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Old 16th November 2010, 04:55 PM   #1
toufu is offline toufu  United States
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Default 2 Color LED's?

I order a few 2 color LED's. What if voltage is applied to both colors, which color gets to light up?

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Old 16th November 2010, 04:57 PM   #2
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Old 16th November 2010, 05:23 PM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
there are 3 leads.
The middle one is common to both LEDs.
The outers feed to the alternate colours.
Equal light output from both will give an average colour. Equal light might not be exactly equal current.
Changing the current ratio between the 2LEDs will give a range of colours.
Turning one or other off gives just one colour.
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Old 16th November 2010, 05:42 PM   #4
Pano is offline Pano  United States
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Yep. So for example a red & green LED with both lit will look yellow. (mostly).
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Old 16th November 2010, 05:56 PM   #5
toufu is offline toufu  United States
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Ok, yellow is not too bad...
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Old 16th November 2010, 06:15 PM   #6
tomchr is offline tomchr  United States
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Recall that for red LEDs the forward drop is about 1.8~2.0 V, whereas, for the green it's 2.2~2.4 V. Generally speaking anyway. For specific numbers see the data sheet for the LED of your choice. Also note that LEDs need to be driven by a current, i.e. you need a resistor in series with the LED. If you know the driving voltage and the forward drop you can calculate the voltage across the resistor. Figure each LED needs 5~10 mA and use ohm's law to figure out the resistance.

~Tom
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Old 16th November 2010, 07:56 PM   #7
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Hello, thought I would chime in here, remember that not all two color LED's are tri-lead. I recall Radio Shack(Tandy) used to carry two lead ones. Depending on which way the power supply was hooked up, they would emit one or the other color. These were good for indicating incorrect polarity.

Peace,

Dave
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Old 16th November 2010, 08:36 PM   #8
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Quote:
Originally Posted by dave_gerecke View Post
Hello, thought I would chime in here, remember that not all two color LED's are tri-lead. I recall Radio Shack(Tandy) used to carry two lead ones. Depending on which way the power supply was hooked up, they would emit one or the other color. These were good for indicating incorrect polarity.

Peace,

Dave
A nice demo with those 2-lead LEDs is to show how they work with DC and then apply AC (correct voltage/dropping resistor) via a long-ish wire. Whirl in a darkened room and you see arcs with the two contrasting colors.
Something from my schoolhouse days......

John
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Old 17th November 2010, 10:58 AM   #9
DF96 is offline DF96  England
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I use 2-lead bi-colour LEDs to indicate output stage bias via a bridge arrangement. Red means too much current, green not enough, LED off means just right. Simpler than fiddling about with a meter.
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