Resistors for LED

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rsgunter

[H]ard|Gawd
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To make a long story short, I need to replace a resistor for the LED on the switch.
Here's what I have: http://www.performance-pcs.com/catalog/index.php?main_page=product_info&cPath=118&products_id=4056

From my worklog: http://www.hardforum.com/showthread.php?t=1187751&page=4
"I ordered 2 pre-wired Bulgin Switches and when I recieved them I noticed that the LED resistor was very fragile, and I'll be damned if I sat the switch on a table and the resistor broke right at the tip, so then i was going to install the other one, and it was broken too, the wiring was never actually touched by my hand, just barely touched because it had to lay on a table. The other one I'm not sure about it's breakage, but it wasn't dropped or even remotely handled improperly."

From performance-pcs the switches that aren't wired require a 67ohm resistor, and their ratings are this:LEDs (ratings @ 20mA) Orange: 1500 mcd 3.0 V

The Bulgin like I have is as follows:
LEDs (ratings @ 20mA) Blue: 1500 mcd 3.4 V

Here's a picture if it helps:
IMG_1920.jpg



What ohm resistor do I need?


***I don't know shit about electrical work so please don't make fun. :p
***If mobo headers are 3.3 volts and the LED is 3.4v why would it need resistance?
***I can do my own soldering, I'm not that n00b, lol.
 
I can't say what resistor without knowing the voltage source. But you can calculate it:
Resistor = (sourcevoltage - 3.4)/.020.
The result is in ohms. Choose the closest resistor value that is equal or higher.

For example, if your source is 5V:
(5-3.4)/.020 = 80. So 82 ohms is close.

If your source is 12V:
(12-3.4)/.020 = 430. So use 430 or 470 ohms.
 
The voltage is coming from the motherboard header, which is apparently 3.3v.
 
I didn't even see the "?" thing lol.

1/8W 68Ohm resistor! Thanks so much!!!

Edit: Radio Shack only has 1/2 and 1/4W. I'm a sad panda now. Can't get anything until Tuesday. :(
 
I needed to do the same thing some time ago. Comes in handy if you don't want to burn out an LED.;)
 
rsgunter said:
I didn't even see the "?" thing lol.

1/8W 68Ohm resistor! Thanks so much!!!

Edit: Radio Shack only has 1/2 and 1/4W. I'm a sad panda now. Can't get anything until Tuesday. :(
Click to expand...

Either of those will be fine. That's just the power that the resistor can dissipate before burning up, essentially, so buying a higher power resistor just increases the safe margin before it burns out.

"If mobo headers are 3.3 volts and the LED is 3.4v why would it need resistance?"

LEDs need a resistor connected in series to limit the current flowing through the LED. Remember that LED means Light Emitting Diode, so without a resistor you would have a voltage connected essentially directly to ground once the diode becomes forward biased and begins to conduct. This means that the voltage source will attempt to drive lots of current down the wire.

V=IR ---> I=V/R

With no resistor, the resistance is effectively very small and near zero. This results in very high current, something that can burn up wires or short out the power supply.
 
Indeed! Thanks so much for the link!

The guy at radio shack just told me I can "play with the ohms a bit". Does that mean I can go a little higher or lower or however that works? :p

The wizard said this:
"1/8W or greater 68 ohm resistor."
 
The bigger the resistor, the dimmer the LED. You should keep it at least 68 ohms to maintain good life on the LED.
 
JC634 said:
The bigger the resistor, the dimmer the LED. You should keep it at least 68 ohms to maintain good life on the LED.
Click to expand...

Seems like sound advice. Guess I'll have to find one of the damn things now. :)

Thanks again for your quick responses!
 
rsgunter said:
The voltage is coming from the motherboard header, which is apparently 3.3v.
Click to expand...
It's far, far more likely to be 5V, and with a blue (or white) LED any resistor between 82 ohms and 470 ohms will do. The difference in brightness is negligible - it's an indicator, not a searchlight. 180 ohms looks nice.
 
Well straight from the performance-pcs website it states the motheboard header is 3.3v.

What's type should I get? I see like carbon filmed and stuff. Just wondering. :)
 
do you mean 1/4W???

We have already said that 1/4 or 1/2 watt is fine. Just use those. You could use a 4 watt resistor if you want... it would be expensive and wasteful, but you should get the idea.
 
rsgunter said:
My radio shack only has 1/4" :(
Click to expand...

Lol, I'm such a dumbass, I've been dealing with a lot of measurements lately, but I did mean 1/4W.

@ Top Nurse
That's what I'm looking for, lol. 1/8. :p

Well dammit I'm getting all blonde joke confused now. :p I asked radio shack specifically if I could use their 1/4" 68ohm and they said, "no, it will burn up your LED." It didn't make much sense to me. *sigh*

So basically it's whatever I want over 1/8W 68ohm or equal to it?
 
The watt rating is how much power can be pushed thru the resistor w/o dying. If Radio Shack has 1/4W resistors, this means that they can handle twice the current for a set voltage.

In the end, just get the 68 or higher ohm resistor. 1/8W will almost certainly be fine, 1/4 will be just that much better.

BTW, the guy at Radio Shack hasn't a clue what he is talking about.
 
Are you just replacing the case power LED with the LED in the Bulgin switch, using the motherboard power LED header? That doesn't need any extra resistor, there's already one fitted to every motherboard to suit the usual amber or green LED on most cases. Amber/green are 2V LEDs, so a 3V-3.4V blue will get slightly less current but still be plenty bright. Same goes for replacing the case drive activity LED, they're straight swaps.

http://led.linear1.org/how-do-i-replace-an-led-with-a-brighter-led/
 
rsgunter said:
Do any of them? I didn't want to argue with him on the phone, there's no purpose. That's why I'm checking with you guys. :)
Click to expand...

My older sister and brother-in-law use to own a Radio Shack store. Here's their qualifications:
Him = former department store credit manager
Her = RN (registered nurse)
 
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