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Topic: Constant Current LED Driver and LED module with resistors (Read 1 time) previous topic - next topic

lokidude

I was planning to drive a 12V RGB led module with a MAX6971 open-drain, constant-current-sinking LED driver but I'm not sure now if it will work.

The 12V RGB module consists of 3 5050 type RGB leds and current limiting resistors, Common Anode.  So, If I hook up 12v directly to the anode and any of the R, G or B leds it lights up full brightness and the current is limited by the resistor.

My question is can I use the MAX6971 to drive these and control the brightness even though the current limiting resistor is there?  Will the MAX6971 just see the combination of the LED and resistor as a bigger Vf and still allow me to control the total current or do I need to find either a different LED module or rig up a PWM setup instead of the constant current driver. 

I prefer a constant current setup but I'm just not sure if I can do this with the built in current limiting resistors.

Any electronics expert out there that can give me an answer?

Grumpy_Mike

Quote
I prefer a constant current setup but I'm just not sure if I can do this with the built in current limiting resistors.

With a built in resistor you can only have the current up to the maximum that the resistor will let you.
You could use a higher voltage than 12V but I don't see where that would get you. Constant current regulators are only normally used on high power LEDs not on strips like this.

lokidude


With a built in resistor you can only have the current up to the maximum that the resistor will let you.
You could use a higher voltage than 12V but I don't see where that would get you. Constant current regulators are only normally used on high power LEDs not on strips like this.


I understand I'd still be limited to the max current at 12v with the current limiting resistor ( lets say 40ma ) but the constant current driver could still be used to limit it to less current with no ill effects on the chip?  To put it another way, I would be able to do RGB mixing using the drivers current limiting up to the maximum current allowed by the resistor at the 12v anode drive is that correct?

I suppose I could just wire it up and see what happens but why fry a perfectly good chip via my own dumb moves.

The goal of this adventure is to use these cheap 3 led RGB modules ( I'm sure you've seen them on ebay ) to build a large 7 segment display ( one module per segment ) that can be programmed for any color you wish, I suppose you could even make each segment a different color if you had an application where that made sense.

fungus

Just use a MOSFET to switch them. You don't need the constant-current part.
Advanced Arduino

Grumpy_Mike


Just use a MOSFET to switch them. You don't need the constant-current part.


That is switch them with PWM

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