I'm going to be installing a Lionel fan-driven smoke unit. The ones with the rectifier built in for the fan.

610 8057-200

I've always run them off of lower ac voltage, but in this case I have to run it off of 18 volts constant AC.  I want to lower it down to about constant 12v AC.

Probably a stupid question, but what is the best way to do that. I've used resistors in plenty of DC circuit's, but never in an AC circuit. Will it resistor even work in an AC circuit?  I tried a few and had no results.

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Throw a diode in, that cuts the power to what it would see in a TMCC locomotive, it only passes one half of the waveform.  For fine tuning of the smoke volume a bit lower, add diodes, each additional diode will drop the voltage about .7 volts.  I use this technique for a variable smoke regulator board.  In my case I used bridge rectifiers for reasons that will likely go a bit over your head, but just single diodes are the ticket in your case.

Steam Smoke Intensity Controller - SMT Rev. 1.1 Schematic

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Yes, a single diode will greatly cut the power to the smoke unit, and that's basically what TMCC does with the triac, they output half-wave track power.  That smoke unit comes with a 27 ohm resistor, so with the diode, you should get reasonable smoke but not an inferno on 18VAC.

Note that the diode has to be oriented correctly, the cathode (stripe) faces the smoke unit, the anode (no stripe) is the hot side of track power.  This is necessary to allow the fan power supply to operate properly.

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