10/07/2012.

### Turnouts motor control

I plan to power the motors with half-wave rectified voltage. Let me explain: a two-position inverter has its common point connected to an alternative power supply (transformer). Its other two terminals are connected to those of the motor by two diodes, in opposite directions to one another. In one position, the motor receives a voltage whose mean value is positive, and thus rotates until it reaches the end position. In the other position of the switch, the motor receives a voltage whose mean value is negative and therefore turns in the opposite direction. Here is the diagram.

Note: the inverter may be a relay SPDT contact; the diodes are in close proximity or even directly welded thereon.

This system has several merits: first, the simplicity of the wiring, a single wire per turnout, plus a common wire to all motors! Then, by placing a capacitor (not polarized, but we can cope with this, see below) at the terminals of the motor, not only is the current smoothed, but the average voltage and thus the speed of the motor can be adjusted by changing the value of the capacity, which is appreciable when one considers the little performance homogeneity of these motors. With a 15V transformer output, some motors require no capacitor, others require up to 1000µF!

For such capacities, the non polarized capacitors are bulky, difficult to find and expensive (example: €8.50 for 200 µF / 35 V, instead of €1.50 for a polarized one). The current is reversed as a function of the turnout control, and a conventional polarized capacitor doesn’t like to be connected upside down. Solution: connect two capacitors in opposition, as shown in the diagram above.