# Finding the proper Resistor Wattage

Discussion in 'General Electronics Chat' started by vladthegreat1, Jan 6, 2015.

Dec 26, 2014
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Hey Folks So currently have a LED strip that consists of 2 LEDs with 2 resistors in this exact layout

I have the choice to use 2 types of resistors (this is an SMD circuit if that matters)

The circuit originally was 2 LEDs that had 2 SMD Resistors marked 151 * upon looking that up i found they were 150ohm resistors 5% . Now i have 2 types of resistors i can put on this board they are:

150R 150ohm 151 +/-5% 1/10W RoHs

150R 150ohm 151 +/-5% 1/8W RoHs

My question is what wattage do i really need for my resistors?

The specs for the LED's are Forward voltage of between 2.1-3.4 with forward current at 30.

I do not know why this layout is this way, it was made by an actual company i am just modifying it and trying to make sure I get the correct resistor in there. Please let me know.

2. ### #12 Expert

Nov 30, 2010
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What is the battery voltage and what is the actual LED voltage?
I come up with 14.6V for the battery, and the resistors are .135 watts each, but that's just an approximation. (educated guess)

Dec 26, 2014
26
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Sorry I forgot source voltage is 12.16 on the voltmeter but let's just say 12

4. ### #12 Expert

Nov 30, 2010
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and what is the actual LED voltage?
(There is no such thing as a 2.1 volt to 3.4 volt LED)

This is looking like 21.87 ma with a resistor power of 0.0717 watts each, but I'm just sneaking up on it.

And while I'm sneaking up on the exact value, it is customary to install resistors with wattage ratings well in excess of the minimum. It makes them last longer.

Last edited: Jan 6, 2015

Dec 26, 2014
26
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I guess I was asking a general because there is 5 different types of LED I can use for project lowest forward voltage 2.1 (green) highest is white @ 3.4
But either way from what your saying I should be OK with both 1/8 (0.125w) and 1/10 (0.10w) right?

6. ### #12 Expert

Nov 30, 2010
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Worst case is .1056 watts. Use the 1/8th watt resistors.

Dec 26, 2014
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Awesome thank you so much #12 you gave been a great help

8. ### #12 Expert

Nov 30, 2010
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That's what the, "like" button is for.

9. ### cmartinez AAC Fanatic!

Jan 17, 2007
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Or the tip jar at most bars...

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