Zener Regulation Resistor

R = (Vin − Vz) / (Iz + Iload)

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Formula

R = (Vin - Vz) / (Iz + Iload)

Description

A Zener diode regulator uses a series resistor to drop the excess voltage between the input supply and the Zener voltage. The resistor must carry both the Zener current (needed to keep the diode in its breakdown region) and the load current. The Zener current should be at least 5-10mA for stable regulation. When the load current increases, the Zener current decreases by the same amount, maintaining a roughly constant voltage. This simple regulator is suitable only for low-current, low-precision applications due to its poor efficiency and load regulation.

Variables

  • R — Series resistor value (Ω)
  • Vin — Input supply voltage (V)
  • Vz — Zener breakdown voltage (V)
  • Iz — Minimum Zener current for regulation (A)
  • Iload — Maximum expected load current (A)

Practical Notes

Check that the resistor power rating exceeds (Vin - Vz)² / R, and that the Zener power rating exceeds Vz × (Iz + Iload). For better regulation, use a Zener-fed transistor regulator or an IC voltage regulator. Zener voltage has a temperature coefficient that can be positive or negative depending on the voltage rating.

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