Switch machines

[lionelsoni]

Although the caption of the first picture in Dennis Brennan's February, 2011, CTT article, "Switch machine wiring", says that it "doesn't have to be complicated", simplicity wasn't the impression that the article left me with.  I got to wondering whether the whole thing, activation and indication, couldn't be done with a single wire, by encoding the switch position as voltage polarity and stealing Lionel's venerable trick of piggybacking the indication on the activation wires.

One thing I noticed is that Brennan doesn't make any provision for shutting off the solenoid after the turnout throws,  even though the switch machine provides contacts that appear useful for that.  In fact, he issues a warning to "[t]ouch and release immediately.  A prolonged contact will burn out a coil."  Yet it looks like contact pairs 9-6 and 8-5 would handily disconnect each of the coils after throwing.  So I propose that those should be connected in series with terminals 2 and 3 (or perhaps 3 and 2, since I can't tell from the picture which coil opens which contact).

After that, I would insert a 1N4001 diode in series with each contact pair, with one contact facing a cathode and the other an anode facing the contact.  Then I would tie the free ends of the diodes together and to a single wire leading to the control panel.  Terminal 1, the solenoid common, would connect to an AC supply, track voltage or a fixed accessory voltage.

At the control panel, I would put another diode pair, again with the diodes facing opposite directions, and then arrange to throw the switch by connecting the free end of one or the other diode to common.  Here we can make a choice among the momentary SPDT switch that Brennan prefers, individual pushbuttons, or my favorite, two studs on the panel and a probe connected to common.

Since polarity of the diode connected through a solenoid coil and its contact to the single wire will determine the polarity of voltage on the wire, all that is needed to indicate the turnout position is a bi-color (red-green) light-emitting diode, which lights one or the other color according to the direction of current, or two individual LEDs in anti-parallel, and a series ballast resistor.

Because the switch-machine contacts are disconnecting the coils quickly, just like a Lionel 022 or a K-Line turnout, there is no reason not to use anti-derailing control rails.  These should not be connected directly to the coils' terminals 2 and 3, but rather through a duplicate of the diode pair used to throw the turnout from the control panel.

It might be prudent to connect a transient-voltage suppressor between the single wire and common, to snub any inductive spike from the solenoid coils.

[DennisB-1]

Bob,

While I can appreciate your comments, I believe I took a simple, direct and reliable solution to twin coil switch machine wiring. I purposely avoided using the mechanical contacts as you suggest to cut off power. I feel it's unnecessary. Why introduce another mechanical connection that may get out of adjustment? The contacts are much better suited for signaling. However, by wiring all the contacts to the barrier strip as illustrated, one can easily do as you suggest.

As you noted, I recommend using SPDT Lever switch-spring return to center off (or two momentary contact switches.) These switches are simply press and release. I think most will understand that but nevertheless, I issue the caution about prolonged contact . 

If you want to get right down to it, a  foolproof way to insure a positive throw and momentary contact is with a capacitive discharge system but that's beyond the scope of this article.

Obviously there are different approaches for wiring LEDs and such. I presented a  simple solution that has worked reliably for more than 20 years.

 

PS:(The lead photo was designed to look complicated)

 

Dennis Brennan

 

 

 

 

[Texas Pete]

Who has twin-coil switch machines in stock?

Pete

[DennisB-1]

Pete,

After this article was written, the overseas factory that manufactured twin coil switch machines was devastated by a fire. All the tooling was lost and the manufacturing ceased. However, you can still find these machines on ebay.

[Texas Pete]

Thank you, sir. I need just a couple or three more for the HO portion of the program, and the usual suspects all seem to be out of stock. Too bad about that fire.

Pete