Smd Resistor
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What size (ohm) resistor do I need for this project?
Ok I have 7 0603 SMD LEDs and another set with 7 0603 LEDs, both on separate circuits. So 7 0603 SMD LEDs being lit from a 5 volt source how many ohms do I need to resist if each LED is 1/8 watts each? I used this cac: http://www.the12volt.com/ohm/page2.asp and got 28.57143 ohms how many is that? do I round it off?
So I need to know what size resistor to use for seven 0603 SMD1/8 watts (each) LEDs coming from a 5 volt source?
I would be wiring them in parallel, this is for sound reactive LEDs on a PSP so I do not need max light out of them.
Also each circuit would all have the same color LEDs, which would be yellow/amber for one and green for the other.
Subtract the LED forward voltage (call it 3V, can be higher or lower depending on colour) from the supply voltage to find voltage that will be dropped across the resistor.
Divide the voltage drop by the current required - for an 0603 I looked up, the LED would be at 3V / 0.03A, so the resistor drops 2V, making it 66 ohms - for the maximum permissible current.
You would round up or even step up the resistance, to reduce current slightly - in fact, depending on the application, maximum current could be a waste of power - if you are trying to illuminate with LEDs, then you want to get as much out of them as you can, but for indication, you would generally run a 30mA maximum LED at 10mA or at most 20mA.
If you attempt to parallel them, then you would add the LED currents, but even with batch matched LEDs of the same colour, perfect balance is unlikely, resulting in some carrying too much current and others too little.
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July 6th, 2007 in
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