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Step 4Increasing the Brightness
The other useful application of PWM with an LED is that the LED can have a larger than normal current passed through it making it brighter. Normally this current would destroy the LED, but since the LED is only on for a fraction of the time, the average power put through the LED is within tolerance.
The limit of this current is defined on the manufacturer's data sheet for the LED, identified as the forward pulse current. There are also often details regarding the minimum pulse width and duty cycles. Using a white LED as an example, the following specifications are given as:
Forward Current = 30mA
Pulse Forward Current = 150mA
Pulse Width =< 10ms
Duty Cycle =< 1:10
Using the pulse width and duty cycle information, the relaxation oscillator can be recalculated with T=2ln(2)RC
Assuming a 10nF capacitor is used, and wanting TON = 10ms, and TOFF = 1ms, the following calculations can be made, and then the circuit diagram drawn.
The limit of this current is defined on the manufacturer's data sheet for the LED, identified as the forward pulse current. There are also often details regarding the minimum pulse width and duty cycles. Using a white LED as an example, the following specifications are given as:
Forward Current = 30mA
Pulse Forward Current = 150mA
Pulse Width =< 10ms
Duty Cycle =< 1:10
Using the pulse width and duty cycle information, the relaxation oscillator can be recalculated with T=2ln(2)RC
Assuming a 10nF capacitor is used, and wanting TON = 10ms, and TOFF = 1ms, the following calculations can be made, and then the circuit diagram drawn.
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