DCC and Live chassis

[The Fatadder]

Picking up on a recurring topic from Tony Wright’s thread about the additional risk to decoders if using a live chassis on a steam locomotive and DCC, I am very interested to hear the experiences from those who have actually gone down this route.

Especially the best practices in order to minimise the risk (or if there are specific decoder choices which are less sensitive to short circuits.)

 

I have a PDK 47xx which is nearing completion, with the kit purchased 2nd hand containing one half set of non-insulated wheels. The kit has a resin boiler so has minimal risk of motor shorts, along with a safe place to fit the decoder.

 

I have never made a loco with a live chassis before, so not really sure what to expect. While I could just buy more insulated wheels, I like the ideal of the simplicity of the live chassis, (and not buying the new wheels will pay for the decoder)

 

Thanks in advance

Rich

[RAF96]

Some of the Ringfields used the motor housing as a chassis and live feed to one brush and they are easily made ‘safe’.

 

It all boils down to isolating the motor brushes from the wheel pickups. Once you have this ‘gap’ then insert the decoder.

 

It doesnt matter if the current path from the wheels is a wire or a live chassis as long as you can isolate the motor brushes from this path.

 

Physical insulation of the decoder against shorts follows standard practice, i.e. don’t let it touch metal - live or not.

 

Rob

Edited February 8, 2018 by RAFHAAA96

[Phil S]

IF you have metal couplings - as used to be common - which are not electrically isolated from the chassis - they weren't normally - THEN when 2 locos come into contact on the track  ( a permitted move with dcc to reproduce banking, piloting or simply an engine shed area ) you will have a 50% chance of getting a short circuit across the track - as the 2 locos could be live to different rails.

 

This is most likely therefore to apply to older locos - Hornby Dublo or Kitbuilt  but also many Triang / Triang Hornby chassis where the Mk3 (or earlier) metal version of the coupling allows the metalwork of adjacent locos to touch.  One subtle reason for manufacturers changing to plastic couplings once Zero-1 was launched!

 

Strictly - as the short circuit current flow would be rail- chassis - other chassis - other rail, it should not have much, if any more than normal, current flowing THROUGH either decoder  (assuming the motor is fully isolated from the chassis - as it should be) BUT IF YOU HAVE LIGHTING which uses the return path through the chassis (as 6-pin decoders do) - then you may still have unexpected problems.    

[The Fatadder]

I hadn’t thought about the coupling side of things, however my intention is to try and isolate the brass body from the frames. The model is fitted with screw couplings soldered to the bufferbeam which would potentially be live. Likewise the (metal) buffers are also soldered into place and would preove a further risk.

 

The layout does have a fair amount of moves with an assisting engine, so will bare this in mind

[Crosland]

I haven't read the other thread (link?) so I don't know what problems are being alluded to, but there's no inherent, nor additional, problem with live chassis if everything is correctly isolated as it should be.

 

Some decoders are wrapped in heatshrink, other you can sit in a little plasticard tray, for example.