Saturday, 10 January 2009

Commercially wound coils for DCC controlled couplings

The biggest drawback with the DCC controlled couplings I've been experimenting with is the need to wind coils. Its time consuming and difficult work (I need a close up TV camera and it takes over an hour of continuous concentration to wind a single coil at the lathe). So, the hunt has been on for ready to use coils.

( Fanfare....)

Plantraco Microflight of Canada sell a "Nano Actuator Kit", which is a 3.8mm dia coil and a couple of tiny magnets. Not only does this save winding ones own, but it comes with a few neat side-effect benefits. When I enquired, it was 10 coils minimum for shipping to the UK, but I recommend checking before ordering.

The coil is 75 ohm and designed for 4.2v. So, two in series is 150ohm and nominally 8.4v. Bench tests suggest they are fine with two in series driven from the 12v of a standard DCC function output for durations of up to 5 seconds (I think they'd be fine for longer, just not tried). Alternatively, they also seem fine on half-wave (using a track pickup for the positive supply rather than blue wire). Compared to my home-wound coils which all needed current limiting resistors, this is a big benefit; no longer need to find space to hide a resistor.

If really worried a 56ohm or 68ohm resistor in series with a pair of coils would bring the current back down to within the maker's specification.

Having an internal diameter of 3mm, they will slide fit over brass tube of 3mm dia (the tube needed a tiny amount taking off with abrasive paper), this makes mounting easy, and also the construction of a hinge pivot quite simple, see the diagram below.

The brass tube is cross-drilled to take the shaft (0.35mm dia), and then the "ears" carefully filed to shape. The shaft is inserted, and fixed to the screwhead (solder/glue to choice), and then the magnet glued to the back of the screwhead (if the screw is steel, the magnet self-sticks!). Finally, the coil is slid into position - ideally a bit further towards the pivot that the diagram shows.
In deciding the shape of the "screwhead" part, give thought to whatever counterweight is necessary to return the coupling to its rest position - a piece of 2mm square bar with a slit might be a better shape in some cases, in others the counterweight needs to be away from the shaft movement.

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