Pulse width and average current
Dim a light by switching quickly
PWM means Pulse Width Modulation: changing the fraction of each cycle spent on. This fraction is duty cycle. LED means Light Emitting Diode, a light-producing semiconductor diode. Frequency in hertz (Hz) counts cycles per second.
EXPERIMENT BENCH
Change the circuit settings, predict the readings, then check a challenge. These are learning models, with the assumptions explained in each lesson.
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Why it works
Ideal 5 V pulses, LED drop 2 V, fixed resistor. Switching edges, visual response and thermal effects are omitted; brightness is not calibrated.
Peak current = (5 − 2)/R. Average current = peak × duty/100. More frequency means shorter cycles, not more average current at fixed duty. The peak formula describes an on pulse. At 0% duty there is no on pulse and the displayed peak is zero.
Difficulty
Build · Build it
Power on with 330 Ω, 50% duty and 100 Hz. Compare peak and average current.
Need a hint?
330 Ω; 50%; 100 Hz
Explore · Change one thing
Obtain 2–3 mA average, at most 20 mA peak, and a frequency of at least 100 Hz.
Need a hint?
330 Ω; 25%; 100 Hz → 2.27 mA
Solve it · Solve the challenge
Obtain 8–10 mA average, at most 20 mA peak, and a frequency of at least 200 Hz.
Need a hint?
200 Ω; 60%; 200 Hz → 9 mA
Extra reading and circuit recipes
Dim a light by switching quickly
A useful pattern
At 25% duty, the current is on for one quarter of each cycle; its peak is unchanged.
Beyond this model
A real driver must tolerate the pulse current. Fast switching does not automatically make an excessive peak safe.
A useful trap to avoid
PWM does not remove the need for an LED current-limiting resistor.
Duty stays at 50%; frequency doubles. Average current…
- doubles
- halves
- stays unchanged
Read more
The same fraction of time is on. Only the length of each cycle changes.