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Topic: What will happen if I drive LEDs in parallel from a TLC5947? (Read 1 time) previous topic - next topic

scswift

http://www.ti.com/general/docs/lit/getliterature.tsp?genericPartNumber=tlc5947&fileType=pdf

The TLC5947 is a constant current driver.  I'm just wondering, if I decided to run two LEDs in parallel, and I tell it to output 20mA, I assume it would divide that 20mA between the LEDs...  But assuming the voltage drop of the two LEDs isn't identical, what would happen?  Would one LED be dimmer than the other as I suspect, with the other drawing a larger share of the current?

And come to think of it... If I were to try to measure the voltage drop across the LEDs, what would I read, with the anodes and cathodes connected like that?  Would it be the average of the two voltage drops?  Or the smaller of the two?

fungus


Would one LED be dimmer than the other as I suspect, with the other drawing a larger share of the current?


Yes.


And come to think of it... If I were to try to measure the voltage drop across the LEDs, what would I read, with the anodes and cathodes connected like that?  Would it be the average of the two voltage drops?  Or the smaller of the two?


The voltage drop across both LEDs will be the same, only the amps will be different.
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Boffin1

If you measured the voltage accross two LEDs in parallel, your meter probes would be connected to the same points, so one voltage.

You are right that the LED with the lowest forward voltage (Vf) would take the most current, and be brighter.

You could use resistors in series with each LED, of a lower value than if you were not using  a constant current driver, but not having resistors is one of the advantages of that type of driver....
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dc42

If the two LEDs are exactly the same type, then you may find that they share the current quite well. I once took apart a toy light sabre, and it had 16 green LEDs connected in parallel, with no resistors to balance the current. Amazingly, the brightness was fairly uniform across all the LEDs.
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fungus


If the two LEDs are exactly the same type, then you may find that they share the current quite well. I once took apart a toy light sabre, and it had 16 green LEDs connected in parallel, with no resistors to balance the current. Amazingly, the brightness was fairly uniform across all the LEDs.


They might have been sorted/binned at the factory.

I'm sure I've seen LED datasheets that mention you can order them in matched batches.

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