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Old 12-25-2012, 10:51 PM   #13
DNSB
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Quote:
Originally Posted by xuyn2003 View Post
OK, I understand, the flashing rate will not change at different level, but the on-off time was adjusted in one cycle.

Another question, will PWM ,have a minimal on time limit? For example, it had to be on for at least 0.1 microsecond, if the time is less than 0.1 microsecond, it will not be turn on at all?
The minimum on time limit depends on the resolution of the controller. A dedicated controller can easily have an on time of 0.1 microseconds, a controller being run by a microprocessor in software which is handling other tasks is likely to have a longer minimum pulse width. The other limit on the minimum duty cycle would be the LED itself as you generally need higher current at the shorter duty cycles to have the LED turn on and there is a minimum pulse width below which the LED will not turn on. Those numbers will depend on the LED manufacturing process and materials.

Using my 1KHz cycle time, an LED being turned on for 0.1 microseconds then off for 999.9 microseconds is not going to generate very much light. It's more likely to have a minimum pulse width (AKA duty cycle) determined by the minimum acceptable light level and then however many steps the designers felt would be useful between that duty cycle and the maximum duty cycle. If an external PWM controller is used, some of the parameters may be hard-coded into the PWM controller while others would be set by the microprocessor. If not PWM controller is present, the microprocessor would be doing all the control functions.

Regards,
David
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