@gunrunnerjohn posted:To prevent damage, you could use a PTC on the gateman so that if the power lingers too long, it would be shut off by the PTC.
Exactly.
OGR being a discussion forum, I think it worthwhile to discuss the topic. The instructions state the gateman works best with 12-14V AC. In other words, it essentially assumes you have a train transformer with traditional Auxiliary/Accessory AC output separate from variable AC track voltage. But what if entering the hobby via 18V AC command control with fixed-output AC bricks that don't have a 12-14V AC output? There have been many threads asking how to reduce the 18V AC from a fixed-output brick to a lower AC voltage for use with accessories. The stock answer is to re-purpose a spare AC transformer bought some time in the last century. A common question is to inquire whether the accessory operates off DC. Many accessories operate just fine on DC. Manuals don't even mention DC not because they can't or won't operate on DC, but because they were built in the last century when low-voltage regulated DC power supplies were expensive - requiring solid-state electronic components like transistors that were quite the novelty. 
Much easier to use AC since it only requires fiddling with the windings of a transformer and no expensive electronic parts.
I'm curious if anyone has actually measured the gateman solenoid's electrical parameters. 
While the instructions say not to use 18V DC, it does not explicitly say it does not work on DC. Obviously, the 18V DC reference is because of the LionChief low-power DC wall-wart. How much AC current does it draw at 14V AC? What is the Ohms/resistance of the coil?
The magnetic field or pulling power of the solenoid coil is proportional to the current flowing at the time. It does not matter if the applied voltage is AC or DC. That the current in the coil is smaller for 18V AC vs. 18V DC is a separate matter. Obviously some coil accessories depend on the alternating magnetic field to buzz or vibrate the accessory to operate. I would be interested in hearing someone who has damaged their solenoid plunger by magnetizing because they used DC.
