help with resistor....

umm.....

i was at radioshack.. and i picked up some red leds. here are the stats:

2.6vdc
28mA
10mcd

and when I got to the resistor, i didn't know what to get.. i tried the linear led calculator, and for the 5v power supply, i have to get a 100ohm resistor..and for the 12 volt, i have to get a 390 phm resistor..

now the questions i have are:
1. what is the power rating for the motherboard connections for the power on and the activity led??
2. When I was looking at the resistor, all of them were divided up into catglories like 1/2 watt, 1 watt, and 1/4 watt. Which one am I suppose to get??

edit: some examples:
http://www.radioshack.com/product.a...name=CTLG_011_002_014_002&product_id=271-1131
and
http://www.radioshack.com/product.a...name=CTLG_011_002_014_001&product_id=271-1311

crazykidz said:

now the questions i have are:
1. what is the power rating for the motherboard connections for the power on and the activity led??
2. When I was looking at the resistor, all of them were divided up into catglories like 1/2 watt, 1 watt, and 1/4 watt. Which one am I suppose to get??

Click to expand...

#1: I have no clue
#2: for most LED applications, the 1/4 watt will be fine. I myself get 1/2 watt, because I use them for more than LEDs. 1 is a bit over the top for just that, unless you have some high-current items on the same circuit.

To figure out if you need the 1/4, 1/2, or 1 watt resistor, just take your current used in Amps and multiply it to the voltage you will pass through it. In your case, the red leds allow 28mA = 0.028. Multiply it by 5 or 12 for whichever voltage you are using.

So
0.028 x 5 = .14 Watt (so 1/4 Watt resistor)
0.028 x 12 = .336 Watt (so 1/2 Watt resistor). You can get away with using the 1/4 Watt resistor. I think the LED will be a little brighter because the resistance of the resistor will be lower because it heated up a little.

thx for the help, im gonna go again tomarrow for the resitor and some othr tools...

still anyone know what resistor i have to use on the led for the mobo cconnections?? the voltage on the mobo connections??

crazykidz said:

still anyone know what resistor i have to use on the led for the mobo cconnections?? the voltage on the mobo connections??

Click to expand...

none - that is the point - the mobo has built in resistors that assume your using 15ma green (1.5v) LEDs

theshadow27 said:

none - that is the point - the mobo has built in resistors that assume your using 15ma green (1.5v) LEDs

Click to expand...

QFT. I've never ever seen a casee with resistors in series with the HDD/Power LEDs.

ugh, do some math.
2.6vdc
28mA

R1 = ( 5V - VLED ) / ILED

( 5V - 2.6V ) / .028A = 86 Ohms

That is the absolute minimum you should go in resistance.

Wattage over R1:
Wr1 = ( 5V - VLED ) * ILED

2.4V * .028A = .0672W