Zener Diodes and Voltage References
A Zener diode is a p-n junction designed to operate predictably in reverse breakdown. Used correctly, it can clamp a signal, create a simple shunt regulator, or provide a low-cost voltage reference. Used without a current limit, it quickly overheats.
Learning Objectives
By the end of this lesson, you should be able to explain reverse breakdown, size the series resistor in a Zener regulator, check diode power dissipation, compare Zeners with precision voltage references, and identify common protection mistakes.
Breakdown Is the Useful Region
A normal signal diode is usually kept away from reverse breakdown. A Zener diode is built so that reverse current can flow at a specified voltage, called the Zener voltage (V_Z), as long as the current and temperature stay within the datasheet limits.
Two physical mechanisms are involved:
- Zener effect: dominant below about 5 V, with a negative temperature coefficient.
- Avalanche effect: dominant above about 6 V, with a positive temperature coefficient.
- Near 5.1 V to 5.6 V: the coefficients partly cancel, so these values are often more temperature stable.
The diode is not an ideal voltage source. Its voltage changes with current, temperature, tolerance, and dynamic impedance.
Shunt Regulator Circuit
A basic Zener regulator places a resistor in series with the supply and the Zener across the load. The resistor must carry both load current and Zener current.
layout direction=LR gap=90
VIN: Connector_Generic:Conn_01x02 value="Unregulated input"
RS: Device:R value="Rs" rotate=270
DZ: Device:D_Zener value="Vz" rotate=0
RL: Device:R value="Load" rotate=0
group INPUT label="Input" direction=TB {
VIN
}
group LIMIT label="Current limit" direction=LR {
RS
}
group SHUNT label="Regulated output" direction=TB {
DZ RL
}
VIN.1 --> RS.1 color=#b91c1c
RS.2 --> local:VOUT color=#b91c1c
DZ.K --> local:VOUT color=#b91c1c
RL.1 --> local:VOUT color=#b91c1c
VIN.2 --> global:0V
DZ.A --> global:0V
RL.2 --> global:0V
The series resistor is not optional. It limits current when the input voltage is high or the load is disconnected.
Design Equations
For a shunt regulator:
[
I_R = \frac{V_{IN} - V_Z}{R_S}
]
[
I_Z = I_R - I_L
]
[
P_Z = V_Z I_Z
]
Choose (R_S) so that:
- at minimum input voltage and maximum load current, (I_Z) remains above the datasheet knee current (I_{ZK});
- at maximum input voltage and minimum load current, (I_Z) remains below the safe current;
- (P_Z) stays below the diode power rating with thermal margin.
Worked Example
Requirement: create about 5.1 V from a 9 V to 12 V input for a load that draws 10 mA to 25 mA. Use a 5.1 V Zener that needs at least 5 mA for regulation.
At the worst low-input condition:
[
R_S \le \frac{9 - 5.1}{25mA + 5mA} = 130\Omega
]
Pick (R_S = 120\Omega). At 12 V with the load disconnected:
[
I_Z = \frac{12 - 5.1}{120} = 57.5mA
]
[
P_Z = 5.1 \times 57.5mA = 293mW
]
A 500 mW diode is marginal in a hot enclosure; a 1 W part or a different regulator is safer.
Voltage Reference vs Regulator
A Zener shunt regulator can power a small load, but it is not a precision reference by default. For ADCs, DACs, and measurement circuits, compare these properties:
| Property | Zener diode | Precision reference IC |
|---|---|---|
| Accuracy | Often 2% to 10% | Often 0.05% to 1% |
| Noise | Can be high | Usually specified |
| Current | Needs bias current | Often lower |
| Temperature drift | Strongly voltage-dependent | Usually specified in ppm/deg C |
| Cost | Very low | Higher |
Use a precision reference when measurement accuracy matters. Use a Zener when low cost and approximate clamping are enough.
Protection and Clamping
Zeners can protect inputs by conducting during overvoltage. For fast transients and ESD, a TVS diode is usually more appropriate because it is rated for surge energy. For slow overvoltage, a Zener plus current-limiting resistor can protect an ADC or logic input, but it must not inject unsafe current into the device rails.
Common Mistakes
- Connecting a Zener directly across a supply without a series resistor or current-limited source.
- Assuming (V_Z) is exact at every current.
- Forgetting worst-case no-load power dissipation.
- Using a small signal Zener where a surge-rated TVS diode is required.
- Feeding a precision ADC from a noisy, unbuffered Zener reference.
Summary
Zener diodes are useful in reverse breakdown when current is controlled. A reliable design checks (R_S), (I_Z), (P_Z), tolerance, temperature drift, and transient energy. For accurate measurement references, a dedicated reference IC is often the better engineering choice.
Further Reading
- Vishay, "Zener Diodes" application notes.
- Analog Devices, "Voltage Reference Design Considerations."
- Texas Instruments, "TVS Diode and Zener Diode Selection" application notes.