LED resistor questions.

[ajnin]

Legend
VS = supply voltage
VD = voltage drop
VL = LED voltage
I = LED current
P = watt

Formulas Used
R = (VS - VL) / I*
VD = VS - VL
P = VD * I (is this right?)

Values
VS = 4.6
VL = 3.6
I = 500mA / .5 A
VD = 1
R = 2
P = 1 (is this right?)

http://led.linear1.org/led.wiz tells me that the above is correct. Now I am trying to figure our how to get the dissipation from both the led and the resistor.

As far as I know, R.dissipation = I * VS. But that give me a value of 2.3, while the above calculator suggests 550.

So my question is, how do I figure the dissipation of both resistor and then the LED?

[Jedi-Loreen]

Why do you think you need to know that? You don't. I've never even heard anyone bring up the word "dissipation" in this context.

You're making this harder than it is.

[ajnin]

I am making a calculator for my droid. I figured that I would be complete as possible and try to learn a bit in the process.

[Sunrider]

Are you sure your values are correct? 3.6v sounds high for running a led at 500ma.

[ajnin]

Another question, are my formulas correct for P or is the calculator linked to correct? It suggests a 1 watt resistor my formula suggest a 1/2 w. I am a bit confused.

And I think this is right for the dissipation values.

VL.dissipation= VL*I = 1.8 watts
R.dissipation=VD*I = .5 watts

Thanks in advance for any help...

[ajnin]

Are you sure your values are correct? 3.6v sounds high for running a led at 500ma.

Your right, most the leds would want 350ma. It isn't about the values given just the process to get to the end. TRying to make sure my formulas are all correct.

[ajnin]

So how about those formulas, do they all look right?

[Rhyen Skytracker]

You are correct with the resistance formula, it is:

(Vs-Vf)/LED current = resistance
4.6V - 3.6V = 1 / .5 = 2
So you would need a 2 ohm resistor.

[ajnin]

Thanks Rhyen.

So something else... http://ledz.com/?p=zz.led.resistor.calculator this calculator say that watts should be .5 as does ohms law, but the first calculator give a value of 1 watt... am I missing something?

Also, does any one have a list of common resistors and their color codes?

[Rhyen Skytracker]

Determine the amount of current (Amps) and voltage (Volts) you wish to calculate power by. Multiply the number of amps and volts together. The product will give you the amount of power. For example: 500(mA) x 3.6(V)= (1.8 w)

they don't make a 1.8 W resistor and the most common ones that close would be a 1 or 3 watt. You will be fine with a 1 watt.

Edit: you can always use the resistor chart here to see the most common ones for LEDs. http://www.thecustomsabershop.com/wi...er/wiring.aspx