to light an LED with a 12V, what resistor value we need?

[Azhar]

What is the ideal value for the current limiting resistor connected in series with an LED in a circuit runs on 12v? 
And what value of a resistor we need to light the LED with the following voltages? 220V, 5V and 24V?
Thanks in advance!

[h1386343]

Red                   30mA   1.7V
Bright red     30mA   2.0V
Yellow            30mA        2.1V
Green            25mA    2.2V
Blue                   30mA    4.5V
Red                   30mA   1.85V
Red                   30mA   1.7V

 R = (VS - VL) / I

VS = supply voltage
VL = LED voltage
I = LED current

[mariush]

LEDs have two properties that you care about:

1. forward voltage drop - how much voltage has to go across the led to light up
2. maximum current - how much current the LED can handle.

The forward voltage drop varies depending on the chemicals/materials used inside the LED but typically, they're the values h1386343 listed in the post above (Except maybe blue which seems a bit too much, should be about 3-3.3v imho).  If the LED doesn't get this much voltage, it may not light up.
The maximum current is exactly that, the maximum the LED can tolerate. If you give it more current than the value the datasheet says the LED can handle, the LED may destroy itself or it may become so bright the color changes (for example red becomes orange).
Even though a datasheet may say the LED supports up to - for example - 20 mA, it doesn't mean you should configure the circuit to give it 20mA. 20 mA for that LED may be super bright. You might find 10mA is easier on the eyes and bright enough for your needs.

You have the simple formula:

Voltage =  Current x Resistance 

Voltage is the voltage you supply to the circuit, from which you take out the forward voltage of the LED.
Current is how much current you want to allow through the LED  in Amperes :  1A = 1000 mA  so  10 mA = 0.01A , 100mA = 0.1A etc

So if your input voltage is 5v and you want to use a LED with a forward voltage of 2.2v at a maximum of 10 mA, then your formula becomes :

5v - 2.2v  = 0.01 x R

so R becomes  (5-2.2) / 0.01  = 280 ohms. 

This 280 ohms is not a standard value, so you can use 270 ohms or 300 ohms, which are easy to find.  A lower resistor value means a bit more current is allowed through led, a higher value means less current is allowed.

If you want to chain several leds, you just take out the forward voltages from the input voltage. With several leds however, the current remains the same, so for example if you have 5 leds you want to use 10mA each, you need to use 50mA in your formula.

ps. You might also be interested in knowing how big of a resistor you need to use.
The power dissipated in a resistor by limiting the current is determined with the formula  P = I x I x R

So if we go with the example above where we go with a 270 ohm resistor and 10mA goes through it, then the power will be 0.01 x 0.01 x 270 = 0.027 watts  , which means a 0.125w resistor (1/8w) is more than enough (for this.

[digsys]

What is the ideal value for the current limiting resistor connected in series with an LED in a circuit runs on 12v?
And what value of a resistor we need to light the LED with the following voltages? 220V, 5V and 24V?   

I always use higher efficiency / lower powers leds, ~100-200mcd. They really don't cost much more and you only need 1-2mA to drive them.
Either way, using the suggested formulas -
5V : 5 - 1.2V (Led drop) = 3.8V (across the resistor) // Use 1K8 ~ 2mA (high effic) or 390R ~ 10mA (std-ish)
24V : 24 -1.2 = 22.8V // Use ~ 10K (high effic) or 2K2 (std-ish)
220V : Now we have a PROBLEM Houston !! Is that 240VAC ?? EITHER WAY so, this is now a dangerous area, and you'll need a lot
more protection / safety components, especially if the Led protrudes from a front panel.
Please be more specific with the 220V, so we can advise better.

[Neilm]

If you want to chain several leds, you just take out the forward voltages from the input voltage. With several leds however, the current remains the same, so for example if you have 5 leds you want to use 10mA each, you need to use 50mA in your formula.

If chaining several in series, remember that there is a tolerance on the forward voltage that can be quite variable. if you are not careful when selecting the number of LEDs vs the voltage you could easily find that you will get some LEDs not lighting.

If using a LED with an AC supply, you must have a diode to take the reverse voltage - an LED will not take it. If you are not sure when messing with mains - DON'T.

Neil

[Azhar]

Thank you all! this was helpful! 

[IanB]

If you want to chain several leds, you just take out the forward voltages from the input voltage. With several leds however, the current remains the same, so for example if you have 5 leds you want to use 10mA each, you need to use 50mA in your formula.

Surely you made a typo here? Since in a series circuit the current remains the same, you need to use 10 mA in the formula, however you need to adjust the total Vf as Vf(tot) = 5 x Vf(led).

[mariush]

Yeah, you're right  ... i mixed series with parallel in my head ... was planning on writing about both in that paragraph.

So
in series current remains the same ... you have 10mA but you have to take out n x forward voltage from input voltage
in parallel,  forward voltage remains the same  ... you have  n x 10mA but you only take out 1x forward voltage from input voltage (though have some tolerance, some leds may be better than others)

Apologies if I confused someone.