LED resistor questions.

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?

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.

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.

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

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...

Quote:

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Originally Posted by Sunrider

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

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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.

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

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.

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?

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

Or just Google "resistor color code". :rolleyes:

My google fu is awesome, yet my small-electronics fu lacks...

When asking about the common resistors I am simply trying to figure out what to suggest to the end user of my little android app. Since I don't really know and you guys, my peers. do. So I asked. Something about no stupid questions only stupid answers.

Dissipation is a needed figure is it not? It is the heat which a resistor generates. Knowing this could prove helpful.

And your right it is easy.... stick a flashlight in a piece of pvc and some poylcarb in the other end, but learning something in the process is far more valuable than making something easy.


Your a pretty helpful guy Jedi-Loreen.....:rolleyes:

Quote:

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Originally Posted by ajnin

Your a pretty helpful guy Jedi-Loreen.....:rolleyes:

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Well, considering that I'm not a guy......

I take some offense at that statement, since I'm female. :|

I missed the part where you said "common" resistors.


You don't really need to know what the resistor heat dissipation figure is in this application.

As long as you use the correct wattage resistor you don't have to worry about frying the resistor. But, if you are using a lot of circuits and several of the components are generating a good bit of heat then you would need to consider adding fans or better heat sinks to get rid of the heat. For most of what we do with sabers we don't have to worry about that.