# Strip LED question

Discussion in 'General Electronics Chat' started by tracecom, Mar 18, 2013.

1. ### tracecom Thread Starter AAC Fanatic!

Apr 16, 2010
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This is the third time I have written this post and had it fail to post, so I suppose it's that "title too long" gremlin.

So, before I wrote the whole thing again and lost it, I posted the first sentence and then edited the post.

Suppose I have a prefabricated strip of LEDs wired as shown in Figure A, and powered by 12 VDC. Each string of 3 LEDs draws 20 mA, for a total of 80 mA per strip of 12 LEDs. Now, suppose that the same strip must be powered by 15 VDC as shown in Figure B, but the current should not increase. I believe I could add one resistor (R9) and restrict the total current to 80 mA, but would that cause problems with the individual strings of 3?

What is the best way to hold the current at 20 mA per string of 3 LEDs given that I cannot change the value of the resistor in each string?

Thanks.

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Last edited: Mar 18, 2013
2. ### spinnaker AAC Fanatic!

Oct 29, 2009
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Are just complaining or do you have a question?

3. ### tracecom Thread Starter AAC Fanatic!

Apr 16, 2010
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Both, but I do wonder why "they" don't fix that title length problem.

4. ### timescope Member

Dec 14, 2011
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For 15v, add a resistor in series: R= (15v - 12v)/ 80mA = 37.5 ohms instead of the 150 ohms in figure B.

Timescope

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5. ### JohnInTX Moderator

Jun 26, 2012
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With a power rating of 1/2watt.

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6. ### tracecom Thread Starter AAC Fanatic!

Apr 16, 2010
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And that doesn't violate the rule against using a single current limiting resistor for LEDs in parallel? (As you can tell, I'm a tinkerer, not an engineer.)

7. ### ErnieM AAC Fanatic!

Apr 24, 2011
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No because each leg has a resistor so the rule does not apply.

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8. ### tracecom Thread Starter AAC Fanatic!

Apr 16, 2010
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Thanks to you all for tolerating (and answering) a naive question.

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9. ### Metalmann Active Member

Dec 8, 2012
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Not naive, Bro; I was wondering the same thing!