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I have a circuit where a LED is connected in series through a resistor to a PWM output signal.

The pulse width modulation (PWM) signal has two states (ON and OFF) where ON is 5V and OFF is 0V. The LED brightness can be controlled using this ON-OFF PWM modulation.

The output across the LED is constant while in the ON state. The duty cycle of a PWM signal determines how long the ON the state is ON.

How come the brightness varies when the voltage across the LED is the same?

Why does brightness seem to depend on the average voltage and not on the voltage across the LED at that instance?

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  • \$\begingroup\$ LED brightness depends on current, not voltage. \$\endgroup\$
    – Finbarr
    Apr 14, 2021 at 8:31

2 Answers 2

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The apparent brightness of a PWM driven LED to human visual system is a very logarithmic curve.

It means you can easily see a big difference between PWM signals of 1%, 2% and 3%, but will not see hardly any difference at all between PWM signals of 97%, 98% and 99%.

The average brightness of the LED to a light meter will still be linear in respect to the PWM duty cycle.

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  • \$\begingroup\$ Thank you for the clarification. \$\endgroup\$ Apr 14, 2021 at 10:19
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It is caused by your eyes, because when the duty cycle is small (for example 20%) and you have a higher frequency (few kHz or more), the LED is on only for small amount of time. Your eyes can't take such high speed changes, so you sees it as a smaller brightness.

The LED is actually as bright as it would be in long term on state. You have got 5V and a resitstor, so the voltage drop accros the LED is something about 3V and the current will be around 5-20 mA. But due to the fast switching frequency it seems like the LED is not that bright.

Try to use smaller frequency less than 400 Hz and you will see how the LED will flash.

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  • \$\begingroup\$ Thank you for the clarification. \$\endgroup\$ Apr 14, 2021 at 10:19

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