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Divider, LED and zener calculator

Three problems from one law

The question is always the same: what resistor to fit. The only difference is what sets it — the LED current, the divider's output voltage or the zener current. There is no sense in three separate pages for these, but there is sense in working them out together with the nearest E24 values and the power ratings.

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Three problems from one Ohm's law
Task
Supply voltage (V)
LED type
Forward voltage (V)
Current (mA)
Result
Resistor
Nearest E24
Power in the resistor
Power in the LED

📐 The formulas​

LED:

R = (V_supply − V_f) / I_f
P(R) = (V_supply − V_f) · I_f

Voltage divider:

V_out = V_in · R2 / (R1 + R2)
with a load:  R2′ = R2·R_load / (R2 + R_load)

The divider current should be at least ten times the load current.

Zener shunt regulator:

R_ballast = (V_in − V_z) / (I_z + I_load)
P_z (worst case) = V_z · (V_in − V_z)/R_ballast      ← with no load

Typical LED forward voltages:​

red 2.0 V · yellow 2.1 V · green 2.2 V · blue and white 3.2 V · infrared 1.4 V

Where this calculator stops:
  • An LED's forward voltage depends on current and on the individual device: datasheets always quote a range
  • A divider is fit for measurement or biasing but not for supplying power: any load pulls it down
  • A zener shunt regulator has poor efficiency and poor regulation under a varying load
  • The worst case for the zener is no load at all, and that is shown on a line of its own

🎯 In practice​

  • Front-panel indication — the series resistor for an LED.
  • A microcontroller ADC input — a divider from 12 V down to 3.3 V.
  • A reference voltage for a comparator or for biasing — a zener.
Practical advice:
  1. Run an LED at 5–10 mA: modern bright ones are quite bright enough and will run cooler.
  2. In a divider feeding an ADC, put a capacitor across R2 — it removes pickup.
  3. A zener drawing more than 20 mA usually means you need a proper regulator.
  4. The nearest E24 values are shown alongside, with the result they actually give.

© 2026 UR3PKI · CyberDev.Space · Content licensed under CC BY-NC-SA 4.0.

How to cite this calculator
UR3PKI. «Divider, LED and zener calculator». CyberDev.Space. https://cyberdev.space/en/radio/calculators/others/basic_circuits (licence CC BY-NC-SA 4.0).

The licence lets you use this material freely, including in teaching materials, but only with attribution to the author and a link to the source.

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