Pulse-width modulation switches a signal fully on and fully off, fast, and varies the fraction of time it is on. Laptops use PWM to set fan speed, screen and keyboard backlight brightness, and inside every switching regulator. A multimeter in DC mode cannot show the pulses, but it shows their average — and that average reveals the duty cycle. This calculator converts between the two.
How to use it
- Enter the signal's high level — usually the logic supply of the chip driving it, often 3.3 V or 5 V.
- Enter the duty cycle, or leave it empty and enter the DC voltage your meter shows on the PWM line.
- Optionally enter the frequency to see the period and the on- and off-times.
The formulas
- Average voltage: V_avg = V_high × duty
- Duty cycle from a reading: duty = V_avg ÷ V_high
- Period: T = 1 ÷ f; on-time = duty × T; off-time = (1 − duty) × T
This works for clean square signals. Meters average fast signals well, but some cheap meters read low on high-frequency PWM; true-RMS meters may also behave differently from what the average suggests.
Worked example: a fan that will not speed up
A four-wire laptop fan stays slow under load. With the laptop busy, the fan's PWM pin reads 0.66 V on a 3.3 V signal: a 20 % duty cycle, even though the processor is hot. The EC is asking for low speed — so the fan and its wiring are not the problem; the temperature reading or EC control is. Fan circuits are covered in Laptop Fan Not Spinning: Circuit Diagnosis, and duty cycle in general under duty cycle.
Where it matters on a laptop board
- Fan PWM — checking what speed the EC requests.
- Backlight PWM — confirming the brightness command reaches the panel connector, as in No Backlight on a Laptop Screen: Diagnosis Guide.
- Switch nodes of regulators, where the duty cycle should match Vout ÷ Vin.
FAQ
Why does my meter show a steady voltage on a PWM signal?
It averages the pulses. The average is proportional to the duty cycle.
Can a meter measure duty cycle directly?
Many meters have a duty-cycle (%) mode and a frequency mode. Use them when available; the average-voltage method works on any meter.
Is 0 % or 100 % duty normal?
For a fan, 0 % may mean "stop" or "minimum", depending on the fan. A backlight PWM stuck at 0 % means no brightness command.