Hornby Turntable - DCC and DC Operation

[Fishmanoz]

I have a number of questions about the operation of the Hornby Turntable. They are:

When originally supplied in DC configuration, how is it wired? Is the rotating track live itself or only made live when connected to a live turnout/turnouts?

I've

read the Hornby instructions for DCC conversion using a loco decoder. It says the contacts at the ends of the (rotating) rails must be removed or a short circuit will result. What causes this short to occur - from where to where?

In DCC, are the turnouts

made live by the roating rail turning to them, or must a separate connection be made?

Finally, if by rotating from one turnout to another would effectively make a reversing loop, is a short caused or is the loop isolated by the different positions of

the rotating rail as it goes from one turnout to the other?

Sorry for the complicated questions. If someone has a wiring diagram for the turntable in DC mode and DCC mode with the contacts removed, that will answer all of my questions.

[Fishmanoz]

roating should be rotating

[Rog RJ]

"When originally supplied in DC configuration, how is it wired? Is the rotating track live itself or only made live when connected to a live turnout/turnouts?"

The rotating track (called the bridge) is connected to the input track (the 2nd

track clockwise from the control hut) which has two contacts that collect power from the input track.


"[i]I've read the Hornby instructions for DCC conversion using a loco decoder. It says the contacts at the ends of the (rotating) rails must be

removed or a short circuit will result. What causes this short to occur - from where to where?[/i]"

From the bridge rails to the exit rails. As the bridge rotates, rails of opposite polarity come into contact via the contacts at the ends of the bridge

rails.


"In DCC, are the turnouts made live by the roating rail turning to them, or must a separate connection be made?"

If you remove the contacts from the end of the bridge rails then there is no connection from the input track, via

the bridge, to the exit tracks so additional track feeds would be required. Having power feeds to all track pieces (preferably soldered connections) is best practice for DCC in any case.


"[i]Finally, if by rotating from one turnout to another would

effectively make a reversing loop, is a short caused or is the loop isolated by the different positions of the rotating rail as it goes from one turnout to the other?[/i]"

I don't quite understand this question but I can't see how a reversing loop would

occur if the contacts on the ends of the bridge rails have been removed.

The Hornby method of conversion is a bit brutal and will invalidate the guarantee. There are a few different ways of converting for DCC and may not require removal of the bridge

contacts. Try a forum search, it has been discussed many times previously.

[Blackbird]

Hi, Rog (RJ) has covered much of your questions regarding the use of the R070 turntable on DCC. But if I can expand: Using the T/T with DCC comes into two categories;
1st; Control of T/T by DCC, the turntable motor will require a DCC decoder wired

to the motor and supply wired directly to the track, busmain (if employed) or controller. This is (unless you wish to be totally DCC) unnessasary as a basic (dedicated) DC controller can be used (cheaper than a DCC decoder).
2nd; Track isolation, as a DCC

system track is permenently 'live', the tracks will short circuit the system when the bridge is rotated as the 'LH rail makes contact with the RH rail via the bridge contacts. This is why Hornby advise removing the contacts (invalidate any warranty?) This

method will of course work, but then all outlet tracks will need to be separatly wired to the system!
A much simpler method is to use a DCC T/T track adaptor. This was available on e-bay but to aid those interested, it was a simple adaption of the X918

outlet track, by gapping the rails between the under rail connection and the T/T, short circuits are prevented. The track connection is still made to the bridge via the under rail connectors, and hence fed to the aligned output. However, if any of the outlets

are connected to the main system, then that outlet track connected to the T/T will also need to be 'gapped'.
Hope this is of help.

[Fishmanoz]

Thanks Blackbird for all the info - I've read the other old thread too. I still am a little confused on the TT circuit but I'm assuming the under track connections on the inlet track are the supply to the bridge? And the gapped adaptor has isolators in

both rails between those connections and the bridge? Let me know if I have that wrong please.

That gets me to outlets. Your system will be fine on these where they are not connected elsewhere in the layout. If they are connected elsewhere and that

connection has the polarity reversed (my reference was to a reversing loop situation in my original post), then there is still an isolation issue. But a simple gapping still has the problem of a loco shorting over it as it passes surely?

I had been

thinking that an Alternative is to remove the supply to the bridge and rely on connections made to each of the inputs. Yes, it means more wiring but solves the polarity issue. Do you think that will work? The bridge will only be live to the track it is connected

to at whichever polarity that track has. I realise that it will mean a loco on the bridge has no connection to dcc while it is rotating.

[Blackbird]

Hi Fishmanoz,
You are correct in your understanding. The DCC T/T track adaptor was designed as a 'simple' conversion method to negate the mutilation of the T/T (removing the contacts) and allow the bridge to supply current to the aligned outlet without

the necessity of additional wiring. Any outlet feeding back to the main circuit would also need an adaptor track. You are correct that an outlet feeding back to the main circuit with reversed 'polarity' would cause a 'reverse loop', but this can be overcome

by planning the setup. Remember, half of the outlets are automatically reversed by the T/T rotation, the pickup plate is segmented.
Your suggestion of removing the feed to the T/T bridge and supplying all outlets will also work, but with a DCC system we

would like the additions; lights, sound and smoke to continue on the rotation of the loco.

[Fishmanoz]

Thanks Blackbird, it's good to know my logic is working.

Just 2 final questions - how do you avoid a short when the loco is bridging over the gapping in the adaptor track when crossing from bridge to outlet track? And if the pickup plate is segemented,

will the polarity at any outlet from the bridge depend on which end of the bridge is connected to it or not?

I've asked the same of Customer Care. IT will be interesting to see their reply.

[Blackbird]

Hi Fishmanoz,
You can't avoid a 'short circuit' via loco bridging if the outlet connected to the main is reversed. This is where planning (checking) is employed to avoid it.
Half of the outlet track feeds are automatically reversed by the bridge

rotation. Without going back to a T/T and checking, I can't now say exactly where the division is, but approximatly, the opposite side to the live feed input track. A quick check with a multimeter will confirm. Given the T/T configuration, it is unlikely that

you will have a track with opposite polarity (same side as input) connected to the T/T.

[Fishmanoz]

Blackbird said:

Hi Fishmanoz,
You can't avoid a 'short circuit' via loco bridging if the outlet connected to the main is reversed. This is where planning (checking) is employed to avoid it.
Half of the outlet track feeds are automatically

reversed by the bridge rotation. Without going back to a T/T and checking, I can't now say exactly where the division is, but approximatly, the opposite side to the live feed input track. A quick check with a multimeter will confirm. Given the T/T configuration,

it is unlikely that you will have a track with opposite polarity (same side as input) connected to the T/T.

[Fishmanoz]

Sorry for the blank Blackbird and thanks again. My proposed layout at present has connections of inlet, 3 dead end outlets and a connection to a reverse polarity outlet going elsewhere in the layout, all on adjacent tracks and so within 90 degrees. It's

easy though - don't connect power to the bridge via the inlet track but do it directly either on the connections or the split ring contacts. Then you can move the TT around as you wish until you get the correct polarity on the desired outlets - would only

be a problem this way if different polarities are interspersed with each other.

By the way, Customer Care said in part:

You are correct in thinking that the bridge receives power via the contacts under the inlet rail. The turntable reverses its

polarity at 180 degrees, I think you will find the website below very useful in explaining the turntable and DCC:

http://www.brian-lambert.co.uk/DCC.htm#Turntables .

The Brian Lambert site has a good explnantion with diagrams.

[Blackbird]

Hi Fishmanoz, Sorry for delay in reply.
Feeding the T/T bridge directly from the DCC main (supply) would negate the use of a DCC T/T track adaptor, however, all of the outlets that are connected to the DCC supply will need both rails isolating to prevent

a short circuit. The short circuit occurs when the RH rail on the T/T bridge makes contact with the LH rail (both permanently live with DCC) via the bridge contacts. As the segmented pickup plate gives automatic changeover to the half of the outlets on the

opposite side of the T/T, I don't think that a reverse loop module will be needed for your proposed configuration. To confirm, check electrical continuity with a test meter.
The T/T DCC track adaptor was designed as a 'simple' conversion of the basic T/T

for DCC use.

[Fishmanoz]

Thanks Blackbird, I think I get all of that now. Yes didn't intend to use the inlet, just appropriate polarity outlets with isolators. Brian Lambert shows all tracks with isolators too.

[davawt]

Hi there
I am new to DCC and TTs,
I have just bought a Hornby R070 TT and a decoder, I am using DCC to run my trains at the moment, can I avoid the messing by only using 1/3ed of the TT outlets I will be using (1) input and three outputs all from

the same side with just one space between the input and the first output, followed by the other two out puts. Would this save me any removing of the TT table contacts. The TT will only ever turn about 1/3rd of a circut then back again.
Thanks for your time,

take care, bye
Dave

[davawt]

davawt said:

Hi there
I am new to DCC and TTs,
I have just bought a Hornby R070 TT and a decoder, I am using DCC to run my trains at the moment, can I avoid the messing by only using 1/3ed of the TT outlets I will be using (1) input

and three outputs all from the same side with just one space between the input and the first output, followed by the other two out puts. Would this save me any removing of the TT table contacts. The TT will only ever turn about 1/3rd of a circut then back

again.
Thanks for your time, take care, bye
Dave

[davawt]

Hi guys
I am now more upto date.... thanks to a web search...
DCC is (always live) so the contacts on the TT table short as they pass the tracks.
Only way is to remove table contacts and feed power to all oulets.
Problem solved....
Take

care, bye
Dave

[Fishmanoz]

Hi Dave, you can certainly remove the TT contacts you need to but I and many others suggest you don't. Instead, you can use isolating fishplates on the TT entry tracks, then you have to power each track as you say. Also, I suggest the input track is not

used to run locos, just as the way to power the bridge. this will work fine and you don't have to butcher the TT. The only precaution you need to take is to not run or stand locos across the isolating fishplates and onto the entry tracks while you are rotating

the TT.
Someone on this forum, can't remember who, also produced a modified isolated entry track - you'll find detail by doing a forum search, I did - but I believe they are no longer available.

[davawt]

Hi Fishmanoz
Would I follow this info even if I was using a DCC decoder (loco type) to turn the TT?
Many thanks for the info, take care, bye
Dave

[Fishmanoz]

Post a new reply...

[Fishmanoz]

Hi Dave, the answer is definitely yes. The TT motor is an independent circuit not connected to track power in the first instance, so you can power it anyway you like, including via a decoder.
Once you connect it via a decoder, it is of course connected

to track (DCC) power, just like your DCC locos and accessory decoders, and only responds when addressed.

[davawt]

Fishmanoz said:

Hi Dave, the answer is definitely yes. The TT motor is an independent circuit not connected to track power in the first instance, so you can power it anyway you like, including via a decoder.
Once you connect it via a decoder,

it is of course connected to track (DCC) power, just like your DCC locos and accessory decoders, and only responds when addressed.

Thanks again Fishmanoz
I am getting the parts this week - R070 TT, Decoder, Wires.
Hopefully up and running

by the weekend.
I am so glad I found this forum and your help.
Take care, bye
Dave

[davawt]

Post a new reply...

[davawt]

Well guys I am now running my TT and my trains (HO) all by DCC.
Bit of solder here and there but mostly just a bit of trail and error and it works.
Thanks for all the help from here and other forums.
Take care, by
Dave

[Fishmanoz]

Well done Dave.