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-   -   SMD LEDs and Resistor Question (http://www.diychatroom.com/f18/smd-leds-resistor-question-163146/)

 mattypark 11-12-2012 09:06 PM

SMD LEDs and Resistor Question

Hello I just have a question about resistors. I am working on a project using 0805 SMD LEDs.

My question is - can I get away with using just one 500 ohm resistor and wire each leg to it, or does each leg need to be on its own resistor?

Also, does it matter which side the resistor is on?

Project Details:

Source Voltage = 12v
Diode Forward Voltage = 3.2v
Diode Forward Current = 20mA
12 LEDs wired in series as follows:

http://s18.postimage.org/6osmib7dl/Snip20121112_2.png

My question is - can I get away with using just one 500 ohm resistor and wiring each leg to it, or does each leg need to be on its own resistor?

Also, does it matter which side the resistor is on?

Thank you!

 ddawg16 11-12-2012 09:26 PM

By my calculation.....30 ohms

But your making it more complicated than it needs to be....put a 4th LED inseries....

 mattypark 11-12-2012 09:35 PM

Thank you - they're rated at 3.2v each, so I was unsure if I could do 4 in a series.

If I use the 3v variety and 12v source, then wire 4 in series, I wonder if I would need a resistor. The online calculator tool I was using recommends a 1 ohm resistor even in that case?

 AllanJ 11-12-2012 09:43 PM

With the four banks of 3 LEDs each all tied to one shared 30 ohm resistor instead of each bank having its own 120 ohm resistor, you can run into instability problems if one of the LEDs should burn out. Then only 3 banks of LEDs would remain in the circuit and each bank would get about 27 ma instead of 20 ma of current flowing through it.

It does not matter whether the resistor is before, after, or between the LEDs in its respective bank.

 joed 11-12-2012 10:42 PM

Your 12v source is only nominal. If you are using a car battery the voltage is actually closer to 13.5 volts.

 mpoulton 11-13-2012 10:56 AM

You need a significant difference between the supply voltage and the sum of the series LED voltages in order to maintain current stability. The LED forward voltage is a rough number that varied a lot depending on conditions. You need enough voltage drop across the resistor to ensure that the current remains acceptable regardless of the changes in LED voltage. 25% or so is usually a good minimum value. Each series string of LEDs needs its own resistor since the voltage across each string will vary, so they will not share current well.

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