LED brightness with less than rated voltage

Using this calculator, 750 ohm resistor would work.

LEDs that are marketed for 12 or 24VAC are actually LEDs with a diode and a resistor already wired in series.  John's use of the LED calculator is correct for 18v DC source (not AC). and no other resistor in the circuit.  Not knowing what resistor is already in the circuit, it is difficult to know for sure what additional resistor to use.  Once you set the safe minimum resistor size, you will be stuck with whatever minimum voltage the LED lights.

An easier approach is to use a diode (converts to pulsed DC) and a CL2 LED driver in series with a plain LED.  The CL2 maintains a safe 20ma current supplied to the LED over a wide voltage range (5-90v).

CL2s are about a buck apiece, diodes maybe 10 cents, and LEDs 2-3 cents in quantity shipped from China.  This is far less expensive than the 12 or 24v AC LEDs.

Bob

@VinceL posted:

I would like to get some LED's for a control panel that will work in conjunction with a toggle switch that turns power on/off on a siding.  I have found some nice panel mounted green LED's that are rated for 24vac. Using 18 volts for the track (DCS and Legacy), will the 18 volts be sufficient to illuminate the panel LED's (kind of like lower voltage illumination I see on Fastrack switches)?

18 volts will be fine, you will probably not be able to see the difference between 18 volts and 24 volts on those.  The light output of an LED is linear to the current through the LED, but your eye does not respond in a linear way to light intensity, so it's likely you'll see little or no difference.

GRJ, I figured the LED's running with 18 volts would be bright enough.  Thanks for confirming.  I have some scheduled to be delivered tomorrow.  I will give them a test.

I received 10 panel mount 24VAC Green LED's today.  They illuminate starting at about 5 volts, by 10 volts (to my eyes) they are almost at maximum brightness.  They are going to work fine with 18 volts.

Like I said, they'll be sufficient.