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Topic: Powering LEDs using transistors controlled by arduino mega (Read 1 time) previous topic - next topic

Getronicle

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Assuming an hfe of 100, you need at least 1/100th of the LED current into the transistor base.   A reasonable rule-of-thumb is 1/50th to 1/20th (2%-5%) to make sure the transistor turns all the way on ;(saturates)


This is mainly what I was getting at. If I wanted a 625 mA current, then providing 1/20th of that value to the base I would need a total arduino output of 31.25  mA. I would guess that this is well within the limits of the arduino?

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Do you know why it's bad practice?   ...The current (around 25mA in your case) gets divided among the parallel LEDs, but it does not necessarily get divided evenly, depending on variations in LED characteristics.  That means some LEDs may be brighter than others.


I understand this principle, however I'm not too worried because even if one LED pulls a lot of the current it needs to be at least 20 mA to fail. Seeing as the LEDs have the same specifications I don't imagine a single LED pulling more >20 mA, leaving <5 mA for the other 3. Maybe I'm wrong on this though :P

CrossRoads - Thanks for that information. However I think I would prefer to stick with the original parallel configuration because, for me, preserving as much brightness as possible is more important than total power consumption for the moment - based on the issues mentioned by DVDdoug. I'll probably apply your advice to another project I'd like to do.

Thanks for the help.

CrossRoads

Two LEDs in parallel each drawing 6.25mA will be the same brightness as 2 LEDs in series each sharing the same 6.25mA.
Drawing twice the current will certainly age the battery faster.
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

Getronicle


Two LEDs in parallel each drawing 6.25mA will be the same brightness as 2 LEDs in series each sharing the same 6.25mA.
Drawing twice the current will certainly age the battery faster.



I understand this and would implement it but I'm actually using something I built before which was powered directly by a battery and I'd rather not build something completely from scratch. I'm pretty much intervening where I can in the circuit. When I want to build another I'll probably do as you advised.

CrossRoads

Makes sense if you have something existing already.
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

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