PWM and duty cycle: how switching on and off sets the power

LED dimmers, fan speed controls, chargers, inverters: they all share one idea. Set the power by switching the load on and off thousands of times a second.

What does it do? In PWM the switch is either fully on or fully off; the frequency stays fixed and only the pulse width changes. The fraction of each period spent on is the duty cycle (D). The average voltage the load sees is D × the supply voltage: at 12 V, 25 % duty gives 3 V and 75 % gives 9 V. Because the switch never sits in between, it barely heats up; it's far more efficient than dropping power with a resistor or linear regulator.

Tap the dashed parts for what they do, how to measure them and how they usually fail. Numbered points are scope measurements.

Phase 1/3: 25 % duty

The switch is on for only a quarter of each period. On average the load sees a quarter of 12 V, i.e. 3 V: the LED is dim, the motor is slow.

1. Load voltage, 25 %

Square wave: short at 12 V, long at 0 V. Average 3 V.

On the scope

Multimeter3.0 V (average)

🛠 Fault scenarios: pick a symptom and see what to measure, in order, on the schematic

🔍 How to spot it on a board

•A PWM generator (NE555, microcontroller or SMPS IC) and a MOSFET/transistor driven by it
•With motors and relay coils, a reverse freewheeling diode across the load
•Turning the pot changes the pulse width, not the frequency

🔧 How to measure it

•A multimeter shows the average: 50 % duty on 12 V reads ≈6 V.
•To see the real wave, scope the gate and the load terminal.
•Some multimeters have a 'Duty %' range that reads the duty cycle directly.

⚠ Common faults

•Shorted MOSFET: the load is always at full power, no control.
•PWM output stuck low or an open gate resistor: the load never runs.
•Low gate voltage (e.g. a standard MOSFET on 5 V): the MOSFET doesn't fully turn on and heats up.
•Missing or open freewheeling diode: with a motor load the MOSFET soon fails.

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Go further

Where it's used: the buck converter PWM with an NE555 Component: MOSFET How it works: motor drivers Concept: sine PWM