Almost every model railway has points and they can be operated using point motors.

If these are solenoid point motors, then a capacitor discharge unit can be used to ensure the points throw fully.

After the initial surge, the current flowing will be less than 42mA which wonít cause any harm to the point motor.


Text Box: Parts list
1x 560R 1W resistor
1x 1k resistor
1x LED
1x 4700uF 35v capacitor
1x NPN transistor
6x 1N4007 diode

This CDU is designed to be run from 14-25v AC or DC.

Pay attention to the polarity of the components to avoid damaging them.

Any LED with a forward voltage of 2-3v can be used. The current varies from 9-21mA depending on the supply voltage.

Circuitry explained
The power supply is rectified using the four diodes connected to provide unsmoothed DC.

If there is no load on the output, the current will flow into the base of the transistor allowing the capacitor to charge.

When you throw a point motor, the capacitor will discharge through the solenoidís coil and the transistor will be switched off.

If the coil is left connected, it will receive a miniscule current of not more than 42mA which will do it no harm.

The two diodes nearest the output protect the circuit components from back EMF which could damage them.


The LED lights when the capacitor is fully charged so is best displayed on the control panel but can be omitted if it is not required..

If it stays off after a point motor has been fired, the point motor is still connected in the circuit.

Neither the CDU nor the solenoid will be damaged by this but no other motors can be fired until the load is removed.


I have not built this circuit and therefore I cannot guarantee that it works or that no components will be damaged.

Graeme Kilgour