[analog31]

An amusing feature of solenoid valves is that the inductance increases as the plunger is pulled into the coil. When driven by AC, it causes an initial "kick" of current that gets the slider moving, followed by a lower holding current. Thus it fortuitously optimizes the current budget of the power supply.

Also, when a plunger gets stuck, the coil continues to draw a lot of current, and could burn up. This is a failure mode of solenoid valves in industrial processes. You can get DC solenoid valves, but the coils are bigger and they're more expensive. It means you shouldn't run an AC solenoid on DC unless you're really thinking hard about what you're doing.

And speaking of failure modes, another amusing feature of solenoid valve systems is the sheer number of ways that they can fail. One thing that's always counterintuitive to beginners is that a valve or its control circuit (or firmware) can get stuck in the "on" position. This is true of solid state and mechanical relays. A well engineered system plans for this possibility. "Stuck" could mean releasing a lot of energy (e.g., if controlling a heater or motor), or flooding your garden, or your house.