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connecting a 3-pin bi-colour LED

Graham

Nov 10, 2009
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Hi all,
I have a red-green LED of the 3-pin variety (as opposed to the ones with 2 pins that use reversed voltage to switch colour). I am wondering if I need 2 limiting resistors (one on each anode), or if a single resistor on the cathode is sufficient? As it is technically the same as two separate LEDs with their cathodes tied together, and therefore both can be on simultaneously (resulting in a yellowish hue), I am guessing that each side should really have its own resistor? But for practical purposes, is it really necessary?
I realise that I could use phased PWM to rapidly alternate between the two sides, and by ensuring only one or the other is on, get away with a single resistor - but do I need to go to these lengths?
Thanks
Graham
 

Harald Kapp

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Nov 17, 2011
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I am guessing that each side should really have its own resistor?
You're guessing right. Otherwise changing the power suppl yto one diode will affect the current through the other diode as bot share the same resistor. You don't want that.

Also due to differences in the voltage drop between red and green LEDs you may want to use different resistor values for equal brightness of the two colors.
 

Graham

Nov 10, 2009
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Nov 10, 2009
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you may want to use different resistor values for equal brightness of the two colors.

Good point - thanks Harold.
 
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