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Capacitive Discharge Unit

Capacitive Discharge Unit (DSU)

“Snap Switches” usually don’t. If you’re still using your throttle’s “accessory” terminal, or even a hefty transformer you’ve substituted, then you’re supplying your turnouts with a measly 12 volts AC. When you press a turnout control button, you probably hear a wimpy “click” followed by a buzz — but all too often, the points don’t press solidly against the opposite rail and a cursed derailment ensues as the locomotive (or a feisty boxcar) “picks the points.”

Sound familiar? Then you’ll want to spend a few hours building this simple Capacitive Discharge Power Supply. This Supply will deliver a hefty 35 volt pulse of DC power to the switch motor, resulting in a resounding “SNAP,” no buzz, and points solidly lodged against the new outer rail. If you add the optional “Ready” circuit (and I recommend you do so), you’ll also have a convenient indication of when the supply has “recycled” and is ready to take on the next turnout (it will take 2-3 seconds from the time you release the turnout control button until the Supply recharges); if you’re anything like me and often change the direction of several turnouts at once, the “Ready” indicator is very useful for pacing yourself and ensuring that you don’t wind up with any partially- diverted turnouts. I use this circuit with both Atlas and Peco turnout motors (not on the same layout — but you could), and they’re quite happy and work so much better that I often wonder why I didn’t do this long ago! (Note: if you’re using stall-motors such as Tortoise, this circuit is NOT for you — Capacitive Discharge is for solenoid-actuated motors ONLY).

Construction is simple and quite forgiving, and can be accomplished in just an evening or two (depending upon your experience with such construction projects). Parts selection is not particularly critical, and substitutions can be made as needed (but try and stick with the resistor values shown — especially for the Ready circuit). All parts are readily available at your local Radio Shack and/or from your well-stocked parts box.

You may not need or want the on/off switch (SW1), but I do suggest including the slow-blow fuse. Including the Power On indicator is strictly up to you; I have a primary “AC On” indicator on my control panel, so I have no use for it. Everything can be mounted on a piece of “perf board” (Radio Shack 276-1395A, or equiv.) approx. 2.75″ x 3″. To protect the heat-sensitive transistors Q1 and Q2 from the stress of soldering, I’ve mounted them both in a socket designed for an 8-pin mini-DIP IC package, with the leads oriented as shown. I suggest mounting the LED in plain sight on your control panel, and the rest of the circuitry under the layout (or otherwise out of the way). Be careful of the 115 VAC input power! Dangerous shocks can result; ensure that the power is OFF when connecting to the AC mains. Carefully insulate any connections carrying AC mains power. If in doubt, get help from an “electrical friend.”

This Supply has enough “oomph” to activate two switch motors simultaneously, so crossovers are no problem. If you have distributed control panels and want to have more than one “Ready” indi- cator, you can simply put two LED1s in series (even 3 in series should be OK). If you really need more than three Ready indicators, send me an Email and we’ll try and work something out that will meet your specific needs). The color of LED1 is a matter of personal choice, perhaps influ- enced by what’s already on your control panel; it should be a color that stands out clearly; I started with green, but converted later to a more-distinctive (in my case) yellow.



With the components shown in the Parts List, there will be approx. 35 volts DC across capacitor C1. This charge is tranferred via R2 onto capacitor C2. R2 limits the steady-state current which flows thru your switch motor to prevent burn-out if the control button is held down too long; it also limits the charging rate of C2 — hence the “Ready” circuit. Resistor R1 provides a discharge path for C1 and C2 when power is turned off. Depressing SWX, your existing turnout control, causes capacitor C2 to “dump” its charge into the switch motor, causing the solenoid to snap smartly. As soon as you release the control , C2 begins to recharge.

The “Ready” circuit is a simple differential comparator which compares the voltage on C1 to the voltage on C2. When the turnout control is depressed and C2 is fully discharged, Q1 turns “on” and Q2 turns “off,” causing LED1 to extinguish. As C2 begins to exponentially recharge, voltage divider R3/R4 will cause Q1 to turn off and Q2 to turn on when the voltage across C2 reaches approx. 80% of C1’s voltage. As soon as Q2 turns on, LED1 is illuminated and the circuit is ready to go again.


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