[Steven Everett]

I am trying to add an LED as an ON indicator to a toggle switch. Mouser offers a plug for an LED with a built-in resistor. My problem is I don't know how to figure out the resistor size I need. I will be switching 15volts on and off and the LED only needs 2.2 volts at 20mA. Can someone help me out with this?
Thanks,
steve

 

[JK1dotNET]

Here's how to figure this:
The voltage across the resistor will be:
15 (Supply)-2.2(Drop across the LED)=12.8V
You need 20mA to go through the resistor (since it's in series with the LED) so using part of Ohm's law, V=IR,
12.8V=0.020A(x Ohms)
and solve for x.
640 Ohms.

 

[JK1dotNET]

Oh, and be sure the resistor can handle a quarter watt since
IV=P
0.020A*12.8V=0.256Watts.

 

[tangent]

[size=xx-small]Duplicate post. Nixed.[/size]

 

[tangent]

I only have one thing to add to JK's discourse, and that's that the 20 mA number you found is almost certainly an "absolute maximum" rating, and not a spec on how much power it needs. If you give it less power, it will glow less brightly and last longer. Many LEDs will reach near-maximum brightness with only a few milliamps, even if they'll actually tolerate much more.

If you want to try before you buy, go to Radio Shack, get the 500-piece carbon resistor assortment -- this will set you back less than $8, and will last you for a looong time, since for audio you'll mostly use carbon resistors only for playing. Anyway, get a pair of 9V batteries, wire them in series, and put the LED across the terminals with a resistor in series with one leg. Experiment with different values over that 640 ohm number JK gave until you get a good balance between brightness and power drain. Personally, I'd start with 1K, and go up to maybe 15K.

 

[Steven Everett]

Thanks a lot guys for the replies. I will use the LED plug without the built-in resistor and experiment with resistors until I find the right ones. I ask the fellows at work the same question, they are railroaders, and they said keep putting in bigger and bigger resistors until the LED's quit blowing up. Now you see why I come here for answers.
steve

 

[tangent]

Quote:

they said keep putting in bigger and bigger resistors until the LED's quit blowing up.

LOL!

Like my answer is any different -- put in smaller and smaller resistors until you get the right balance between brightness and battery drain. On a railroad engine, you have insane amounts of electrical power available -- I know, my dad's an electrician for a mine where they have an electric rail system to move the coal around. But in a portable amp, you're trying to conserve all the power you can without compromising the sound.