Elite Manual - Warning about decoder damage.

[Chrissaf]

Hello, I'm new to the forum and this is my first post. I have taken the plunge and 'migrated' from my ageing Zero 1 to an Elite based system which is most definitely a far more comprehensive product than Zero 1, which did, however, serve me well within its limitations. I have tried searching this forum and elsewhere on the www to answer a concern I have about a warning in the Elite manual which says

DO NOT REMOVE THE LOCO FROM THE TRACK WHILE IT IS STILL RUNNING. FAILURE TO DO THIS MAY DAMAGE THE LOCOMOTIVE DECODER.

Now, whilst I would not be purposely removing a running loco from the track, derailments are bound to occur which would replicate the scenario in the warning. Is there a real risk that decoder damage may occur? If so is the damage likely to be physical such as component failure or maybe a corruption of set up parameters that could be rectified by recoding? Thanks in advance for any light DCC experienced members can shed on my concerns.

[Chrissaf]

Anything that has the capability to produce a power surge has the capability to potentially damage a decoder. Hence the term 'MAY' in the quoted statement.

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With DCC, the track voltage is typically 28 volts peak to peak. A 'short circuit' from running against incorrect point positions or sparks that could be generated when lifting a loco drawing traction current i.e still moving under electric motor power but lifted off the track. Can cause 'ringing' of the 7Khz DCC track signal. Ringing is a form of positive feedback that can cause the 28 volt peak to peak track voltage to magnify through oscillation to more than 2 or 3 times its nominal value. Once the ringing exceeds about 60 volts 'peak to peak' you are in decoder component break down failure territory. Even if the failure is only just enough to cause the decoder to lose configuration (needing a factory reset to recover from the corruption), components can still be weakened such that the decoder then becomes extremely sensitive to even minimal shorts that only produce low levels of ringing.

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This example DCC track voltage scope image below, shows a DCC signal with  'ringing' on the leading edge of the digital signal pulse. The ringing is the tall spike highlighted in yellow. Note also the other yellow highlight that states that the measured 'peak to peak' voltage of this ringing DCC waveform is 69.6 volts.

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You can mitigate the effect of 'ringing' by fitting a DCC 'snubber' across your DCC track signal. A snubber can be easily made from a 0.1uF Ceramic capacitor (50 volt rated or better) in series with a 2W 100 Ohm resistor. See example 'snubber' in image below. Note that this resistor will get warm to the touch, this is normal. The other thing to note, is that the 'snubber' will draw about 100mA continually from your DCC controller power supply. This may impact on the number of DCC locos you can run simultaneously:

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TIP: As this is your 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, scroll down and write your reply in the reply text box at the bottom of the page and click the Green 'Reply' button.

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Particularly as my reply includes an image. If you use the blue button, any reply you write, may be held back for image approval. Even though it is already a previously published image.

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[96RAF]

Welcome

A lot depends upon the robustness of the decoder. Some take badly to the spike they may see during a derail, and others never notice.

 

The effects of a derail can be as simple as a decoder losing its address and requiring a readdress to a complete fryup requiring a new decoder but this latter event is very rare. Quote often you just rerail and carry on.

 

Likely Hornby just covering their butt.

 

The R8247s shown on Chris’s diagram are accessory decoders as he has pinched his drawing from another of this excellent explanations. The track bus would normally just be connected to your Elite.

[Seacommander]

Thanks Chrissaf and RAF96. I did wonder whether the risk was something to do with the possibility of voltage spikes generated when a circuit is suddenly interrupted. As it is very unlikely that I will ever approach the 3-4 amp track current draw available from the Elite I think I can easily afford the 100ma draw from a snubber. Also, I would expect/hope that Hornby took into account the potential risk in the design phase of the decoders and that RAF96's comment about 'butt covering'' is closer to the reality of any damage occuring in the vast majority of cases. Thanks again.

[Chrissaf]

Not a loco decoder, but Hornby R8247 Accessory Decoders are notorious for being damaged by 'short circuit' induced ringing. Rob was correct, my snubber drawing was taken from a previous post of mine about R8247's being damaged and/or configurations corrupted due to shorts on the track. Hence why there is T take off point right next to the snubber for the feed to the R8247s. Since fitting my snubber in 2013....I have never had a 'short' induced ringing issue since.

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PS - I fitted my snubber because one of my three R8247s was permanently weakened after a short, such that any short, no matter how slight. Caused my R8247 to lose configuration and needed to be 'factory reset' before it would accept the DCC address being put back into it. This errant R8247 was subsequently replaced with a new one.

[96RAF]

That is one very good reason for having your points if DCC operated on a separate bus. Mine are/were like this and never had a problem as any shorts on track never affected them.