My layout which is DCC has a reversing loop on which the polarity is controlled by the switch of a Turtoise switch machine. When a train enters the reversing loop, I stop it, switch the turnout and put the power back on the engine.
What could happen if I don't stop the engine before throwing the switch?
Guy
Modeling CNR in the 50's
If your system is competently wired, and your DCC base station is in good order, nothing except a cut-off of track power and a short indication by that base station...beeps, light, or both. IOW, the first metal tire to bridge the important gap will cause a hard short, but the base station's short detection circuitry/wizadry should cut off power to the rails in a few milliseconds, short enough an interval to prevent fatal heating in the decoder.
Crandell
I use the contacts on Tortoise machines to control signals. There is a distinct "dead time" between the time when one signal light goes out and the other comes on. This is about one second, probably just a bit shorter. I run my Tortoise machines on 9 volts, and the delay would be longer at a lower voltage.
The engine in the loop will stop suddenly and then resume. The lights will go out, and if it's a sound engine, the sound decoder may reset so that you get the startup sequence when power comes back. Depending on the speed and characteristics of the train, the train's momentum may cause the cars to bunch up and then re-stretch, and might even cause a derailment.
It takes an iron man to play with a toy iron horse.
To avoid the dead zone using Tortoise contacts to flip a reverse loop polarity, use the Tortoise contacts to drive a relay which is used to reverse the track polarity. That way it will pick up or drop out instantly, no delay time with no power.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
Hi Randy,
"a relay which is used to reverse the track polarity"
Ca you give me more info about thatkind of relay? Where can I buy it? Which manufacturer?
Well, you need a DPDT relay, it will have the saem contacts as a DPDT toggle, and will be wired to the tracks the same way you would wire a toggle switch for manual control.
Then, pick a coil voltage. You need to use one of the Tortoise contacts, witha power supply capable of meeting the coil voltage and current. Pick a set of Tortoise contacts, say 2-3-4 (pin 4 is the common). Wire one sode of the power supply to one side of the relay coil. Wire the other side of the power supply to Tortoise pin 4. Wire Tortoise pin 2 or 3 (one or the other, NOT both) to the other relay coil connection.
Witht he Tortoise in one position, the relay will be powered and the contacts pulled in, when the Tortoise moves the other way, the contacts drop out.
Best practices say that if there is a position that the Tortoise is in most of the time, that should be the one where the relay coil is NOT powered. If it's 50/50, it makes no difference.
If the track polarity ends up wrong, you can either flip the power in to the relay contacts, or switch the relay coil wire from Tortoise pin 2 to pin 3.
Whether you use the contacts of the Tortoise to control the loop directly, or through a relay, you are still committing yourself to running around the loop in only one direction - unless that's not important.
For how little they cost, the investment in an Auto-Reverser module would solve all your trouble, and you'd be able to run through the reverse loop in either direction with no problems.
Mark.
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Why would that be? If the turnout is set straight, the contacts would set the loop polarity to match the main coming in the straight route. If the turnout is lined to the diverging route, the contacts would set the loop polarity to match the main from the diverging side.
Just like using a manual toggle switch, you can enter on either side and leave on the other.
This has been a topic of discussion on the Digitrax list - instead of using auto reversers, which wait until there is a short and then fix it, why not avoid the short altogether, and use switch machine contacts and/or a relay? Never a short - unless you run into the turnout lined wrong, in which case there's a short and derailment. It works for balloon loops, wyes, and other reversing arrangments. The only time it would mean always going one way around the loop is if you used a spring switch, and that would require a reverser, since there would be no contacts.
I'm not sure whether a relay would work all that well. You can't use a latching relay, because they are not designed to take constant input on their coils, much like a twin-coil switch machine. But, if you use a non-latching relay, what happens in that "dead zone" where there is no power on either coil? Does the relay hold its contacts long enough to bridge the gap, or do you get a dead zone there, too?
I don't know a lot about the various kinds of relays available, so maybe this is a non-issue.
MisterBeasley ...if you use a non-latching relay, what happens in that "dead zone" where there is no power on either coil? ...
...if you use a non-latching relay, what happens in that "dead zone" where there is no power on either coil?
...
A non-latching relay only has one coil, when it is energized, it's flipped one way and has a spring return to the other position when not energized. You would use one set of contacts on the Tortoise to energize the relay at one end of it's travel and it would not be energized at the other end.
Another option is to use two latching relays(such as Atlas Snap Relays) or a single latching relay with four sets of contacts. Flip the relay(s) using buttons or a momentary toggle and have the relay(s) flip the loop phase and the power to the tortoise.
Yet another option, and actually the simplest, is to get a 4PDT toggle switch and use two poles to throw the turnout and two poles to swap the loop phase. The only problem with this method is making sure you throw the toggle switch quick enough to not cause the loco to stop and reset, especially if you have sound equipped locos. In this case you really want a switch that does NOT have a center off position to make it easier to throw quickly.
rrinker Why would that be? If the turnout is set straight, the contacts would set the loop polarity to match the main coming in the straight route. If the turnout is lined to the diverging route, the contacts would set the loop polarity to match the main from the diverging side. Just like using a manual toggle switch, you can enter on either side and leave on the other. This has been a topic of discussion on the Digitrax list - instead of using auto reversers, which wait until there is a short and then fix it, why not avoid the short altogether, and use switch machine contacts and/or a relay? Never a short - unless you run into the turnout lined wrong, in which case there's a short and derailment. It works for balloon loops, wyes, and other reversing arrangments. The only time it would mean always going one way around the loop is if you used a spring switch, and that would require a reverser, since there would be no contacts. --Randy
You're right Randy. I wasn't thinking it through - you see, I have my two reverse loops wired that way in my two staging yards and we must go through the loop in one direction only. But the reason isn't because of the polarity, but because the exiting points of all the turnouts coming out of the yard throat in the reverse loop staging are free-floating and going in the loop in the opposite direction would surely spell disaster !
So, yes - the relay DOES work .... it's my staging yard layout within the reverse loop and the way it's configured that prevents me from using it in the opposite direction to enter.
Thank you for the answers.