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different voltages for leds?

leds on ebay,at different online shops and,found in old electronics.can any led work with say, 12 volt car power,[dc, with the appropiate resistors]disconnect it and hook it up to ac house[again,with the appropiate resistors]current?should i look for certain voltages sources when buying or just strive for the voltage the led calls for?also,is 12 volt dc car power the same as 12 volt computer power?the fans and such on computers [as far as i can tell]run on 12 volts.might seem like a silly question,but then i'm REALLY not very conversant with all this led tech.in other words,can i run an led off any[ac or dc]current as long as i use the correct resistor?thanks for schooling me on this.i'll be forever gratefull

LED stands for light emitting diode. They are a diode that emits light when a positive DC voltage of about 2 volts (for red diodes, other colors require higher forward voltages between 3 and 4 volts) . Being a diode they also block current when reversed biased, however they can only block when biased with a relatively low voltage (typical spec is 5 volts min.). 

With a DC voltage source you can wire a LED across the + and - terminals, if you include a series resistances that limits the current. The current is typically limited to 20 - 30 mA depending on the size and type of LED. You find the appropriate resistor value by taking the DC voltage of the supply subtracting the 2 to 4 volts of the forward biased diode then divide that number by .02 (i.e., 20 milliamps). So for a car system the voltage is about 13.5V when charging, the resistor would be (13.5-2)/.02 = 570 ohms, typical value used would be 620 ohms. You need to take care not to put the LED in backwards.

To use a AC voltage source like your house mains you have to include a rectifing diode in series with the resistor and the LED. This is the circuit you see in the little LED night lights. The resistor should be based on the max voltage of the rectified AC power. For 120V AC this is about 168 Vpeak. So the resistor would be 168 / .02 or about 8400 ohms. A typical value seen in the little LED night lights is 10000 ohms (10k).

The difference in car 12 volt system and a computer 12 volt system is the max current available (and noise but that is a different matter).

I hope this clears things up.