# In DCC world, how can I tell if the turntable polarity is correct?



## Dennis461 (Jan 5, 2018)

In DCC world, how can I tell if the turntable polarity is correct?
[N, north rail, S south rail]
I can connect a LED w/resistor from land based rail N to turntable based rail N.
The LED will light if TT polarity does not match, LED is not-lit if polarity is correct.

BUT

If a rotate the TT, the LED will be wrong, because N became S.
My TT has slip rings and does not flip polarity.

I am converting from DC system and my current LED warning system no longer works.


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## 65446 (Sep 22, 2018)

I'm thinking: Once engine is sitting on the approach track, flip it's polarity to match the TT's, This way you won't be standing there waiting for the TT to do a 180 !...You can mark the TT in some fashion which lets you know if it is or is not matching approach track polarity.. If 'yes' then roll right onto it..If 'no', flip approach track polarity before rolling onto TT..
Of course, then there's the question of the stall-track polarity matching the TT's .....
But maybe it all can be done using a Hex Frog Juicer, instead..........I think......M


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## Severn (May 13, 2016)

My reading knowledge. In dcc it's not really polarity but phase. The pwm signals on one rail are mirror opposite in the other. The turntable track should match the feeder tracks.


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## AmFlyer (Mar 16, 2012)

I use Legacy with AC track power, but in two rail a TT is just a reverse loop. The track power to the TT is connected through a PSX-AR-AC, the same as a reverse loop would be. The PSX-AR-AC is just the high current AC version of the DCC reverse loop controller. So the power to the slip rings in your case would be the output from the controller.


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## Dennis461 (Jan 5, 2018)

Here is a pic..
The doghouse on the TT, the light USED to come on when polarity of track was correct for loco movement, while LOCO was on TT, movement towards doghouse.
Now with DCC, the light is always on, regardless of switching the PHASES. The TT has a DPDT switch connecting it to MAIN.

If I leave the DPDT alone, doghouse end of TT matches polarity of the left hand lead tracks, and the track with slope back tender, drive right onto and off the TT. I also have to determine/regulate the DPDT of main layout, as I may not need to switch it with DCC.

I am trying hard to NOT buy an AR1 
Still learning DCC, currently only the 2526 has a decoder.


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## AmFlyer (Mar 16, 2012)

Very nice scene with the TT and Roundhouse. The AR's make operations so much nicer. I have 5 of them. Three for reverse loops, one for the TT and one for a 22.5deg crossing to flip the power to the metal frogs.


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## mesenteria (Oct 29, 2015)

A modern scale turntable 'should' have what is termed a 'split rail' design. That means the wipers feeder the wires up to the bridge rails get their feed from TWO pairs of rails. The two sets of rails are wired opposite each other, meaning the turning bridge moves over a dead spot, dead to the wipers, and then the wipers contact the other rails simultaneously, but they're phase-reversed. This allows the bridge to turn the loco safely, and then for the loco to trundle off the bridge in the original direction of movement, but oriented correctly to the bay or parking tracks.


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## Lemonhawk (Sep 24, 2013)

If your DCC, then think of the turntable as a reverse loop and just put an AR1 or what ever reverse loop controller on the turntable wires. You could also eliminate the need for split slip rings and use some actual slip rings.
www.amazon.com/WTXRHW-Electric-Collector-Rotary-Diameter/dp/B08MKVNML5/ref=asc_df_B08MKVNML5/?tag=hyprod-20&linkCode=df0&hvadid=475715987248&hvpos=&hvnetw=g&hvrand=13931013757505736074&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9012383&hvtargid=pla-1095980427028&psc=1


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## Dennis461 (Jan 5, 2018)

I re-wired my roundhouse tracks to match the 'other' end of the TT, Green circled tracks.
This makes it a simple matter to put an engine away, drive onto the TT, rotate to align with a stall, and park it.
The red X is a 15" radius which is rarely used.
Yellow is in phase, left end, green is in phase right end.


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## Severn (May 13, 2016)

Pretty interesting stuff. I don't have a turntable but see the pitfalls depending on how it's all connected and what the turntable can do ... You might have a real set of problems to solve.


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## DonR (Oct 18, 2012)

You have apparently changed wiring from first pic.
If I read you right, you had the best circuit if you are
not installing a reverse loop controller.

The way I saw it and would wire it:

All roundhouse terminal tracks wired to be in phase with lead ito turntable.
DPDT switch for power to TT track.

If TT DPDT is set to match lead then you could simply drive
loco onto TT revolve to selected terminal track then drive off without
flipping any switch. To return to lead track reverse what you
just did. If you revolved forward to do that you would be out
of phase again. Just always be sure the doghouse is at
the lead track.

However, if you wanted to TURN LOCO AROUND
you would BACK it onto TT, revolve TT. FLIP DPDT (to rematch lead)
and drive off. TT would be in phase for next time it is used.

That works... but, get an auto reverser and be done with it.

If you have converted to DCC remove the DPDT switch you
used to control the 'main' polarity as DC. There is no need for it.

Don


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