FAQ - What are my options for installing a DC Analogue R070 Turntable on a Digital DCC layout?

[Chrissaf]

Edited by Moderator 96RAF to reflect Hornby web page updates.

Introduction.

The Hornby R070 [motorised] and R410 [un-motorised, but can be converted to motorised using the R411 motor kit] turntables are designed for use on DC Analogue powered layouts.

The key difference to Digital DCC being that on a DC Analogue layout, the rotating bridge track will not normally have any power connected to the bridge rails as it rotates. However, on a Digital DCC layout all track will normally be live powered at all times, including when the turntable bridge rotates.

The Hornby turntable includes mechanisms both wiring and physical to ensure that the bridge gets power when it is required. This DC Analogue design feature built into the Hornby turntable is however counter-productive when utilised on a Digital DCC layout as it creates a ‘short circuit’ as the turntable bridge rotates – see image below that demonstrates this ‘short circuit’ issue.

Fig 1 - Hornby turntable when used on a DCC layout with ‘always live’ track. The dotted lines represent physical wires that are located under the turntable that extend track power to the rotating bridge via a ‘split slip ring’.

Hornby’s solution to this Digital Conversion issue was to physically modify the turntable to remove the bridge contacts to electrically isolate the bridge rails from the turntable ‘Inlet Track’ – see extract from Hornby’s previous (now removed) conversion page below:

Hornby’s documented conversion approach was very drastic and removed the beneficial aspects of the bridge rail contacts, thus the forum cannot endorse Hornby’s old method. Instead, the forum’s conversion approach is to make appropriate use of ‘Insulated Rail Joiners’ to achieve the same end goal. This alternative approach has the benefit that the bridge rail contacts can remain in-situ to perform their secondary function to promote robust rail alignment.

Before moving on to ‘Solution Mode’, it is worth describing another aspect of the Hornby turntable design - as this has a bearing on the conversion solutions documented later in this tutorial.

The Hornby turntables deploy a ‘Split Slip Ring’ design to transfer power onto the rotating bridge rails. The ‘Split Slip Ring’ performs two distinct functions.

1. It transfers power from the layout onto the bridge rails to allow a locomotive to be driven off the bridge into a dead end unpowered storage outlet track piece.
2. The ‘Split Slip Ring’ reverses the power on the bridge rails as it passes the half way point in its full 360° rotation. If it didn’t do this, then the bridge rails would be opposite polarity when the bridge returned back to the ‘Inlet Track’ after performing a 180° rotation and create a ‘short circuit’.

This bridge power reversal is documented in the series of images below. Note how the bridge briefly loses track power when it traverses the half way point in its 360° rotation. This brief power loss has absolutely no impact on a DC Analogue layout as the bridge is unpowered anyway as it rotates.

But consider a Digital DCC loco which may have lights or more importantly ‘sound’. There will be a power break to the locomotive DCC decoder as the bridge rotates. Any lights and/or sound will therefore experience a break. The ONLY workaround for this is to use a DCC decoder in the loco with a ‘Stay Alive’ implementation.

Fig 2 – Split Slip Ring design.

Notice how the original Blue ‘A rail’ now becomes a Red ‘A rail’ and vice-versa for Rail B as the bridge passes the half way rotation position.

Solution 1 – Suitable for basic DCC powered locomotives.

This solution is the simplest of the solutions presented in this tutorial. It takes as its premise that the locomotive fleet is just DCC powered for motor control and they do not have lights or sound fitted OR the premise that the layout ‘builder / user’ is prepared to forego those locomotive functions in order to simplify the turntable installation to the bare bones.

This solution leaves the bridge rail contacts in-situ, the only modification that is required is very simple as the turntable happens to use spade terminals in the bridge power transfer wiring.

Fig 3 – Basic DCC Solution.

To modify the Hornby turntable for the above solution [note that this part of the modification is performed as part of ALL the solutions documented in this tutorial] turn the turntable over to reveal its underside.

You will see two wires extending from the centre split slip ring area to the nominated ‘Inlet Track’. These wires are normally held in place and covered by a length of PVC tape.

Unplug the ‘spade terminals’ from the inlet track contacts. These two wires need to be extended to your DCC track supply. You can buy ‘male’ spade terminals to suit the Hornby ‘female’ spade terminals to make this task easier and also less destructive.

Now go ahead and install the turntable on the layout. Making sure that you deploy ‘Insulated Rail Joiners’ where the layout track joins to the turntable ‘Inlet Track’.

Mode of Operation.

Whilst referring back to Fig 3 above, consider a locomotive being driven onto the bridge from the main dedicated lower right inlet track.

The bridge rails are getting their DCC power from the previously extended bridge wires to the DCC controller track output [this could be either a full DCC BUS implementation or a small DCC wiring BUS just dedicated for use by the turntable].

As DCC power is present on both the bridge rails AND the inlet track from the main layout, the locomotive will drive across the ‘Insulated Rail Joiners’. If the controller ‘short circuit’ trips then those extended bridge wires need to be reversed.

Whilst the locomotive is stationary on the bridge rails, the locomotive is still receiving power via the extended bridge wires. As there is no additional DCC power wiring to the individual turntable siding outlet tracks, these will be electrically dead. That is, until the rotating bridge lines up with them. The bridge rotation is halted at the destination outlet storage siding track. The power from the bridge rails is now extended via the bridge rail contacts, which have NOT been removed as per Hornby’s instructions, to power the destination outlet siding track piece.

The locomotive can now be driven off the turntable bridge into the destination outlet storage track. If the bridge rotation is then started again and continued for another task, the outlet storage track with the parked locomotive will lose power and become dead track.

In order to drive a locomotive from an outlet storage track where the locomotive has been stored back onto the bridge, the bridge first needs to be rotated to align with that outlet. This automatically reconnects the DCC power to the outlet via the end of bridge rail contacts to facilitate that task.

Notes:

Although the outlet siding tracks have no power on them except when aligned with the rotating bridge; I have included the ‘blue & brown’ colours on the rails to denote the power polarity [DCC phase] that will be present when the power is applied via the rotating bridge alignment.

The term polarity is not actually applicable to DCC as DCC is an alternating voltage, the correct term to use with DCC power is ‘phase’, but most modellers find it easier to understand and use the ‘polarity’ term, hence why I have used both terms in this tutorial.

The ‘blue & brown’ rail colours change on the rails connected around the turntable due to the effect of the ‘split slip ring’ design discussed previously in Fig 2.

Fig 3 above shows the bridge rotation motor being controlled by a DCC decoder. This is optional and discussed in this tutorials ADDENDUM.

Solution 2 – DCC dropper wiring additions to maintain outlet track power at all times.

Although a little more complicated, the extra work required is worthwhile as this solution allows parked locomotives on the outlet storage dead end siding tracks to sit there with their ‘lights on’ and ‘sounds’ functional.

Fig 4 – Full DCC dropper wiring implementation.

In this solution, in addition to the ‘Inlet Track’ wire modification, all the connected turntable dead end siding tracks will require ‘Insulated Rail Joiners [iRJs]’ to be fitted to eliminate the ‘short circuit’ issue documented in Fig 1.

Special care needs to be taken with attaching the DCC power droppers to the outlet tracks to ensure that they are connected the right way round. Get one of these the wrong way round and you will get a ‘short circuit’ trip display on your controller when trying to drive a locomotive across the IRJs of any incorrectly wired track piece.

Note how the ‘blue & brown’ rail colours change on the tracks located around the turntable due to the effect of the ‘split slip ring’ design discussed previously in Fig 2.

Fig 4 above shows the bridge rotation motor being controlled by a DCC decoder. This is optional and discussed in this tutorials ADDENDUM.

In all other aspects, this solution is basically the same as Solution 1 and therefore not expanded further in this tutorial text.

Solution 3 – Solution to accommodate multiple turntable routes back to the main layout.

In Solutions 1&2, all the outlet tracks of the turntable were dead end siding track pieces that went no further than enough distance to store a locomotive. Solution 3 gives consideration to what the impact would be to the turntable wiring if one or more of those turntable outlet tracks extended from the turntable back into the main layout track plan.

The example layout section below describes such a situation. For ease of drawing, I have just used a red rail colour rather than my previous ‘blue & brown’ colours to denote DCC power rail polarity [DCC phase].

The triangle placed on the turntable bridge represents the front ‘facing forward’ end of a locomotive.

Fig 5 – Turntable in Bridge Position A.

With the turntable bridge in Position A, a standard ‘teardrop reverse loop’ is being accidently created in this example layout scenario. Any ‘reverse loop’ will generate a ‘short circuit’.

Note that this Position A route via the turntable should not be used to traverse a train, only the locomotive in isolation. This is because the RLM protected section of track is restricted to the length of the turntable bridge. One of the reverse loop rules is that this RLM protected section track length should be longer than any train that passes through it. For more detail on this design rule, read my ‘Reverse Loop’ tutorial mentioned at the end of this tutorial section.

Fig 6 – Turntable in Bridge Position B.

With the turntable bridge in Position B, another ‘reverse loop’ is being created that emulates something very close in design to a WYE reverse loop. To see the WYE, look to Fig 5 above which creates a triangle of track with the turntable located at the top.

As can be seen in both of the images above, a ‘short circuit’ is created when the bridge is either in Position A or Position B. The polarity [DCC phase] of the tracks connected to these two turntable outlets are defined & ‘fixed in stone’ by the location of their connection to the main layout. Thus it is impossible to use the rotating bridge auto polarity reversal feature to correct the bridge polarity [DCC phase] to remove the short circuit(s).

The only way you can resolve the ‘short circuits’ in this particular example scenario would be to include a DCC Reverse Loop Module [RLM] into the DCC feed powering the bridge rails. This is shown in the schematic below:

For the purpose of this schematic, I have removed the ‘blue & brown’ polarity [DCC phase] colour indications on the turntable outlet tracks and replaced them with plain black track rails. These rails still, of course, need to have DCC power applied to them. The use of the ‘black’ rail colour is just to signify that the power can be applied EITHER way round and is not sensitive to polarity [DCC phase]. This is because the RLM module will reverse the DCC power on demand as required to match the bridge rail power to any and all tracks it is aligned to.

The implementation of the RLM could also be deployed on Solution 2 in this tutorial. Some ‘Internet’ sources actually promote using a RLM as a generic DCC turntable solution.

As a RLM is being used to control the bridge rail polarity [DCC phase], the inclusion of the IRJs on all turntable connected tracks is even more critical, else the RLM function will be compromised. For more information on RLM modules and ‘teardrop’ & WYE reverse loop theory; read my ‘RLM Tutorial’ that is downloadable from the sticky post at the top of the ‘Hornby DCC’ sub-forum.

ADDENDUM

The three solutions in this tutorial are using a locomotive DCC decoder to operate the electric motor in the turntable hut. This could be a Hornby R8249 decoder, but any basic locomotive decoder will suffice.

If a decoder is used, do remember that you might want to change a CV setting on it after installation; therefore I recommend that some form of connector arrangement is included between the decoder and the DCC track supply. Then it will be possible to disconnect the decoder from the track and reconnect it via a pair of temporary wires to the ‘PROG’ output of your preferred DCC controller for configuration tasks to be undertaken.

A locomotive decoder is preferred to a fixed [permanent] voltage output ‘Accessory Decoder’ as a locomotive controller will enable speed as well as direction rotation control of the turntable bridge. Operating the Hornby turntable at a more sedate pace with a locomotive DCC decoder should make it quieter in operation as well as looking more prototypical.

Of course, although the turntable is being used on a Digital DCC layout, the turntable bridge rotation can still be controlled manually by connecting the turntable motor to a DC Analogue controller or DC supply that is then dedicated for the bridge rotation task.

However if a locomotive DCC decoder is used, then the rotation of the turntable opens up the possibility of automating the rotation of the turntable via DCC software control such as with Hornby RailMaster, which is outside the scope of this tutorial to detail. Although if the demand is evident; a complimentary tutorial for RailMaster turntable control could be written.