DCC Loco Conversions - 8 - Decoders & Me

Background

When I first considered using DCC for driving my trains I knew absolutely nothing, now after 9 years of fun I know enough to retain some sanity in the old grey matter and have some pleasure out of the result, well, most of the time!

After reading an introductory book on the subject and considered what I wanted to do with DCC I looked around and decided on a Lenz Digital+ system. Back in 2012 there was little available that would include the potential for wifi based control for wireless walk around for UK use and German engineering was a plus in my book even though it commands higher prices.

Okay, with a system selected to suit me, you will need to specify your own requirements and find a system that suits you, I decided to start off in a straightforward way so I bought some Lenz decoders. So no system vs decoder mismatches to fog any issues I might have.

I was aware that although system components generally had to be from the same manufacturer, decoders could, for core functionality be from one of many different manufacturers.

My Specification for Decoders

It soon became obvious that just using Lenz decoders for minimal functionality was an overkill, and not cost effective, my specific needs were easily summarised as :-

1) 4 digit addressing to utilise the lower 4 digits of a loco's running number as the DCC id/address as much as possible.
2) BEMF (Ref 8.1) for setting a speed and allowing the decoder to adjust motor voltage to suit prevailing conditions, e.g. gradients.
3) Basic speed control, start, mid and max to constrain loco performance to a realistic maximum and use the available speed steps effectively, 28 steps my preference.
4) Acceleration and deceleration settings, to add some realism to handling.
5) Two function outputs for loco head codes where fitted with illumination.

Back then not all decoders catered for 4 digit addressing and BEMF is a facility I needed for two reasons :-

1) Much improved low speed control particularly with pre-DCC locos for shunting.
2) The mainline is a folded figure of 8 (Ref 8.2) with 1:48 gradients so BEMF allows what are almost dynamic background trains to run without operator intervention on the grades.

It is important to decide on the parameters that matter to you, including whether sound is required before selecting a family of decoder models. I have a wide range of DCC ready and pre-DCC locomotives and certainly no one family of decoders suits all locos for reasons I shall illustrate in this series of articles.

As a reminder I do not fit sound to my locos so I will not being saying much about it except space becomes even more critical as where to install the speaker. Also on many pre-DCC locos of the Tri-ang and British Trix era the mechanisms are so loud, be it clunky 3 pole motors cogging at low speed, teeth clatter from coarse cut metal gear trains, worn rattling gearbox bearings, serrated wheel rumble or the noise amplification that comes from both plastic and metal loco bodies, any sound effects could be drowned out.

Next: Tools & Materials.

Jim

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References

8.1 BEMF - Back Electro-motive Force

This is well explained here from an overview to the highly technical :-
https://dccwiki.com/Back_EMF

8.2 Realising a Dream - 15 - Folded Eight

My mainline track configuration for watching the trains go by:-
https://platform1mrc.com/p1mrc/index.php?threads/realising-a-dream-15-folded-eight.5581/

 

Hi Paul, thanks for your input.

I covered these issues in my "Is it Worth the Effort - Part 4 - 14.3", but a reminder to members is worthwhile.

Long ago I stopped doing a stall test because a well serviced X04 motor (and chassis) or it's contemporaries can be as low as 0.25A rising to maybe 0.5A worst case before wheel slipping occurs, which despite being something like 10x the current demanded by modern locos means that I rarely fit a 2A peak decoder, 1A is usually adequate. These older Tri-ang locos also relied on magnetic grip on steel rail, (however I would not like to use DCC with steel rail, can be fun enough on nickel silver rail), so they are more likely to slip sooner on NS rail anyway.

Ventilation of decoders in an old loco is also easier as there is often plenty of space.

I always check motor current before fitting a decoder, excess current in vintage locos is often due to an oil sodden commutator, heavy carbon deposits in the commutator slots or due to magnetic field collapse, and occasionally a partially burnt out winding due to the latter.

In my experience a complete stall is only likely in the event of a valve gear jam, by e.g. a screw being lost or quartering issues due to a wheel slip on an axle, then that is often very noticeable and very rare, so as long as the current taken under load stays below 70% of the decoders continuous rating all is well.

Valve gear lock is very rare with these battleship builds, earlier Hornby 9Fs crude but trouble free, meanwhile I have had two Railroad 9Fs which just destroyed their valve gears for reasons unknown.

Being too cautious on ratings or using premium short circuit protected decoders can result in excessively large, and expensive decoders which may not fit.

However if you are fitting expensive sound decoders then you do need to be very much more cautious, but fitting those to vintage locos seems to be wasteful as the loco mechanisms are often very noisy for reasons I gave above.

Jim