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LED stripe resistor

BobK

Jan 5, 2010
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I'll tell you what. Buy one of the PWM LED or motor controllers off Ebay and then look at what it outputs. You will see a square wave with varying duty cycle. Then buy an LED driver or switch mode power supply and you will see a constant current or voltage produced. They are not the same thing.

Bob
 

kellys_eye

Jun 25, 2010
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It is not outputting a fixed 12V if it's dimming an LED strip. Adding an inductor and capacitor is expected, standard practice on LED drivers.
Of course it's not outputting a 'fixed' 12V - that's the whole point of a PWM dimmer - the average DC output varies as a function of the pulse width however the 'peak' voltage still remains at 12V hence the LED will always have sufficient forward voltage to conduct (emit). Current is still limited by the series dropper resistor.

The most basic PWM LED dimmer you can get is based on the 555 - and it doesn't use any any inductors. Nor is that a requirement of such a controller.

Are you 'conflating' the 'M' (modulation) part of the PWM statement? True, the PW signal is not 'modulated' (i.e. controlled by an external or feedback signal) so the 'M' bit can be argued - but that's just nitpicking.

I suppose the specific designation would be 'variable pulse width controller' but I've never seen it TLA'd as 'VPC' - they've always been known, if even colloquially, as PWM.
 
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Irv

Jun 7, 2017
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Jun 7, 2017
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I have not yet found an LED that can't *appear* to be dimmed by switching it on and off (using full rated voltage). All you need to do is to switch it on and off fast enough. The percentage of time that it is on compared to the time it is off is what makes it seem to be dim to our eyes. Any on/off flashing more than 20 or 30 times per second is going to be averaged out by our eyes / brain. So PWM works.
 
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