'Stay Alive' problem

[EnGee]

Hi All

 

I am quite new to model railways so apologise if this has already been asked. I have done lots of research but have been unable to find any reference to this problem. Well it's more of an annoyance than a major problem but I'm hoping someone can point me in the right direction.

 

I have the Mixed Freight DCC fitted set with some extra track. I have fitted the 'Stay Alive' system as described and explained excellently by Chrissaf. It works fantastically, so thanks very much Chrissaf. Don't see why they are not all fitted as standard by the manufacturer, but that's probably another story!!

 

Anyway, it all works perfectly OK during the day, trains start from anywhere, trains run slowly over points, etc. The problem arises when I switch off the power to the controller at the end of the day. About half a second after switching off, both trains jump forward about an inch. Not a major problem unless one of trains happens to be at the end of a track and starts headbutting the loft wall!

 

So I have a theory. When the track power is switched off the 'Stay Alive' cap will supply the decoder with its' required voltage for about half a second. After that the decoder gives up and shuts down and then any excess voltage left in the cap is fed directly to the track which makes the trains jump forward until the cap is fully discharged.

 

So my questions -

Is my theory correct or am I just talking rubbish!?

Is there any way this unwanted lunge can be avoided?

 

Thanks for any help.

[Fishmanoz]

I suspect you are sort of right NG.  I think that what is happening is that the stay-alive is maintaining volts in the decoder as it is still charged (forget about the track being involved) and the decoder reads this DC, goes into DC mode and moves off until the capacitor is discharged.

 

You can stop this by turning off DC Running in CV29 of your decoders, but not if you are still using the Select as your controller (Select cannot adjust CVs, at least not until a firmware update expected later this year).  Do you have a local model shop, friend or club near you who can do the adjustment for you?

[Chrissaf]

Thank you for your vote of confidence in my previous posts regarding 'Stay Alives' Since this is your first published post, then you have been what I would term a 'secret admirer' of my forum work. That is to say, taking it all in with little contribution. No issue with that at all, but sometimes getting some positive feedback at the time when it was appropriate can be nice.

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Anyway back to your question. I agree entirely with Fishy above. When you turn off the DCC signal, the decoder then only sees the DC voltage of the 'stay alive' capacitor. The decoder thinks it is then on a DC Analogue controller and responds appropriately in the manner you have described. The 'Stay Alive' is discharging directly into the decoder....not the track. As Fishy says, you need a DCC controller that can amend CV29 to 'turn off' this behaviour.

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The Mixed Freight Set comes with a Select Controller that cannot currently amend CV29. However if you can live a little longer with this issue. The Select is due a major firmware upgrade this coming Autumn (about September / October) that will allow you to 'Read and Write' CVs including CV29. The Select upgrade will be a 'return to Hornby' upgrade. Pricing for it has not been announced. Up until now Select firmware upgrades have been priced at £15. However, this version 1.6 upgrade is significantly changing the Select product. The upgrade will also make the Select compatible with RailMaster via a new 'Select a Link' cable. So I would expect the 1.6 upgrade price to be set accordingly.

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Since you have a Select, the loco addresses you are using have to be in the range 01 to 59. This means that your current value of CV29 in your decoders is almost certainly a value of 6. To turn off this DC behaviour, you need to change the value of CV29 to 2.

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As this is your very first post I offer you this posting tip.

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TIP: As this is your very first post, just be aware that the 'Blue Button with the White Arrow' is not a 'Reply to this post' button. If you want to reply to any of the posts, scroll down and write your reply in the reply text box at the bottom of the page and click the Green 'Reply' button.

[EnGee]

Thank you both for your answers. I actually meant to write 'motor' not 'track' in my original post, just a senior moment! I knew it had nothing to do with the track as I got exactly the same effect when picking up the train, which could be slightly alarming! Anyway glad to see I was sort of on the right track (no pun intended!)

So CV29 it is then. Just need to work out how to do it.

Thanks again.

[pd91223]

There may be a local model railway club or model shop that would be prepared to do this for you. Alternatively, there may be a forum member who lives near you? Where are you based?

[EnGee]

Thanks for the suggestion pd. I am based in Newbury and do know there is a Newbury mrc. I must get round to joining!

 

Thanks again.

[Flashbang]

You could try a 1M0 (1,000,000 Ohm) or higher value resistor connected directly across the SA capacitor. This should remove the charge but not effect the operation.

[EnGee]

Flashbang:

Erm... an interesting thought, I too came up this idea but discarded it so would be interested in your logic.

 

I decided that putting a resistor across the cap would merely reduce the time before the decoder run out of power, at which point the remaining power in the cap would revert straight to the motor as before. The amount of current that a 1meg resistor would use, compared to the motor, would be negligable. Therefore I believe the train would still 'lunge' forward as before.

 

I have not tried this in practice and it is only a theory so happy to be proved wrong.

Anyone else any thoughts on this idea?

 

Thanks for everyones help.

[Chrissaf]

I think it is a 'catch 22' situation. You need a relatively low value resistor to discharge the capacitor before the decoder can react when the DCC track power is switched off, but a high enough value so that you don't leak away significant charge from the capacitor during normal operation reducing its effectiveness. What I call 'leaky bucket' syndrome. Like you (NG), I would suspect that 1MegOhm would not discharge the cap fast enough to prevent 'power off' lunge. I would also suspect that a value derived at through experimentation where the 'power off lunge' issue was then resolved, would result in a resistor value that was too low and created a 'leaky bucket' scenario affecting SA performance.

[EnGee]

Thanks Chris, my thoughts exactly. So CV29 it is then!

[Chrissaf]

NG,

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I bring your attention to a new thread started today. It would appear that requests to upgrade the Select firmware are now being implemented with the 'yet to be officially launched' version 1.6 firmware.

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https://www.hornby.com/uk-en/forum/select-16-is-here/?p=1

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Just to remind you that Select version 1.6 gives you the ability to amend CV29. The main issue I see is that the 'user interface' will be completely different in V1.6 and not covered in any of the current Select user manual documentation downloads. It would appear that Hornby include some limited documentation with the returned upgraded Select, but it may not cover in detail how to use it. Rob (RAF96) could give guidance on that aspect as he has seen an early release of version 1.6.

[EnGee]

All sorted (well, nearly all, see new thread!),  Select upgraded and CV29 changed to "2" so no more 'lurching', and thank you all for your help.

 

The documentation that came back with it was OK and I was able to work out how to change things fairly easily thanks to the info in this thread BUT where is this information? How on earth does a 'normal' person know that CV29 should be set to 2? Is there a list of what each CV does?

[Chrissaf]

How on earth does a 'normal' person know that CV29 should be set to 2? Is there a list of what each CV does?

 

To save others responding unnecessarily to EnGee's question above. It has been fully answered to EnGee's satisfaction in his other posted CV thread.

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The answer can be viewed in my reply timed at 16:25 in this other thread:

https://www.hornby.com/uk-en/forum/post/view/topic_id/28229/?p=1

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