LED question from a newbie

[magic77]

Hi hopefully someone can put me out of my mysery

I want to wire up 4 LEDs in series and run them off a 12V line in my PC.

Now accoring to one site its average that a blue LED is 3.4V which from what I worked out would leave the LEDs underpowered which is fine as dont want them to be so bright

is this correct and finally how can you work out the voltage of the LEDs

thankss

[audioguru]

Hi Majic,
Welcome to our forum. ;D
Leds work with current, not necessarily just voltage.
A blue LED could operate at 3V or 4V, they are all a little different. If it is 3V and you apply 3.5V to it then it will burn out if you don't use a resistor in series to limit its current.
If you connect 4 blue LEDs in series and their actual voltage (not their max rated voltage) is 3.4V then with a 12.0V supply they will each have only 3V across them and will be very dim. But if their actual voltage is only 3V then they might burn out without a current limiting resistor.

You can measure the voltage of an LED if you connect a 390 ohm resistor in series with it to limit its current then connect the LED wire and the resistor wire to a 12V supply. If the LED has 3.4V (measure with a voltmeter) then the resistor has the remaining 8.6V and Ohm's Law calculates the current at 22mA. If you use 330 ohms then the current will be 26mA which is close to the usual max of 30mA.
With a 390 ohm current limiting resistor in series with a 2V red LED, with a 12V supply the current is 25.6mA.