Friday, 30 January 2009

Zimo "stay alive" capacitors

Zimo offer the option of adding "stay alive" capacitors to their chips, which will keep the chip alive when the loco is not connected to the track. This can be a bit of dirt or similar getting in the way. Its a cheaper DIY equivalent to the Lenz "UPS" add-on for Lenz Gold decoders. Lenz use "superCaps" which have massive capacitance and can keep a loco running for several seconds, but superCaps are expensive and need extra circuits to work on 12v.

I have a Simplex in 7mm scale, from an old Impetus kit. Its rigid underframe, 4 wheels, very short wheelbase. As its rigid, there is a fair chance only three wheels are on the track at any moment in time, so its a bit sensitive to tiny bits of dirt.

As there is lots of space inside, I have tried a 4700uF capacitor connected to the Zimo MX63, with the additional diode, resistors and choke recommended by Zimo. This seems to keep the wheels running for about 1/4 of a second before the chip decides there is insufficient power for the motor. The chip stays alive for about 12 seconds before the capacitor has discharged through a resistor. This is a massive improvement in practical running - if a loco has bad pickup for 1/4 second its not going to run at all. It will sail over turnouts where it previously would stutter or stall. Obviously 1/4 second doesn't allow silly stunts, such as running along a workbench, but practical considerations suggest that is an easy way to break a loco !

4700uF is a big lump for anything other than 7mm scale. In smaller stuff, I've tried 100uF and 220uF capacitors and it does help even if its only keeping things alive for tiny amounts of time.

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.