John's Jottings

159. Electrickery - September 2019

Posted on August 31, 2019 at 10:35 AM

One of the members of the Barton Bridge club was adamant: Cab control with common return just did not work. From many years’ experience he has concluded it was the cause of all sorts of electrical problems. If a controller failed, it had to be because the system used common return, even if the cause was actually a broken wire. If the electric light signals operated incorrectly, it had to be a direct result of using common return, even if the reason was a fault in a relay that was powered by a transformer unconnected to any of the traction circuits.

Another of the club’s members, who always installed common-return wiring, had drawn out beautifully coloured circuit diagrams showing all the possible combinations of polarity. But they didn’t persuade him. No amount of explanation would convince the sceptic otherwise

He seemed particularly concerned that parts of the return wires could be carrying 24 volts – the combined voltage of two 12 volts controllers working in parallel. Would the insulation of the wires withstand such a voltage?

“But surely supplies in parallel increase the current, not the voltage,” Graham commented. When challenged, it seems the fellow worried about that as well.

“I don’t want my wiring getting hot and starting a fire underneath a baseboard,” he explained with genuine concern.

“You just use a slightly heavier duty wire for the return wire,” Graham advised. “But unless you are using really ancient motors in your locos, or operating locos larger than O-gauge, there should be no problem these days. After all, DCC puts a roughly 20 volt supply into the track with up to 5 amps, and normal layout wire is adequate for their ‘bus’ wires.”

The fact that for over five decades, a great many layouts have successfully operated with common return cab control was not sufficient evidence to make the disbeliever even consider changing his mind. He was wedded to the concept: One function - two wires. Always.

His own layouts had two-way section switches on their control panels, one connected to each controller, but there were always two wires going out to each section: a feed and a return. This increased both the length of wiring and the number of terminal blocks he required, and hence the costs incurred. But it was a price the fellow was prepared to pay.

“Simplicity of the wiring concept leads to easier fault-finding,” he proclaimed. “If a section goes down, then there are just two wires to examine. If a common return wire is looped right round the baseboards, you don’t know where to start looking for the break, do you?”

“Just bridging each of the rail breaks in turn with the tip of a metal screwdriver will quickly tell you if the problem is with the feed or the return rail,” Graham advised. “Then there’s only the continuity of one wire to examine. Surely that simplifies fault-finding even more, doesn’t it?”

“Doesn’t every modeller have his own way of designing wiring?” our chairman suggested. “There are some basic rules imposed by the very nature of electricity, but how they are implemented is up to the individual modeller. Isn’t it all a matter of the level of technical understanding, how well he can cope with setting out and reading complex wiring-diagrams, and the mechanics of installation that determines each modellers’ style of wiring? In the end there’s seldom just one right way.” And we all agreed with that analysis.

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