Turn table

[andel1]

Hi all

I'm using a loco decoder on my turn table but would like to use an accessory decoder but they all seem to use three wires so what would be the best way to do it 

[Chrissaf]

The simple answer is you can't.......

The R8247 Accessory Decoder is not designed to operate a TT

There are some 'non Hornby' decoders designed specifically for TTs, but in the Hornby world you're doing it the only way you can. The 'loco' decoder gives you fine motor control granularity. The R8247 provides either a pulse or a permanent on voltage that would result in little or no control of the TT position. Also, the R8247 voltage output is not reversible unlike a 'Loco' decoder so you wouldn't be able to drive the TT in the other direction using it.

[andel1]

Thanks Chris  greatly appreciated 

[96RAF]

@Chris

I hadn't really thought about polarity coming out of an R8247 being both the same way round when the port is set to always on, but logically common © is always common and is fed via the signal-light-etc by two positives from the (+) and (-) ports.

 

Every day another little light comes on....thanks.

[Fishmanoz]

It would be possible to design a TT to operate with an accessory decoder, bi-polar type like the 8247.  You would need a diode arrangement on the +ve and -ve outputs such that the polarity to the motor would reverse dependent on which direction you selected, and you would have to set the dwell time of the decoder to the track transit time of the TT.  And the motor would have to operate within the continuous current limit of the decoder.  

 

But the R070 is very old and designed for DC so it doesn't have and of this.  So the simple workaround and only solution is to work it as a loco with a loco decoder.  Also remember if using RM to operate it to set the motor speed and dwell time correctly in your ini file so the motor stops after it has moved on one track position.

 

It would be pretty simple for anyone with electronics knowledge to make this mod.  Then if operating with RM, page 93 tells you how to set up TTs operating either with loco or accessory decoders.  Not surprising as other brands designed for DCC operate via accessory decoders.

[Chrissaf]

You would need a diode arrangement on the +ve and -ve outputs such that the polarity to the motor would reverse dependent on which direction you selected.

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I gave this some thought a fair while ago, having sat down with a pad of paper and a pencil. It was like the 'krypton factor' (an insolvable puzzle). After an hour of trying, I came to the conclusion that whilst the positive output was fixed on the single 'C' terminal. It was impossible to perform this voltage reversal trick based purely on diodes without resorting to transistors and/or ICs. If anybody can post a doable diode based schematic then I would be interested to see it.

 

[Rog RJ]

There is a very simple circuit that only uses two resistors that enables Tortoise point motors to be used with Lenz accessory decoders that might work with Hornby accessory decoders and turntable here -  http://www.rmweb.co.uk/forum/viewtopic.php?f=10&t=46494

[Rog RJ]

There is a very simple circuit that only uses two resistors that enables Tortoise point motors to be used with Lenz accessory decoders that might work with Hornby accessory decoders and turntable here -  http://www.rmweb.co.uk/forum/viewtopic.php?f=10&t=46494

 

Edit.  The website may be slow to respond

[96RAF]

@Rog

The forum confirms it works, so I suppose it relies on the + and - terminals never being energised together and presumably relying on a diode inside the acc decoder behind those terminals to prevent a back short.http://www.rmweb.co.uk/forum/download/file.php?id=84929

[96RAF]

I posted that picture from there onto here Rog but its held moderation at the moment.

[Chrissaf]

I had a look at that circuit. Interesting concept. However there is one major difference between a Tortoise point motor and a TT motor. The Tortise is a low current stall motor. Thus when you continue to supply voltage the motor will just harmlessly stall when the point is fully switched over drawing a low current until you reverse the direction of the decoder output.

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In the case of the TT motor, this motor needs a third 'power off' condition, the other two conditions being 'drive TT clockwise' and 'drive TT anti-clockwise'. The circuit with the two resistors may provide the drive clockwise and anti-clockwise condition, but there is NO third 'power off' condition. Thus if you tried to implement this circuit for a TT it will never be stationary it will always be on the move, either clockwise or anti-clockwise depending upon which way the R8247 is switched.

[Chrissaf]

PS - wish the edit button was functional.

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Just realised that not having a 'power off' option would also make the R8247 unsuitable for a TT, even if there was a 'diode logic current reversal circuit' that could reverse the voltage across the TT motor. You need a 'power off' condition to stop the TT. The Hornby recommended 'loco' decoder option has the 'power off' capability AS WELL as the drive left and drive right AND drive slow and drive fast.

[Fishmanoz]

Isn't an 8247 only on for its dwell time?

[Chrissaf]

Yes for a pulse. But if configured for 'always on' then it would be permanent.

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I do however see where you are going with the question. Set the 'pulse' dwell time just long enough to move to the next TT inlet/outlet.

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But the dwell time would have to be extremely accurate else I would imagine a loss of alignment synchronisation would be very easy to achieve. Then if synchronisation was lost (almost inevitably in my view) there would be very little scope to reset the alignment using the R8247, it would have to be done physically by hand and risk damaging the gears.

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This thread is an interesting discussion exercise but I still think in the circumstances, using the Hornby recommended 'loco' decoder is the best all round solution.