Easy voltage question.

If i have a 12VDC source, and i have connected a LED that is rated at 5V, and of course I have a resistor connected in serie with the LED. What will the voltage be across the resistor and across the LED?

Cheers!
Lac.

That entirely depends on the size of your resistor. In a circuit like that there are three things you need to know:

a) the supply voltage
b) the current rating for the LED, and therefore the current needed in the circuit
c) the value of your resistor.

Then by using simple ohms law (V=I.R), you can determine everything you need and want to know, including the volt drop across the resistor.

What value resistor are you using?

An LED is a very nonlinear load. It may be 5v at the rated current, but only 5.25v at twice the rated current and 4.75v at half the rated current.

So, you might be able to assume that no matter what the current (within reason), the LED is around 5v. From there, you assume there's 7v across the resistor and you can choose a resistor size that creates the desired current.

If the LED is rated at 5v it will drop 5 volts and the resistor will drop the rest of the supply voltage, so u have to know the current needed to light the LED without damaging it and depending on that current u choose the correct value of the resistor which will drop about 7 volts. :wink: