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Current-limiting resistor(s) on RGB LEDs  


Cinti
(@cinti)
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Rather than adding a current-limiting resistor on the R, G, and B legs of either a common anode or cathode RGB LED, why not use a single resistor on the common cathode or anode?  A small detail but it saves a couple of components and makes thing a bit less messy.


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DroneBot Workshop
(@dronebot-workshop)
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@cinti While that might SOUND like a good idea, it actually has a number of issues.

  1. Technically you should be using different values for the Red, Green, and Blue LEDs, as they each have different dropping voltages. Although a lot of the time we don't do that, but the color linearity suffers. Most of the time the Red segment uses a resistor half the value of the other two.
  2. A much bigger issue is that the current requirements change depending upon which segments you light up. So lighting up all the segments would take three times the current as lighting only one. You could compromise by using a higher value resistor, but the brightness would be all over the place. Individual resistors solve this issue.

 

Also, resistors are probably the cheapest electronic components on the planet, so other than saving PCB space it really wouldn't be saving much.

If you really want to save resistors use a programmable RGB LED instead, they don't require ANY dropping resistors and are much more linear in color and brightness.

😎

Bill

 

"Never trust a computer you can’t throw out a window." — Steve Wozniak


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Cinti
(@cinti)
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Topic starter  

Good points all!  Most of my projects only light a single R, G, or B element.  Those I2C addressable color LEDs are pretty pricey.


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ISH
 ISH
(@ish)
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I like the idea. very interesting yet simple.

Your points are well noted Bill.

I do believe that Cinti I onto something here. In my case, I was struggling for pcb space. I have multi leds of the same colour that only light up one at a time. This sounds like an ideal situation to group like coloured leds that can share a common resistor(freeing up some pcd space).

 

Please confirm if you agree.

 

 

ISH


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teddy
(@teddy)
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Posted by: @cinti

Rather than adding a current-limiting resistor on the R, G, and B legs of either a common anode or cathode RGB LED, why not use a single resistor on the common cathode or anode?  A small detail but it saves a couple of components and makes thing a bit less messy.

That can be done, but the brightness WON'T be the same.

 


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Maxli
(@maxli)
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Posts: 19
 

The brightness might be different.


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