Zener diode voltage regulator: how to design one

How the regulator works

In the circuit, the input voltage V_in feeds a series resistor R_s. The Zener diode sits in reverse across the output, in parallel with the load. Once the reverse voltage reaches V_Z, the Zener conducts and its voltage barely changes as its current varies, so the output stays near V_Z.

The current through R_s is I_s = (V_in − V_out) / R_s, and it splits between the load and the Zener: I_s = I_L + I_Z. When the load takes more current, the Zener takes less, and the other way around. The total through R_s stays roughly constant, which is why the output holds steady.

The regulation only works while the Zener current stays between a minimum and a maximum. Below the knee current the Zener stops regulating, and above its power rating it overheats.

Zener characteristics that matter

Four datasheet values drive the design.

• V_Z, the Zener voltage, is the regulated output voltage.
• r_z, the dynamic resistance, tells you how much the output changes per unit of Zener current: ΔV = r_z × ΔI_Z. A smaller r_z means better regulation.
• The knee current is the minimum current for stable regulation.
• The maximum power rating limits the current to I_Z,max = P_max / V_Z.

Curvo Studio's Zener simulator models the reverse side as follows. Below V_Z a small leakage current flows, growing toward the knee current as the voltage approaches V_Z. Above V_Z the model is linear: I = I_knee + (V − V_Z) / r_z. The BZX55C5V1 preset, the default, has V_Z = 5.1 V, r_z = 17 Ω, a 1 mA knee current and a 500 mW rating.

Sizing the series resistor

Choose R_s so that the Zener always keeps at least a safe minimum current, even at the lowest input voltage and the highest load current.

R_s = (V_in,min − V_Z) / (I_Z,min + I_L,max)

Pick I_Z,min comfortably above the knee current, for example a few milliamps, so regulation holds when the input or the temperature changes.

Then check the other extreme: the highest input voltage with the lightest load, usually no load. All the current goes through the Zener:

I_Z,max = (V_in,max − V_Z) / R_s − I_L,min

The Zener power is P_Z = V_Z × I_Z,max, and it must stay below the diode's rating. The resistor dissipates P_R = (V_in,max − V_Z)² / R_s, so choose a resistor rated above that.

If the check fails, increase R_s, which lowers the current and reduces the load range you can supply, or use a Zener with a higher power rating.

Worked example: 5.1 V regulator

Design a regulator with the BZX55C5V1 (V_Z = 5.1 V, 500 mW, 1 mA knee) for an input of 9 V to 12 V and a load of 0 to 20 mA. Choose I_Z,min = 5 mA.

1. R_s = (9 − 5.1) / (0.005 + 0.020) = 156 Ω. Use the standard value 150 Ω, which keeps the minimum Zener current slightly higher. 2. Worst case for the Zener, 12 V input and no load: I_Z = (12 − 5.1) / 150 = 46 mA, so P_Z = 5.1 × 0.046 ≈ 235 mW. That is below 500 mW. 3. Resistor power at 12 V: (12 − 5.1)² / 150 ≈ 0.32 W, so use a 0.5 W resistor. 4. Check the minimum case, 9 V in and 20 mA load: I_Z = 3.9 / 150 − 0.020 = 6 mA, above the 1 mA knee.

The hand calculation treats the Zener as an ideal 5.1 V source. The simulator's model also includes r_z, so the output moves a little. At 9 V and 20 mA it settles near 5.18 V with about 5.5 mA in the Zener, and at 12 V with no load near 5.79 V with about 41 mA. The output therefore varies over roughly 0.6 V while the input changes by 3 V and the load swings across its whole range.

Load range and limits

The regulator can only supply load currents up to I_L,max = (V_in,min − V_Z) / R_s − I_Z,min. With the 150 Ω resistor at 9 V this is 26 mA − 5 mA = 21 mA. The lowest load resistance it can drive while holding 5.1 V is about 5.1 / 0.021 ≈ 243 Ω.

A Zener regulator is simple but wasteful. The resistor and Zener burn power even when the load draws nothing, and the output still drifts with input and load because r_z is not zero. For larger loads, designers add a transistor or use a dedicated regulator. The Zener is still widely used as a voltage reference and as protection against overvoltage.

The simulator models a fixed power rating. Real Zeners must be derated as the temperature rises, so leave extra margin in a real design.

Try it in the Zener simulator

Open the Zener diode simulator with the BZX55C5V1 preset and set a negative applied voltage, which is reverse bias.

• At −3 V the current is only about 0.04 mA of leakage.
• At −5.1 V the current is the 1 mA knee current.
• At −5.79 V it is about 41 mA, with about 240 mW of dissipation. This matches the 12 V, no-load case above.
• Keep lowering the voltage. Near −6.4 V the current is about 79 mA, the power passes 0.5 W and the simulator flags the device as overloaded.

Compare the curve with the worked example. The slope of the regulation region is 1 / r_z, so changing r_z in the panel changes how flat the output stays.