SMD LEDs & SMD Resistor values

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callen5914
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SMD LEDs & SMD Resistor values

Post by callen5914 » Wed Aug 03, 2016 9:18 pm

Greetings,

I have been playing with circuits now for a bit and usually that means LEDs of the 5mm variant. I have been dying to try out make reflow gear so I have been making some PCB designs for SMD types of parts. Now, I have looked over a couple schematics found on various products that feature SMD LEDs and SMD resistors. This is where I'm troubled greatly. Normally I can hook up a 5mm LED with a 270R/330R and it's all good in the hood.

After looking at your various schematics in 5V circuits I see HIGHer resistor values for SMD components (Like 1KR)!!! What gives right? So yeah I am a rookie and went to some math. But it's not adding up coach!!!

P=V*I

So a 0805 resistor is said to able to handle 1/8 (0.125) watt of power

So if 5*.02 = .1

Couldn't a 330R value still be used? I am using an 0603 Super bright LED in red and an 0805 resistor package... Why use a 1kR?

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leon_heller
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Re: SMD LEDs & SMD Resistor values

Post by leon_heller » Thu Aug 04, 2016 12:00 am

To save power?
Leon Heller
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phalanx
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Re: SMD LEDs & SMD Resistor values

Post by phalanx » Fri Aug 05, 2016 12:12 pm

callen5914 wrote:Couldn't a 330R value still be used? I am using an 0603 Super bright LED in red and an 0805 resistor package... Why use a 1kR?
Besides power savings, the original designer probably didn't need a super bright LED so he lowered the current to make it dimmer.

Since LEDs are current controlled, you don't choose a resistor value by using a power calculation. You first determine the current you need, then figure out what value resistor you need to get that current, and then calculate what power will be dissipated at that current so you can select the appropriate wattage resistor with the value you already calculated.

-Bill

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