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New to LED's: resistance question
jholman76:
OK, so I am new to LED's and resistors.
I want to connect a LED to my PC (preferably through a molex connection) to light my trackball. I have bought a couple from radio shack that are 7000mcd. 25mA 3.3- 3.6V.
I basically want to take 12V or 5V and make it into 3.3-3.6V. Ive tried to figure out various sites that calculate resistance, but nothing is clicking... any help would be appreciated.
-Also I have a couple of LED's I pulled from my old Antec True Blue PSU that died and they have resistors already. I'm just not sure what kind of LED's they are. can I hook them up to a 5V or a 12V and see if they work? what happens if its too much for them?
Thanks.
Joe
weisshaupt:
Is the 3.3v the Reverse Voltage? A LED is basically a Diode- Current will only flow in one direction, so if you hook it up the wrong way (reverse voltage) it gets destroyed.
Normally forward voltage is closer to around .7 - 1.2 Volts or so. 5V-Forward Voltage of the diode = the voltage across the resistor. You want 25 mA current so solve the equation V=IR where V is the "left over voltage" after you subtract the Diode Forward Voltage from the Power Supply voltage, and I = 25 mA. Normally R should be in the range of 100-300 Ohms.
Normally as long as you don't exceed the reverse voltage by hooking them up the wrong way, a LED + Resistor will be fine, as the Resisotr will take on the "left over" voltage ( and might get very hot) The 5 V power supply is the one I would dink with ..
Ed_McCarron:
--- Quote from: weisshaupt on July 07, 2008, 08:53:03 pm ---A LED is basically a Diode- Current will only flow in one direction, so if you hook it up the wrong way (reverse voltage) it gets destroyed.
--- End quote ---
First part, yes. Second part, no. Hook up a diode backwards and current just won't flow - no destruction.
Try this: http://home.cogeco.ca/~rpaisley4/LEDcalc.html
Fill in values. Plugging in your numbers (12v, 3.3v drop, 25ma) gives 349.99 ohms, .22 watts. A 360 ohm 1/4 watt resistor should do the trick.
As for the pre-resistored LEDs, try 'em on 5V first. That way, on the off chance they are geared for 5v, they won't blow. If they're out of a PS, I'd wager they are set for 12v. Can you read the color bands on the resistors?
jholman76:
Yes, I can...
LED<>--+pos------<resistor- gold, brown, red, yellow>----
from the LED, the positive (red) wire goes to the resistor and those are the bands. the red looks like a maroon or burgandy... dark red...
thanks
joe
Namco:
I'm going to bookmark this thread. Awesome answer from Ed to jholman's question, a question that I myself have been meaning to research. So you saved me some time. :cheers: Thanks!
--- Quote from: jholman76 on July 07, 2008, 10:26:24 pm ---Yes, I can...
LED<>--+pos------<resistor- gold, brown, red, yellow>----
from the LED, the positive (red) wire goes to the resistor and those are the bands. the red looks like a maroon or burgandy... dark red...
thanks
joe
--- End quote ---
Using the calculator found here (and reversing the order of the bands): http://www.sizes.com/materls/resistorcolor.htm
I come up with 420 Ohms, too high for 5v, more like 12v... actually 14v, I wonder if it'll work for the 12?
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