Tapelight RGB - Pwm

[Spadez]

Hi,

Sorry for all the questions, this is one of the only days I've had free to work on my project.

I've found something called led tapelight, sometimes called rgb striplight. I was wondering if anyone knows if these LEDs could be controlled with a MOSFET pwm setup like a luxeon for example. I've heard somewhere that Tapelight has specific requirements, like constant current drivers, and I'm not sure if that would be any different from normal rgb LEDs.

James

 

[Spadez]

A little bump for this. The stuff is also called "striplight".

Here is a picture of what I mean:
**broken link removed**

 

[audioguru]

The striplight has nothing special except it might have matched LEDs in parallel or might have a few matched LEDs in series then these strings are in parallel.
If you know the voltage and current required then you can dim them with PWM.

 

[Spadez]

Great!

I know the requirment of voltage and current:

12V
400ma

What I dont know however is I think I read somewhere that these strips require constant current. Is this correct? Will a PWM and mosfet setup allow me to do this or will I need additional components to ensure the current remains level throughout PWM?

 

[audioguru]

We don't know if the LED-strips have built-in resistors to limit the max current.

An ordinary PWM circuit applies as much current as the power supply can deliver to a DC motor or an incandescent light bulb. It will burn out LEDs unless you limit the max current with a resistor or with a constant current source.

An ordinary wall-wart power supply does not limit the current to its rated current.

 

[Spadez]

The tapelight does have resistors on it, however I have been told that it is neccessary to limit the current to ensure I dont blow the LEDs.

I am using a laptop power supply with a voltage regulator. Is it correct that I need to also build into my circuit a current regulator with the PWM to stop the LEDs drawing too much current?

 

[audioguru]

If the strip-LEDs have current-limiting resistors and are spec'd to use a 12V supply then use a 12V supply and their current should be 400mA.
Then use a PWM circuit that has a max 12V output (its input probably needs to be higher than 12V) to dim the LEDs.