[Madcat05]

What would be a good resistor value for this LED from Mouser

Thank You

 

[Madcat05]

anybody?

 

[Megaptera]

Without any information about what you're powering it with, it's impossible to say. But it's pretty easy to figure out.

Look at its specs, and there'll be a rating for milliamps (mA). Probably around 20 or so, though I didn't actually check. Your resistor will be bringing the current down below that value. Voltage (V) = current (I) times resistance (R), you take your amp's V and just do the math (R=V/I) to get the numbers right.

Let's assume you're just trying to power it from a 9V battery, and it's a 20mA LED. I=V/R becomes R = 9V/0.002A = 4500 Ohms, or a 4.5K resistor. Plug in your actual values for V and I, and you get R.

Make sense? It's really just straightforward algebra.

Edit: Also, have patience -- Your bump is less than two hours after your initial post, which is really jumping the gun.

 

[Madcat05]

Yes it makes sense, I've done it before.

I'll be getting an LED from radioshack not Mouser. The mA is rated at 10mA. I'll be using two 9V battery's, so its 18V.

So it's R = 18V/0.001A = 18000 Ohms, right? So it would be a 1.8 ohm resistor.
Correct me if I'm wrong.

 

[Megaptera]

18K, not 1.8, but otherwise you got it. The value doesn't have to be overly precise (too high a resistor and you'll just have a dim LED) so if you have to round up to a 20K resistor, you won't blow anything up. I wouldn't round down too far, though...

 

[Madcat05]

Ok I see what I did. Thanks for your help!

 

[Roan]

If a person was interested in maximizing the brightness of the led, would it be necessary to consider the led's resistance when calculating the resistor value? If so, how much resistance is there across an led?

 

[IdiotTech]

The way i do it is that you need to know what the voltage of the led is.

Say you know that the voltage drop accross the led is 1.8V and 20mA max.
Next say your source is 18V.

So then 18V-1.8V=16.2V

So then you need a resistor that will set the current to 20mV at 16.2V.
16.2V/20mA= 810 ohms.

So then you can just change the values for the voltages and curents.

 

[Earwax]

Quote:

Originally Posted by Megaptera Let's assume you're just trying to power it from a 9V battery, and it's a 20mA LED. I=V/R becomes R = 9V/0.002A = 4500 Ohms, or a 4.5K resistor.

Isn't 20 mA = 0.020 Amp?

 

[Megaptera]

Quote:

Isn't 20 mA = 0.020 Amp?

Whoops, you're right, my mistake. Let's try not to get bogged down by this whole precision nonsense, as long as we're in an order of magnitude or two, right?

So Madcat, divide that resistor value by 10. Don't know where my brain was last night.