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Capacitor Discharge Power Supplies

Capacitor Discharge Power Supplies

This page presents various capacitor discharge power supply circuits for use with twin coil switch machine motors. Also shown are some optional wiring methods that can be used to enhance the availability and control of these supplies.
Basic Capacitor Discharge Units

Circuit A is the simplest type but suffers from a relatively slow recovery rate.

Circuit B is a very popular power supply as it has a very good recovery time. This circuit has a high current surge when the capacitor is charging.

Simple But Effective Switch Machine Power Supply

The first circuit of this page is for a classic Resistor / Capacitor unit but with a few modifications made.

Simple Resistor / Capacitor Unit schematic

The standard bugaboo with this type of circuit is the relatively long charging time of the capacitor. But if you are willing to wait the 1 second that a 2200uF capacitor takes to charge to 90 percent of its maximum voltage when a 220 ohm resistor is used this can be a simple and cost effective power supply. Using a lower resistance charging will shorten the charging time proportionally.

Shown in the top power supply circuit is an indicator circuit that shows when the capacitor is near its full charge. This is an optional feature and can be left out as shown on the second unit.

To increase the availability of switch machine power more than one discharge unit can be connected to a central transformer and rectifier / filter capacitor. This would allow smaller discharge units to be placed around the layout and used to operate machines that are nearby.


The diode in front of the 220 ohm resistor will prevent units from draining the voltage from each other when turnouts are thrown. Although this is unlikely to happen with an adequately sized supply transformer.

If the recharging time is not too important such as for machines that are not thrown often, the value or the 220 ohm resistor could be increased and its wattage reduced. The maximum potential load on the circuit will be reduced accordingly. If not as much pulse current is needed to throw the turnouts then the value of the capacitor could also be reduced.

With a 16 volt AC supply transformer the DC voltage across the capacitor will be about 21 volts and the maximum charging current will be 0.2 amps.

For a circuit of this type the charging time is dependent on the values of the resistor and capacitor used. For example if a 220 ohm resistor and a 2,200 microfarad capacitor are used then the charging time constant would be as follows.

220 Ohms X 2,200uf = 0.484 Seconds (1 Time Constant)

This is the time that the capacitor would take to reach 63 percent of the supply voltage. The time needed to reach approximately 88 percent of the supply voltage is 2 time constants.

2 X 0.484 = 0.986 Seconds (2 Time Constants)

For practical purposes the time to reach the supply voltage is 5 time constants.

If you have a little patience, about one second’s worth, the basic resistor / capacitor power supply can be a very effective and economical system. However if you have the need for speed then one of the more sophisticated supplies is required.

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