To remotely control the direction of a turnout, many modellers use the common snap relay.  This mechanism contains a solenoid which, when momentarily powered, causes a mechancial pin to shift from one position to another.  Reversing of the power pulse pulls the pin back into its original position.  When the actuator pin from the solenoid is connected to the turnout, the active direction of the track can be switched.

The solenoid only requires a momentary pulse of power to move the actuator pin.  Extended application of power simply results in wasted energy in the solenoid coil that is traslated into heat.  To avoid overheating the solenoid, a momentary contact switch, similar in function to switch control boxes offered by model railroad part manufactures are used.

The problem with these parts, along with other inexpensive momentary contact switches, is that they can get stuck in the active position.  The amount of current required for the solenoid can be high, causing a surge when the switch first makes contact.  This surge, over time, can pit the contacts of the switch and eventaully cause them to weld closed.  If left unattended, this closed switch will continue to provide power to the solenoid, causing it to heat up significantly.  Often enclosed in plastic, this heat in the solenoid will eventually melt the encasement and possibly other parts nearby.  There have even been some cases where adjacent materials may combust before the solenoid burns out.

To prevent this from happening, a capacitive discharge switch circuit can be implemented.  The snap relay configuration does not change from the description above except for the power source applied to the momentary contact switch.  Instead of taking power directly from the layout supply, the capacitive discharge switch circuit becomes the source.

The operation of the circuit is actually quite simple.  The source power in this example is from a 16VAC output transformer.  The input power to the transformer, not shown here, is the AC mains.  Use proper safety procautions connecting the transformer primary winding to the household power.  For added safety, connect the transformer to the mains through a 1A fuse.

The output of the transformer is fused at 0.5A, then rectified to DC through the diodes and filtered with the 1000uF capacitor.  The DC source is connected to an LM317 3-terminal adjustable regulator, wired in a current limiting configuration.  The maximum output current of 360mA is set by the 3.3 ohm resistor.  This output current is used to charge the 8000uF capacitor, which becomes the reservour for the high peak output required to actuate the solenoid.  The remaining components control the output of the LED to illuminate only when there is sufficient charge available for operation.

When a switch is closed between the circuit and the solenoid, energy is dumped from the 8000uF capacitor providing sufficient peak current to actuate the unit.  Once the capacitor is fully discharged, current is limited to a maximum of 360mA, which is not enough to cause the solenoid to overheat.  When the switch is released the capacitor recharges at a controlled rate and after approximately one second the circuit is ready to activate once again.

Only one circuit is required for the layout, with its output wired to each snap relay.  Maximum current can be further reduced by increasing the 3.3 ohm resistor accordingly.  Increasing the resistance will reduce maximum current available to the solenoid after the initial peak, but it also increases the charge time for the output capactior, creating a longer delay between successive firing.  Decreasing the resistor will shorten then recovery time, but also allow more current to pass to the solenoid in the event of a stuck switch, thereby allowing more heat to be generated and potentially reducing the affectiveness of the circuit.

The maximum current is determined by the following formula:
I_max = 1.2V / 3.3 (I in Amps)

You are welcome to download a copy of the schematic for your own reference and use.