How to use it
- Enter the input voltage (the unregulated supply) and the Zener voltage you want at the output.
- Enter the load current your circuit draws and the Zener current to keep the diode regulating.
- Read the resistor value, the current through it and the power for the resistor and the Zener.
- Pick the nearest standard resistor value that is lower, so the Zener current does not fall below what you set, and a resistor with a power rating comfortably above the result.
Formulas
- Total current I = I_L + I_Z
- Series resistor R = (Vin − Vz) / I
- Resistor power P_R = (Vin − Vz) × I
- Zener power with load P_Z = Vz × I_Z
- Zener power with no load P_Z max = Vz × I
Worked example
A 12 V supply, a 5.1 V Zener, a 20 mA load and 5 mA of Zener current. The current through the resistor is 25 mA, so R = (12 − 5.1) / 0.025 = 276 Ω. The resistor dissipates 6.9 × 0.025 = 0.1725 W: use a 0.25 W part at the very least, better 0.5 W. With the load connected the Zener dissipates 5.1 × 0.005 = 0.0255 W, but with no load it takes all 25 mA: 5.1 × 0.025 = 0.1275 W.
The nearest lower E24 value is 270 Ω, which gives a little more current: 6.9 / 270 = 25.6 mA.
Tips
- A Zener shunt regulator wastes power. It suits small loads and voltage references; for more than a few tens of milliamps, a linear regulator IC or a switching converter is more efficient.
- Zener voltage has a tolerance (often ±5 %) and changes with current and temperature; for a precise reference choose a dedicated reference diode.
- Add a capacitor across the Zener to reduce noise.