Protects sensitive downstream components (like microcontrollers) from over-voltage spikes.
    • Regulates voltage for low-power circuits without the complexity of an IC regulator.
  • Expected outcome:
    • The output voltage (VOUT) remains clamped at approximately 5.1 V despite the input being 9 V.
    • Connecting a moderate load (470 Ω) decreases Zener current but maintains VOUT at 5.1 V.
    • If the load resistance becomes too low, the regulation fails, and VOUT drops below 5.1 V.
  • Target audience: Electronics students, Level: Medium.

Materials

  • V1: 9 V DC voltage source, function: main power supply.
  • R1: 220 Ω resistor, function: series current limiting (RS).
  • D1: 1N4733 A Zener diode (5.1 V, 1 W), function: shunt voltage regulator.
  • R2: 470 Ω resistor, function: load simulation (RL).
  • M1: Multimeter (Voltmeter mode), function: measure output voltage.
  • M2: Multimeter (Ammeter mode), function: measure Zener current (IZ).

Wiring guide

Construct the circuit using the following connections and SPICE node names (VIN, VOUT, 0):

  • V1 (9 V Supply): Connect Positive terminal to node VIN and Negative terminal to node 0 (GND).
  • R1 (Series Resistor): Connect one terminal to VIN and the other terminal to node VOUT.
  • D1 (Zener Diode): Connect the Cathode (striped end) to node VOUT and the Anode to node 0.
  • R2 (Load Resistor): Connect one terminal to VOUT and the other terminal to node 0.
  • Measurements:
    • To measure VOUT: Connect the Voltmeter Positive probe to VOUT and Negative probe to 0.
    • To measure IZ: Break the connection between D1 Cathode and VOUT, and insert the Ammeter in series (Positive to VOUT, Negative to D1 Cathode).

Conceptual block diagram

Conceptual block diagram — Zener Voltage Regulator
Quick read: inputs → main block → output (actuator or measurement). This summarizes the ASCII schematic below.

Schematic

[ SOURCE ]                 [ LIMITING ]                     [ REGULATION, LOAD & MEASUREMENT ]

                                                                           (Branch 1: Regulation)
                                                                 +---> [ Ammeter M2 ] --> [ D1: Zener 5.1 V ] --> GND
                                                                 |     (Measure Iz)       (Shunt Regulator)
                                                                 |
    [ V1: 9 V DC ] --(VIN)--> [ R1: 220 Ohm ] --(Node VOUT)--> ---+
    (Main Power)             (Series Resistor)                   |         (Branch 2: Load)
                                                                 +---> [ R2: 470 Ohm ] ------------------------> GND
                                                                 |     (Load Simulation)
                                                                 |
                                                                 |         (Branch 3: Monitoring)
                                                                 +---> [ Voltmeter M1 ] -----------------------> GND
                                                                       (Measure Vout)
Schematic (ASCII)

Measurements and tests

Follow these steps to validate the regulator design:

  1. Open Circuit Test (No Load):

    • Temporarily disconnect R2.
    • Measure voltage at VOUT. It should read approximately 5.1 V.
    • Calculate the current flowing through the Zener: IZ = (VIN – VZ) / R1. Expect ≈ 17.7 mA.

  2. Load Regulation Test:

    • Reconnect R2 (470 Ω) between VOUT and 0.
    • Measure VOUT again. It should remain stable at 5.1 V.
    • Observe the Zener current. It should decrease because some current is now diverted through the load RL.
    • Expected Load Current (IL): 5.1 V / 470 Ω ≈ 10.8 mA.
    • Remaining Zener Current: ≈ 17.7 mA – 10.8 mA = 6.9 mA. Since IZ > 0, regulation holds.

  3. Overload Test (Simulation):

    • Replace R2 with a 100 Ω resistor (if available) or simulate a short.
    • Measure VOUT. The voltage will drop significantly below 5.1 V because the load demands more current than R1 can supply while maintaining the Zener breakdown voltage.

SPICE netlist and simulation

Reference SPICE Netlist (ngspice) — excerptFull SPICE netlist (ngspice)

* Practical case: Zener Diode as a Voltage Regulator

* --- Power Supply ---
* V1: 9 V DC voltage source (Main Supply)
V1 VIN 0 DC 9

* --- Components ---
* R1: 220 Ohm Resistor (Series Current Limiting)
* Wiring: Connect one terminal to VIN and the other to VOUT
R1 VIN VOUT 220

* R2: 470 Ohm Resistor (Load Simulation)
* Wiring: Connect one terminal to VOUT and the other to 0 (GND)
R2 VOUT 0 470

* M1: Multimeter (Voltmeter mode)
* Wiring: Positive probe to VOUT, Negative probe to 0
* Implementation: High impedance resistor to simulate voltmeter load
R_M1_Voltmeter VOUT 0 10Meg

* ... (truncated in public view) ...

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* Practical case: Zener Diode as a Voltage Regulator

* --- Power Supply ---
* V1: 9 V DC voltage source (Main Supply)
V1 VIN 0 DC 9

* --- Components ---
* R1: 220 Ohm Resistor (Series Current Limiting)
* Wiring: Connect one terminal to VIN and the other to VOUT
R1 VIN VOUT 220

* R2: 470 Ohm Resistor (Load Simulation)
* Wiring: Connect one terminal to VOUT and the other to 0 (GND)
R2 VOUT 0 470

* M1: Multimeter (Voltmeter mode)
* Wiring: Positive probe to VOUT, Negative probe to 0
* Implementation: High impedance resistor to simulate voltmeter load
R_M1_Voltmeter VOUT 0 10Meg

* M2: Multimeter (Ammeter mode)
* Wiring: Inserted in series between VOUT and D1 Cathode
* Positive to VOUT, Negative to D1 Cathode (Node: VZ_CATHODE)
* Implementation: 0V DC source to measure current
V_M2_Ammeter VOUT VZ_CATHODE DC 0

* D1: 1N4733A Zener Diode (5.1 V, 1 W)
* Wiring: Cathode to VZ_CATHODE, Anode to 0
* Note: Cathode is connected to VOUT through the Ammeter
D1 0 VZ_CATHODE D1N4733A

* --- Models ---
* Model for 1N4733A Zener Diode
* BV=5.1V (Breakdown Voltage), IBV=49mA (Test Current)
.model D1N4733A D(IS=2.5n RS=1 N=1.2 BV=5.1 IBV=49m)

* --- Analysis ---
* Transient analysis (1ms simulation time)
.tran 1u 1ms

* --- Output Directives ---
* Print voltages and Zener current (Iz)
.print tran V(VIN) V(VOUT) I(V_M2_Ammeter)

* Operating Point for initial check
.op

.end

Simulation Results (Transient Analysis)

Simulation Results (Transient Analysis)
Show raw data table (1008 rows) 
Index   time            v(vin)          v(vout)         v_m2_ammeter#br
0	0.000000e+00	9.000000e+00	5.047821e+00	7.223902e-03
1	1.000000e-08	9.000000e+00	5.047805e+00	7.224007e-03
2	2.000000e-08	9.000000e+00	5.047805e+00	7.224007e-03
3	4.000000e-08	9.000000e+00	5.047805e+00	7.224007e-03
4	8.000000e-08	9.000000e+00	5.047805e+00	7.224007e-03
5	1.600000e-07	9.000000e+00	5.047805e+00	7.224007e-03
6	3.200000e-07	9.000000e+00	5.047805e+00	7.224007e-03
7	6.400000e-07	9.000000e+00	5.047805e+00	7.224007e-03
8	1.280000e-06	9.000000e+00	5.047805e+00	7.224007e-03
9	2.280000e-06	9.000000e+00	5.047805e+00	7.224007e-03
10	3.280000e-06	9.000000e+00	5.047805e+00	7.224007e-03
11	4.280000e-06	9.000000e+00	5.047805e+00	7.224007e-03
12	5.280000e-06	9.000000e+00	5.047805e+00	7.224007e-03
13	6.280000e-06	9.000000e+00	5.047805e+00	7.224007e-03
14	7.280000e-06	9.000000e+00	5.047805e+00	7.224007e-03
15	8.280000e-06	9.000000e+00	5.047805e+00	7.224007e-03
16	9.280000e-06	9.000000e+00	5.047805e+00	7.224007e-03
17	1.028000e-05	9.000000e+00	5.047805e+00	7.224007e-03
18	1.128000e-05	9.000000e+00	5.047805e+00	7.224007e-03
19	1.228000e-05	9.000000e+00	5.047805e+00	7.224007e-03
20	1.328000e-05	9.000000e+00	5.047805e+00	7.224007e-03
21	1.428000e-05	9.000000e+00	5.047805e+00	7.224007e-03
22	1.528000e-05	9.000000e+00	5.047805e+00	7.224007e-03
23	1.628000e-05	9.000000e+00	5.047805e+00	7.224007e-03
... (984 more rows) ...

Common mistakes and how to avoid them

  1. Reversing the Zener Diode:
    • Error: Connecting the Anode to VOUT and Cathode to GND.
    • Result: The circuit behaves like a standard diode, clamping the output to ≈ 0.7 V instead of 5.1 V.
    • Solution: Ensure the striped end (Cathode) is connected to the positive potential (VOUT).
  2. Using a Series Resistor (R1) with too high resistance:
    • Error: Using 10 kΩ instead of 220 Ω for R1.
    • Result: When the load (R2) is connected, the voltage drops immediately; the Zener turns off because there isn’t enough current to keep it in breakdown.
    • Solution: Calculate R1 such that enough current flows to satisfy both the load and the minimum Zener bias current (IZK).
  3. Exceeding Zener Power Rating:
    • Error: Removing the load while using a very small R1.
    • Result: All current flows through the Zener, causing it to overheat and potentially burn out.
    • Solution: Ensure PZ = VZ × Izmax is less than the diode’s power rating (e.g., 1 W).

Troubleshooting

  • Symptom: Output voltage is equal to Input voltage (9 V).
    • Cause: Zener diode is open (broken) or not connected.
    • Fix: Check connections to D1 or replace the diode.
  • Symptom: Output voltage is ≈ 0.7 V.
    • Cause: Zener diode is connected in forward bias (backwards).
    • Fix: Reverse the diode orientation.
  • Symptom: Output is 5.1 V without load, but drops to 3 V (or lower) when load is attached.
    • Cause: The load resistance is too low (drawing too much current) or R1 is too high.
    • Fix: Increase the load resistance or recalculate R1 for higher current delivery (watching power limits).

Possible improvements and extensions

  1. Series Pass Transistor: Add an NPN transistor (like a 2N2222) with the Zener controlling the base. This creates a Series Voltage Regulator capable of handling much higher load currents.
  2. Filtering: Add a capacitor (e.g., 10 µF) in parallel with the Zener diode to filter out noise and improve the stability of the voltage reference.

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Quick Quiz

Question 1: What is the primary function of the Zener diode in the described circuit?




Question 2: What is the expected output voltage (VOUT) of this circuit when functioning correctly?




Question 3: Which component is responsible for limiting the current flowing through the Zener diode?




Question 4: How must the Zener diode be biased in this circuit to regulate voltage?




Question 5: What happens to the Zener current when a moderate load (470 Ω) is connected in parallel?




Question 6: Under what condition does the voltage regulation of this circuit fail?




Question 7: Why is this circuit useful for Analog-to-Digital Converters (ADCs)?




Question 8: Which specific Zener diode model is specified in the materials list?




Question 9: What is the primary purpose of the 9 V DC source (V1) in this setup?




Question 10: Besides providing a reference voltage, what is another key use case for this circuit?




Carlos Núñez Zorrilla
Carlos Núñez Zorrilla
Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

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