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A Capacitance Discharge Unit, or CDU for short, is a safer method to power twin coil. or solenoid switch machine, than a momentary contact switch and power supply. The reason it's safer is the CDU delivers a metered pulse of current to the solenoid, and then the energy applied drops to a level low enough to prevent coil burnouts if the switch becomes stuck or is held on too long. The pulse is also of high enough energy to assure positive switching action. The negatives to a CDU is cost and cycle, or charge, time. The cost can be justified by assuming that eventually a switch will become stuck or held too long and a solenoid will burnout causing downtime and replacement costs. The cycle times can easily be held to under 10 seconds which shouldn't be a problem. It takes time on the real roads to manually throw switches, often more than 10 second. Also the CDU's power pulse assures positive switching action which can prevent derailments of you prized locos and rolling stock. So how does a CDU work? A voltage is metered through a limit resistor so that the current, and thus energy, is low enough not to heat the coil even after days of continuous power flow. When the coil is disconnected a capacitor is allowed to charge with the energy that would normally flow into the solenoid coil. In this application the capacitor can be though of as a metered battery which can be discharged instantly. So it stores the charge as its slowly fed through the limit resistor until it reaches the voltage of the power supply. It now can have quite a bit of energy stored, even more instantaneous energy than the power supply by itself could provide. The amount of energy stored is a function of voltage and capacitance. When the switch is closed to the coil the power is discharged into the coil as a pulse which produces a strong magnetic field which moves the points. After the pulse is dissipated the energy is controlled once again by the limit resistor.



If we add a power supply and the correct size components we can easily build a basic replacement supply which will safely operate with most turnout solenoids on the market. Altla's twin coils have a common connector between the coils. Some aftermarket coils like Peco have separate wiring for each coil, but can be made into twin coils by connecting one lead from each coil together to form the common. Which wire doesn't matter. This will switch 1 coil every 7 to 10 seconds. It may power two coils, especially the smaller atlas ones at once, but was designed for one coil at a time. How to switch more at the same switch throw will come in a later installment.



After you have built one of the above you might wish to consider this circuit. The use of a DPDT switch allows the switch to be mounted on the switch panel and the switch position will show route selection. It is a dedicated CDU, so one would be required per turnout, but the components are not really that expensive. It is also the stepping stone to the single switch multi turnout CDU.



So the next solution is powering multiple turnouts such as a crossing where you wish to switch 2 or more sets of points at once. The easy way would seem to be build the above and just use bigger caps. Well, while that will work the charge time increases unless we use smaller resistors. This can lead to problems if a coil burns out at a future time and the resulting drop in resistance causes the rest to burnout in a cascading reaction. There is also the problems of all the solenoids needing to be close to the same inductance. If you were to mix brands you might find that the switches with lower induction rob the ones with higher inductance of power. So one would switch fine, but another would be lazy or not switch at all. There is also the problem of finding bigger caps and their higher pricing and size due to their capacitance increase. The best solution is to build 1 of the above for each solenoid and replace the DPDT switch with a switch with more more poles. But 4PDT switches can be hard to find and expensive. If you are powering 3 switches at once a 6PDT switch can be impossible to find. So you simply replace the switches with cheap DPDT relays and power them with an LM 7805 voltage regulator and use a two dollar on-off switch to change routing. The simplest and cheapest way is still to use the very first circuit and use an Atlas snap switches to control your turnouts, but to switch a route that requires 3 points to change would take around 30 seconds due to charging time. One last note is on the relays. We need to provide a device to protect from reverse voltages in the circuit due to coil collapse in the relay. Some relays have built in diodes to shunt this spike to ground. They can be identified because they will operate only if their coils are correctly polarized, hook them backwards and they are basically a dead short until the magic smoke is released from the built in diode. If you do this, or buy a relay without a diode, you need to install a 1n4001 or better diode to take care of this spike. More CDU and other train circuits can be found at Rob Paisley Train Circuits Homepage which is an excellent source. Fred

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