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4QD controllers
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The same scheme can, in principle, be used with other controllers, provided they are reasonably matched, but 4QD cannot give advise on this.
It should be noted that our VTX and Pro-120 do include, as an option, an expansion connector to simplify double heading, so you should use this with these controllers.
Both controllers are wired normally, but an extra connector is fitted to each so that pins B to F may be linked for double heading. Pin A is effectively battery +ve to feed ignition and reverse, so does not need to be linked.
With this arrangement the controls may be plugged into either loco A or Loco B. Do not plug in two sets of controls, or the result will be confusing - though unlikely to do any damage if the controls are wired properly.
Note the fuse in the pot zero line. This is optional but if this is not fitted, or is too high a value, there is a risk of blowing the earth fuse track(s) on the controllers, depending on how the rest of the machine is wired, or what wiring faults occur.
Consider the situation where the battery negative connections are both earthed to chassis - this may be intentional, or it may be through the bodies of such items as horns and lighting. Now consider that the locos are working - and there is a poor connection to one controller's battery -ve wire.
The controller's negative current will now try and find its way to B- but it will flow through the pot zero link between the controllers and back through the connected chassis. Since the pot earth is a low current connection - something has to fuse! Best make it a fuse fitted for that purpose! Its rating is not important but I suggest the smallest fuse you can easily get which is likely to be somewhere around 500mA.
See Earth fuse: why it blows for more on this.