All about capacitor discharge units

How to control your "points", "turnouts" or "switch machines" on your layout.


Depending upon where you live, you will call a section of track, where one route diverges from another either a "turnout", "set of points", "switch" etc. I normally refer to it as a turnout, or set of points.

Peco HO scale curved points image

An example of a HO scale set of Peco curved points

Solenoid control:

A solenoid style switching mechanism is one way of switching your "points". That is, a coil of fine wire that create a large enough magnetic field when a current passes through them, to switch the blades of your turnout over. 2 coils are used - one for each possible route setting.

Peco points motor

A typical solenoid style points motor

Controlling the solenoids:

The nature of the coil of wire has resulted in a special requirement to switch your turnout from one route to the other. A very short burst of energy (electricity) is required, but cannot be left on, and must be large enough to throw the switch blades. The solution is by way of a "capacitor discharge unit". I don't take credit for the design, it has been around in many variations for as long as I can remember. I am just presenting it in my terms, which will hopefully be understood by somebody!

Basically, a power source charges some capacitors to their capacity. Often, an indicator (such as an LED) shines brightly to show that the unit is ready to do its job. A button or switch is closed, to allow that charge to dissipate through the a solendoid coil and change the point blades over in the process. The circuit does not allow any further current to flow until the switch or button is released, preventing coil burn out or even a fire. Once the button is released, the capacitors charge again, ready for the next use.

Capacitor discharge unit image

An example of a capacitor discharge unit circuit board

Troubleshooting tips:

If your capacitor discharge unit doesn't quite do the job you expect it should do, a few tips below may help -
1. Don't hook too many points motors up to switch at once. Under normal circumstances, you would not expect to throw more than 2 solenoids simultaneously.
2. A large wire gauge should be used between the capacitor discharge unit, the switches and solenoids. The shorter the length of wire the better, but thicker wire will overcome distance issues.
3. Ensure that the blades switch freely, with no obstructions such as ballast in the way.
4. Attached accessories such as clip-on "detectors", will add physical resistance to the movement of the blades. This will greatly reduce the number of solendoids that may be simultaneously activated.
5. A higher power supply voltage feeding into the capacitor discharge unit will also have a bearing on the available burst of energy available. This of course depends upon the design of the circuit - and the capacitor voltage ratings are normally the ruling factor. Generally a maximum voltage might be 24V DC or 15V AC. (The peak voltage of 15V AC is actually 21V, so that is approximately the voltage that the capacitors will charge to.)

So if you are having trouble activating your solenoids - increase wire gauge, increase supply voltage and/or increase capacitance.

Peco accessory add-on switch

An example of a Peco accessory switch. It clips onto the base of the points motor, and can be used as an indicator on your control panel to show route setting, operate signalling, or interlock other points etc

Wiring diagram:

A printable PDF file - simplified diagram showing how to wire up your capacitor discharge unit to your switches and solenoids.


© Robert Parnell