Zener Voltage Reference

Hold a reference node near 5.1 V using a reverse-biased Zener diode and a current-limiting resistor.

V1 supplies 12 V through R1 = 680 Ω. D1 is reverse biased and holds Vref near its 5.1 V breakdown setting. The resistor supplies the sum of Zener current and reference-load current.

$$I_{R1}=\frac{12-V_Z}{680\,\Omega}\approx10.15\,\mathrm{mA}$$ $$I_Z=I_{R1}-I_{\mathrm{reference\ load}}$$

RL = 10 kΩ draws about 0.51 mA, leaving approximately 9.64 mA in D1. The operating-point table shows Vref in green and reverse Zener current Iz in blue. The current direction for Iz is from cathode to anode, so normal regulation gives a positive reading.

Model limits. A reference needs enough resistor current to keep the Zener in breakdown. Below that point, output voltage falls with supply or load. Power ratings, Zener tolerances, noise, temperature drift and transistor heating are outside this example.

Try it. Set V1 DC voltage to 9 V; check that the reference remains near its original voltage. Restore the original values after comparing the results.