A Zener diode is a pressure-relief valve for voltage — it clamps the line exactly where you tell it to. Wire it backward across a supply with a series resistor, and it holds the output at its Zener voltage while burning off the excess as heat. That heat is the design limit: the power dissipated equals the Zener voltage times the Zener current, P = V_z × I_z. Exceed the diode's wattage and the relief valve becomes the failure.

This calculator checks the heat. Type the diode's Zener voltage in volts into Variable A — 5.1 for the classic logic-level regulator — and the current flowing through it in amps into Variable B; the Result box gives the dissipation in watts. An Arduino hobbyist in Oregon regulating a 9-volt battery down for a sensor, for instance, enters 5.1 in Variable A and 0.02 in Variable B, reading 0.102 watts — comfortably inside a standard 0.5-watt Zener's rating. Size the series resistor so the worst-case current keeps the Result under half the diode's rating, and the clamp holds steady for years.

With the pre-filled defaults — Variable A 10, Variable B 5 — the Zener Calculator returns Result 50.0000. Change any input and those results update instantly, so you can see exactly how each number you enter moves the answer.