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The resistance of a point motor coil is a few ohms, so connected direct to 12v DC the current would be several amps, overloading most power supplies or causing the motor coil to overheat. That's why 'passing contact' switches are used so that only a short burst of current is delivered. The CDU often runs at a higher voltage so that the current pulse can deliver more energy, but when the capacitor is discharged, only a relatively small current is allowed to continue flowing through the coil, possibly around 0.25 amp. this is insufficient to overheat the motor coil.
So your relay contacts may have to take a surge current of some amps but this rapidly drops off and they only have to break the small residual current.

The alternative to relays (and a lot cheaper) is to use a 'Diode Matrix' to distribute the CDU output to two or three point motors - this lends itself to 'route switching' for example. The principles of this and CDUs can be found in "Wiring the layout" by Jeff Geary, KRB Publications ISBN 0 9542035 5 0. This book also comes with a useful CD ROM illustrating how to wire up a layout.

John Webb
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