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Calculate resistor wattage

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Phil
Owner, Oznium.com

Joined: Feb 11 2003
Posts: 7719


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Post Thu Feb 23, 2012 1:49 pm

Does anyone know how to calculate the appropriate sized resistor wattage for:

Source: 12v to 14.5v

LED: 3.5v 20mA

The resistor would be 560 ohm, but would the power rating be 1/4 or 1/2 watt?

Trying a handful of online calculators, I get different results.
jdusty914

Joined: May 09 2004
Posts: 1623
Location: Tennessee


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Post Thu Feb 23, 2012 2:19 pm

I found one that says 1/4 W. I tried another one and it is giving me a value that is really close to the 1/4W value. Looks like it would be 1/4W. P=(I*I)*R is the formula. By using this it comes out to be a value of 224m Watts. Hope this helps some.
Phil
Owner, Oznium.com

Joined: Feb 11 2003
Posts: 7719


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Post Thu Feb 23, 2012 2:25 pm

Thanks jdusty914

Any other engineering folk can weigh in?
Chris

Joined: Sep 27 2004
Posts: 2395
Location: NY


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Post Thu Feb 23, 2012 4:03 pm

wheres vettecrazyyyyyyy
corvettecrazy

Joined: Dec 17 2003
Posts: 4357
Location: moved (twice)


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Post Thu Feb 23, 2012 4:10 pm

1/2 watt

Resistor
P=iV
P=(.020A)(14.5-3.5v)
P=0.22 W

LED
P=iV
P=(0.020A)(3.5V)
P=0.07W

Total System Power = 0.29W

Technically you could go with a 1/4W resistor but you would be using ~90% of the power capacity of the resistor. Meaning it will be screaming hot at 14.5v, especially since you are probably going to be heat shrinking the resistor to make it water tight.

Using a 1/2W resistor will be MUCH more reliable and then you should be fine with using a 5% tolerance resistor verses a 1%. Also, using a 1/2W resistor will allow a buffer for cars who have voltage spikes over 14.5V.
A-Ray

Joined: May 01 2005
Posts: 4062
Location: Volunteer State


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Post Thu Feb 23, 2012 6:35 pm

I concur with these guys. Although I only have a small amount of wiring knowledge, and am currently enrolled in a basic electronic class, everything they say mirrors exactly what I've seen/heard/know.
Phil
Owner, Oznium.com

Joined: Feb 11 2003
Posts: 7719


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Post Thu Feb 23, 2012 8:26 pm

Thanks for the feedback guys.

Reason I ask, is got a customer who's been buying our 1/4 watt resistors, and using with some LEDs he's getting elsewhere. Been installing into car dashboards. After a while, it stops working. The LEDs still work, but the resistors are fried.

But... We've sold literally 100,000's of "loose" LEDs, pre-wired LEDs, and resistors, all using 1/4 watt. And never had any serious problem. Been using 1/4 watt only since like 2002.
jmoddes

Joined: Apr 25 2011
Posts: 2142
Location: Kenosha, Wi


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Post Thu Feb 23, 2012 9:55 pm

Phil wrote:
Been using 1/4 watt only since like 2002.



lol
bad venge

Joined: Jul 28 2007
Posts: 3550


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Post Fri Feb 24, 2012 4:39 am

Maybe the other LED's are pulling more current then spec'd blowing the resistors
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