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Topic: Wiring LEDs in series (Read 6890 times)previous topic - next topic

polarbeargarden

Mar 03, 2011, 07:43 pm
Hi, I am very new to the Arduino software and to electronics in general.  I was wondering how many LEDs I can wire in series to one pin on my Uno board. I am also wondering how to connect the lights and what sorts of resistors I would need.

I'm a first year college sculpture student, and this is for a project I'm working on.

Thanks!

johnwasser

#1
Mar 03, 2011, 08:23 pm
I think the limitation will be voltage drop.  Each LED (like any diode) has a voltage drop across it that depends on the semiconductor used (different for different colors).  If the voltage drop is, for example, 1.5V then you could string three in series (4.5V) but not four (6V).  The series resistor should 'soak up' the left-over voltage.  In the case of three 1.5V-drop diodes that draw 20mA you'd need a resistor that will pass 20mA at 0.5V (5V - 4.5V).  Ohm's law gives the relation between Ohms, Volts and Amps so the calculation is easy.

If you want to get around the voltage drop limitation by putting strings of LED's in parallel you will then have to worry about the current limit.  Each pin of the ATMega processor has a maximum current it can source (when HIGH) or sink (when LOW).  The ATmega168 is rated for an absolute maximum of 40.0 mA per pin.  If your LED's draw, say, 18mA you could put two strings in parallel (36 mA << 40 mA) but not three (54 mA >> 40 mA).
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MarkT

#2
Mar 03, 2011, 08:31 pm
For completeness the typical forward voltages for LEDs are about 1.9V for red, 3.5 to 4.0 or so for green blue and white.  Orange and yellow are intermediate.  Thus you can drive 2 red LEDs but that's about it from 5V.  It gets more interesting at 12V of course.
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