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Control the brightness of power LED

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Newbie level 5
Jan 17, 2007
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I want to make a circuit in order to control the brightness of a power LED(5W). I brought a LED driver (PR4101) and its demo broad already. And my circuit is operated at 12Vac (or may be 24Vac) input.

In the PR4101 datasheet (P.10), there is an explanation of "typical application with 12Vac supply and phase cut dimmers". However, I am not understand what is the use of the pin "PWM" as it stated that this pin can control the brightness of LED in the datasheet. As shown in P.10, only "VDI" is connected, but "PWM" is not connected to anything else. Is the brightness of LED really can be controlled even if the PWM pin is not connected? and its brightness is depend on the turn-off/on portion of the incoming voltage only? Actually, I am not sure what the input signal should be so that I can dim the LED. Am I right to think that the brightness can be controlled by the turn-on/off protion of the input signal as shown in P.10 of the datasheet? Is there someone can give me the schematic diagram to show me how to set up the correct input signal for the diagram shown in P.10 or to PR4101 demo board (P.18 ) so that I can control the LED's brightness? Thank you.

PR4101 datasheet:
**broken link removed**

pwm signal led

I think the datasheet implies that the PWM pin is internally pulled high so the boost converter is operational by default.

VrefPWM looks like it is an internally generated reference voltage which sets the threshold which the PWM pin has to exceed before the boost oscillator will start. Looking at it the other way, if you pull the PWM pin below 1V, the LED will go off.

Driving a signal at 500Hz or less will let you control the LED current if you vary its mark/space ratio. 0% (always low) means no LED, 100% (always high) gives full current.


powerled hobby

Thanks Brian.

I have tried to give a 100Hz pulse width signal to the PWM pin of the demo board, and the input (Vin+ and Vin-) is a standard 12Vac. However, the LED flashs seriously. So I don't know what signal should I provide to the PWM or VDI pin? I also tried to use a 5KHz PWM signal to the PWM pin. The flash is less obvious than before (using 100Hz signal). However, it becomes obvious again when the LED go to low brightness level (dim mode). The reason why I try to use so high frequency signal is because I find that the timebase is 50.0us in the PWM measurement diagram shown in P.12 of datasheet. The PWM signal used in this measurement seems to be 3KHz. I don't know why it recommends to use 500Hz or lower for the PWM?

control led brightness

connect the VDI pin with K10 positive node.then drive the pwm pin with pwm signal lower than 500Hz.

leds control brightness

Do as ss13 says first, it might fix the problem.

Otherwise, check these things:

1. The VDI pin resets the chip if the supply voltage drops too low. Make sure your AC supply isn't being loaded too much when the LED is drawing current. You could have oscillation where the supply is high, LED operates, supply drops, VDI resets the chip, supply goes up again, LED operates.... and so on.

2. Depending on how much current you are trying to push through your LEDs, C8 may not be large enough at 100uF. You could have ripple from the AC supply beating with the PWM signal. If this was the case, I would expect 100Hz to give a flash rate of about 1 second. You can confirm this by using a DC supply instead of AC.

3. It could just be a conflict of PWM frequency and the switching frequency although I would think this less likely. If it is, double check that you are feeding your PWM directly between the input pin and ground. It may be worth trying a different ground wire location on the board.

Let us know if this helps.


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