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New to LED's: resistance question

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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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