LED with 24V?

[alderbody]

I'll be using a 24V DC supply for a univibe clone i'm about to build.

I want to use a 3PDT switch and add a LED.

Can i do it? I mean, will the LED be able to handle the 24V?

[brett]

Hi.
Those little resistors that go in series with LEDs don't just limit the current flowing, they take up the excess voltage.
The LED takes 2 volts.  The resistor will have to take 22 volts.  So if you want 10mA to flow in a resistor with 22 volts across it, just use a 2.2K resistor.  For 5mA, use a 4.4K resistor (4.7k is close enough).  Ohm's law:V=IR !!

cheers

[amz-fx]

The resistor will have to take 22 volts.  So if you want 10mA to flow in a resistor with 22 volts across it, just use a 2.2K resistor.  For 5mA, use a 4.4K resistor (4.7k is close enough).  Ohm's law:V=IR !!

10ma with a 22v drop will have almost a quarter-watt being dissipated in the resistor...  while a 1/4w resistor MIGHT take the current, you would want to use a 1/2w just to be on the safe side.  Watch the watts!   

regards, Jack

[Mike Burgundy]

Can you do it? Yes!
Can the LED handle 24V? No.
The trick is in the limiting resistor that's in series with the LED. The LED likes a certain current (most of the time that's max. 20mA), and lets trough that current at a certain voltage dropped across the LED (usually max 3.6V for ultra-brights, this is the "forward voltage"). The easy way to determine the limiting resistor is to calculate (Ohm's law) the value that would allow 20mA to go through at the given supply voltage and use that as your *lowest* value. You could easily double it or more for ultrabrights - you do want to be able to look at it
Lower currents (higher resistance) yield less brightness. If you don't know what your LED's forward voltage and maximum current are, start conservatively with say 3V and 5mA and see if that's bright enough (or too bright - who knows?)
Off the cuff I'd say start with 4 k or 4k7 and maybe tweak that if needed.
also see:
http://www.muzique.com/schem/led.htm

Edit:
Jack's right about the dissipated power. Most modern metal films can handle 1/2W, but it's good to make sure. You could also use two 10k's in parallel to spread the load.