Can anyone tell me what size resistor I will need to run a 12 volt led on 24 volts. Its for a furnace application. Want to install the led's on specific circuits to be able to spot what circuit is causing a problem. Transformer is 24volt 40va. Also since I'll be adding a foot of wire to mount the led's should the resistor be mounted at the lamp or power sorce. Thanks

It's best to use any diode, say a 1N4001, a resistor and an LED in series.

You can probably get away with the closest value to:

R ~= (24/0.010)

2.2 to 2.7K or 2200 to 2700 ohms, 1/4 watt or better

As another example:

Say you had a 5 volt DC power supply, with an LED turned on with a transistor.

R = (5-1.2-0.6)/0.010

Where 5 is the voltage of the supply, 1.2 is the drop across the LED and 0.6 is the drop across the transistor. Power is computed from I^2*R. LED currents are usually 10-50 mA.

The AC case is a little complicated. Even though the LED will rectify, it's not a good idea and since the tolerances are so high, I'm not even going to bother converting to a peak voltage because of the built-in factor of 5, 10-50 mA.