Plantraco Coils in L&Y 0-4-0 loco

The pictures in this post illustrate the use of the commercial Plantraco coils inside a scalefour L&Y electric loco (another High Level kit). I've used two slightly different arrangements, partly because of interior clearance issues, and partly to try out the options.

The first shows the coil mounted on a cylinder. This proved to be fairly troublesome to make, and I probably won't repeat the exercise.

The second shows a more normal "hinged flap" arrangement, similar to that on my 2mm scale Class 04. This was much simpler to build, and is the basis of a prototype etched component. The "nose" sticking out is a piece of lead which ensures the flap falls when the current is turned off, and the horizontal wire at the top limits rotation to 90 degrees.

The loco isn't finished yet; lots of details to add, and I want to make sprung buffers and other extras which require work on the lathe. So video in operation will be a while coming. But it does all work, the couplers are wired in series to a single function output of a Zimo MX620 chip. The chip is in a paper sleeve below the motor. Both couplers rotate 90 degrees, which is far more than I need to release an Alex Jackson. The Alex Jackson's will be fitted onto the outputs of the mechanisms with a bit of fine bore tube.

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.

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.

Coronation class with vertical movement DCC controlled AJ's

The latest from the retrofitting workshop; a High Level Coronation class 0-4-0ST with vertical movement AJ's.

I fitted vertical movement rather than the rotary used in the Armstrong and the 2mm scale 04 because of various space constraints. Primarily, the relatively low footplate height, plus existing underframe components would have made a "through buffer beam" rotating coupling difficult to fit. In addition, I wanted to try out a vertical (normal) movement AJ.

The plan with the vertical movement had been to use the iron core of the coil to hold a permenant magnet in the "closed" position, and energising the coil would repell the magnet to the "open" position.



The main lessons :-

• magnetic attraction back to rest is possible, but didn't quite work in this case due to lack of clearance around an existing brake shaft on the model. The iron core in the coil is too far from the magnets on the flap to pull the coupling back to rest, hence the need to add a counterweight (swings in space near gearbox).


• the power required to pull an AJ down vertically is quite large, there is considerable drag as the tail of the coupling disengages due to the horizontal spring strength in the couplings. So, a more powerful mechanism is required than rotating. This required bigger magnets, and thus created the clearance problems in the first point above.


• operationally the movement is better than rotation. The loco can be programmed to backup and uncouple, but does not need to have "move forward" in the programmed sequence. Therefore the movement is more prototypical, giving the crew time to disconnect the coupling before pulling away.

The DCC chip is a CT Elektronik DCX75, mounted in the bottom of the boiler underneath the motor. The coil is about 35ohms, and a further 33ohm series resistor is used to limit the current to the coil. The coil is wired to "half wave" (ie. to a track pickup rather than decoder blue), the DCX75 does not have a blue wire. Movement control is via various CV's which CT provide for couplings, the JMRI definitions file for this is available from me (and in time will be in a future JMRI release).



I am likely to rebuild this coupling in a few weeks with a further modification; a hollow electromagnet will allow the fixed magnet to move within the core of the coil, this should mean more power with a smaller fixed magnet. It will also make the magnetic "return to rest" more likely to work as the fixed magnet will be nearer the iron component.

Smaller (!) DCC controlled couplers

I've had the 4mm scale loco with DCC controlled AJ couplings since the spring. But my aim has always been at "really small". 2mm scale "small". So, the last few months have seen various ideas tried, rejected, tried again.
The latest is for a 2mm scale Farish 04. Its a standard N item which has had the coupling pocket removed from the plastic keeper plate, plus 2mm finescale wheels (yes I know the spoke pattern is wrong for an 04!).



Picture shows coil on bench held on bit of bluetac. The coil wires are 45swg (0.07mm dia), or about the same as hair. The scale behind is a 1mm divisions.

The movie below shows it in operation. I will turn down the movement both forwards and backwards for real operations (various DCC CV settings) as they are currently a bit too large.

The chip driving the 04 is a CT DCX74. The DCX74 and DCX75 both support movement and coupling control from a single function press. It is intended for Krois uncouplers and I am exploiting the control here. The manuals from CT are fairly cryptic, but I think I have untangled most of the uncoupler issues. (The Zimo MX63 and MX620 are a bit larger, and also have functions for Krois couplers, though fractionally different in the way they operate. The Zimo documentation is a lot easier to follow). For those using JMRI (Decoder Pro), I have put my revised decoder file for the CT DCX chips in the JMRIUsers Yahoo group files area, when its had some more testing, I'll submit it for a future JMRI release.

I can see scope for a slightly larger version of this design (parts easier to handle!), with the frame and swing arm etched in nickel silver for 4mm use. I have a sketch design for the parts, and they should work with either a rotating AJ (like my earlier Armstrong Diesel) or the standard "pivoting" AJ with the pivot perpendicular to the coupling axis. I need to talk to someone who can put the etch bits on the side of a test-sheet, plus try out a few more electromagnet coil size options.

DCC Wagon Coupler finished

Video of the finished wagon, right way up !

Goofy blows a decoder

It had to happen, though its taken a while. But I blew up a DCC decoder yesterday.

I hard-wired the wagon with AJ couplings, and tested with a multimeter. All seemed fine. So I pushed some connectors onto the wheels and the decoder malfunctioned. Much prodding with multimeter and the DCC programmer followed, but the chip was dead.

Head scratching for several hours followed, until I realised a potential short circuit route; if the suspension of the wagon is compressed on one side only, a wheel rim can touch a W-iron. The W-iron base is used to solder the AJ wire. The AJ wire is pushed by the operating cam, and the operating cam is connected to the function output. BANG!!. The answer, by touching the wheel against the W-iron I could have put track current onto a function output wire.

Lessons learned:
a) Should have put the blue common onto the "dangerously exposed" bits of the mechanism, rather than the function output. Better still, should have wired it without a blue, but used half-wave back to the track (red/black) as in a NEM651 socketed decoder. (Check decoder supports this wiring before doing it!).
b) Should insulate EVERYTHING so it cannot short.
c) When fitting AJ couplings, should ensure that the coupling is not directly soldered to a metal W-iron, otherwise a short can go the length of a train. Same must apply to metal buffers attached to metal underframes.


Changes to the test wagon:
1) Lots of insulation added; the operating cam for the uncoupler is now rubber coated, the cam bracket has insulating tape between it and the W-iron. Insulating tape added where memory wire might short against W-iron it crosses. Plasticard added to prevent AJ wire shorting across.
2) Checking whether chip will support half-wave wiring (ie. don't use blue, instead connect to either red or black).


And finally, I will be sending the FL2 back for exchange under TCS' very generous Goof-Proof replacement scheme. (Replacement received in 48 Hours, fitted into wagon, all now working well done Bromsgrove Models for the prompt service)


(signed) Goofy.