Basic Turntable Wiringgreenspun.com : LUSENET : Wiring for DCC : One Thread
I see from one of the previous enquiries that turntables if slip ring fed don't always need a reversing loop controller. I am making a simple hand operated table at the end of a staging yard with only 4 roads leading to it. I was thinking of using something like a double centre shaft, the two being insulated from each other and having the supply transfered to the two shafts by the use of brushes and thereby connected to the turntable rails. I think however that this is not the sort of slip ring you mean. Is it better to pick up the supply from the ends of the feeder rails, providing they do not short in between their proper aligned locations? Don't the locos then have to be reprogrammed for their correct direction of travel after turning? Thanks for any assistance you can provide.
-- Gordon Fred Bailey (email@example.com), July 15, 2003
The beauty of DCC is that when you want the loco to go forward, it will do so regardless of which direction it is pointing. So no, you don't have to reprogram it when turning it around. But if you use the turntable to reverse the locomotives you will need some sort of track polarity reversing control on the turntable itself. This is a track control problem rather than one of controlling the loco.
The simple way is to wire up all other track sections around the turntable in the same fixed polarity as if the turntable wasn't there. Then find a way to ensure that track polarity on the turntable bridge always matches up with the other lead tracks regardless of which way the turntable is 'pointing'. Your idea of using solid slip rings is a correct and valid way to get uninterruptable power to the rails. Attempting to pick off TT track power from ajoining rail stubs is difficult. You would have to contrive some sliding mechanism that will yield a good electrical connection but never jam. And plan on loosing TT power (and sound ??) while the loco rotates.
So back to the slip ring idea. You can use a DPDT toggle switch or position sensing relay to swap track polarity to the slip rings as the turtable rotates past the 1/2 way around point. Select the mechanical alignment so that the track polarity switches where there are no other yard tracks to avoid ambiguity. You can also use an auto-reverser to correct turntable track polarity as a loco enters or exits after the turntable is aligned. Just wire it between the DCC supply and slip rings, just like the DPDT switch or relay.
A third way is to use split slip rings. That is... two slip rings as you discribed above, but each divided into two distinct sections with a small dead spot between the conductive bars. These get wired similar to a DPDT switch up to the TT track rails. Split slip rings are more difficult to construct but will do the job of automatically swaping turntable track polarity as it is rotated. But there is a necessary dead spot in the rotation that will interfere with sound decoders. [The dead spot is necessary to prevent a DCC short circuit as the two separate slip ring brushes transfer from one section to another.]
The solid slip rings and inexpensive DPDT toggle switch solution is simple. Align the toggle handle to point in the direction of the dog- house on one end of the TT and it is almost foolproof. Replace the DPDT switch with a low cost auto-reverser when you tire of making operator errors.
-- Don Vollrath (firstname.lastname@example.org), July 15, 2003.