Zener Diode Voltage Regulator
Calculate the series resistor, zener current, and power dissipation for zener diode regulators.
Component Values
Results
Zener I-V Characteristic
Zener Diode Regulation
A zener diode regulates voltage by operating in reverse breakdown. When reverse-biased beyond its zener voltage (Vz), it clamps the voltage across it to Vz regardless of current variations. The series resistor Rs limits the current flowing through the circuit and absorbs the voltage difference between Vin and Vz.
The design process starts with choosing Rs so that the zener always carries at least 5–10 mA of current, even at maximum load. This minimum current keeps the zener in its regulation zone. If the load current increases, less current flows through the zener; if the load disconnects, all the current flows through the zener — so its power rating must handle the worst case.
Zener regulators are simple but inefficient for high-current loads because Rs dissipates power as heat. They work best for low-current voltage references, protection clamps, and circuits drawing under 100 mA. For higher currents, consider a three-terminal regulator like the 7805 or LM317.
Rs
Rs = (Vin − Vz) / (Iz + Iload)Zener Power (Pz)
Pz = Vz × IzKey Points
- Rs must supply both Iz (min 5 mA) and Iload
- Zener power rating must handle Pz = Vz × (Iz_max) when load disconnects
- Regulation degrades if Iz drops below the minimum knee current
- Choose a zener with Pz rating ≥ 2× the calculated dissipation
Applications
- Low-current voltage references
- Overvoltage protection clamps
- Bias voltage generation for transistor circuits
- Simple voltage regulation for sensors
Formula Reference
Zener diode voltage regulator:
R_series = (Vs – Vz) / (Iz + IL)
P_zener = Vz × Iz (must be < Pmax)
Iz_min ≈ 5–10% of Iz_max (for regulation)
Load regulation:
ΔVout ≈ Vz × ΔIL × (rz / (rz + Rs))
rz = dynamic impedance (from datasheet, typically 5–50Ω)Common Zener Diode Reference
| Part | Vz | Pmax | Iz_max | rz (typ) | Package |
|---|---|---|---|---|---|
| 1N4728 | 3.3V | 1W | 303mA | 10Ω | DO-41 |
| 1N4733 | 5.1V | 1W | 196mA | 7Ω | DO-41 |
| 1N4740 | 10V | 1W | 100mA | 17Ω | DO-41 |
| 1N4744 | 15V | 1W | 66mA | 30Ω | DO-41 |
| BZX84C3V3 | 3.3V | 300mW | 91mA | 28Ω | SOT-23 |
| BZX84C5V1 | 5.1V | 300mW | 59mA | 40Ω | SOT-23 |
| MMBZ5V1 | 5.1V | 350mW | 69mA | 25Ω | SOT-23 |
| 1N5352B | 15V | 5W | 333mA | 3Ω | DO-201 |
Worked Examples
Vs=12V, Vz=5.1V (1N4733), IL=20mA (logic ICs), Iz=10mA:
Rs = (12–5.1)/(10+20)mA = 230Ω → use 220Ω
P_zener = 5.1×10mA = 51mW << 1W ✓ P_Rs = (12–5.1)×30mA = 207mW → use ½W
Vs=5V, Vz=3.3V (BZX84C3V3, SMD), IL=1mA (ADC ref):
Rs = (5–3.3)/(5+1)mA = 283Ω → use 270Ω
Check: P = 3.3×5mA = 16.5mW << 300mW ✓
Design Tips
Zener regulators are inefficient — all excess current burns as heat. Best for low-power loads (< 20mA) and voltage clamping/protection. For better regulation, add a BJT emitter follower: Vout = Vz – 0.7V, IL can be 100× higher. Zener noise: lowest at Vz ≈ 5–6V (below: avalanche noise; above: Zener breakdown noise).
Did you know? The Zener effect was described by Clarence Zener in 1934. Modern "Zener" diodes below about 5.6 V actually use avalanche breakdown (not true Zener effect), but the name stuck. The 5.6 V threshold is where the two mechanisms balance and the temperature coefficient is nearly zero.