Question on led powered by 12 volts 10W

[mossyoak1974]

First off hello all. Im new to trying to build a circuit board and have a question. I am trying to build a board to slowly fade on a 10w led flood light over about a 3 to 5 minute time frame using a 12v source. My question is, is this fesable and what type resistors and capacitors would i need. Thanks for any help.
Zack

 

[barry]

If you're expecting to do this with just "resistors and capacitors", forget it. You'll need the world's biggest capacitor to do it. I suggest you look into using some type of pulse-width modulation scheme. If you are totally new to electronics, as I'm guessing you are, you will need to educate yourself a bit before tackling this problem.

The main issue here is that if you simply use an RC circuit, or even an RC controlling a pass-transistor, you will have to dissipate significant power across your device.

 

[BradtheRad]

This illustrates how an RC circuit can turn on a transistor gradually. It's a resistive drop method rather than a pulsed method. It dissipates a lot of heat (as Barry refers to in post #2). The method isn't ideal but it might be adequate if you only need it once a day for just a few minutes. Heat sinking is required.

By charging a capacitor gradually, it biases a transistor so it turns on gradually.

The transistors are darlington arrangement. This allows you to use a smaller capacitor value. 6,000 uF is an approximate value.

--------------------------------------

Edited to add:

I almost forgot... Since the load is an led, it is off until the applied voltage rises to a certain point. That makes another obstacle to solve. And it makes the PWM method look better and better.

 

[mossyoak1974]

So say i have the light wired to a photocell and then to a motion sensor, would the PWM method be able to fade on the light and stay on until no more motion is detected? Or would it fade on then fade off on a regular basis?

 

[KlausST]

Hi,

I almost forgot... Since the load is an led, it is off until the applied voltage rises to a certain point. That makes another obstacle to solve. And it makes the PWM method look better and better.

Connect the LED in the collector line and calculate the emitter resistor as currnt source resistor.
To improve the system I´d unmount th 10k. Instead a resistor in parallel to the capacitor could give an improvement.

Klaus

- - - Updated - - -

Hi,

maybe you find this useful.


Klaus