SuperClipper Switchable-Resistance Analog Train Controller

Many older model railway enthusiasts will remember the Hammant and Morgan line (H&M) of controllers. These were a triumph of solid British engineering. With a metal casing that withstood just about anything, including me standing on top of them, and with simple, robust, over-rated construction, they last a lifetime. There was the Clipper, the Powermaster, the Duette, the Safety Minor, and a variety of add-ons and hand-held accessories designed to connect the units with transformers. I got a Powermaster for my sixth birthday, but I lost track of it when I left home for college (along with a lot of vintage toys that would probably now sell for a great deal on ebay). The Powermaster and the Safety Junior were autotransformer designs, the others used rheostats. Later on came the Flyer and Commander units, but I have never owned one of these.

The Clipper was the most common model. There have been several incarnations of the Clipper:

I think the middle was the most beautiful piece of design. Sadly, the design is based on a rheostat, and does not give the best of control, especially at low speed, and especially on modern locomotives with efficient motors and rare-earth magnets.

I purchased one of these from (it cost way more to ship than to buy) and at right you see it with the front panel removed to show off the internals. Note the custom thermal cutout at the front of the chassis, and the (selenium?) rectifier riveted to the right-hand side wall. The cutout will isolate in about 1 second by means of a bi-metallic strip at a current of 5 Amperes, although it contributes less than half an Ohm of series resistance. The current is essentially limited by the transformer secondary reactance to 5Amps in any case, so the thermal cutout acts only to preserve the transformer in the event of a prolonged short circuit.

The circuit diagram of the clipper is

One interesting aspect of the design is the "high-resistance/low-resistance" switch. When set to low (meaning that you intend to use a locomotive of the older, low-resistance type) the control merely inserts a variable resistance of up to 50 Ohms. This gives almost no control of modern locomotives. Grounding the tail of the rheostat produces a potentiometer so that more nearly it is voltage that is controlled rather than series resistance. Better, but still not good.

This is the inside view of my new SuperClipper. Installed into the old case, and using all of the old hardware except the rectifier (which I removed to make more room, modern diodes being physically much smaller), it contains a modified version of the NRTC electronics. This allows software definition of the control action, but retains the same old glorious feel of the Clipper knob. The finished product has only a small modification on the front panel so that you can identify that it is the New McCoy:

What is the control action like now? That is the good news. The mechanical feel of the device is essentially unchanged, solid smooth action because the mechanical parts of the rheostat are not changed. They are used as an input device to the microprocessor. This is programmed to provide one of two responses.

In the "low resistance" switch position, the controller acts as a 0-12.5V voltage source with a negative 1 Ohm output resistance. This gives as good a shunting action as any non-feedback design. As the load current increases the output voltage increases slightly to compensate just a little. There is a small amount of "inertia" such that it takes a few hundred milliseconds for the train to follow the knob position. This is enough to stop jerky action but not to provide a simulation of inertia. There is a dead-man timeout, so that if the knob is not touched for 30 minutes the train automatically stops until the knob is moved. If there is a short circuit the electronics shuts off and retries every 5 seconds or so. There is also thermal shutdown for the pass-transistor heatsink, retained from the NRTC-A design.

In the "high-resistance" switch position, the controller acts more like a torque controller, setting locomotive current. This is designed to reproduce the effect that was (presumably) intended by H&M in the first place, namely that the train acts more like a real steam train, requiring active control to maintain a proper speed. In a way, it resembles the ETI-1508 that I describe elsewhere in these pages, but the action is not completely that of a current source, merely a high-resistance supply. The source voltage is still limited to 15V, but it is electrically equivalent to a 60V supply with a high-resistance rheostat. The dead-man timeout remains as well.
The action in high-R mode is just as expected! A loco will speed up on straight sections and slow on curves, if the control is left at a fixed point. On starting a little more throttle is required, and the train kind of takes off a little like a horse out of the starting gate, requiring a swift reduction of the knob to get a smooth pull away from stops, especially with a light rake. On the G-scale layout where I tested it, these effects are visible to the naked eye, in spite of the weight of the train. Of course, the wheels of the loco are locked together, so there is a lot of that drag you feel with a 4WD vehicle running in differential lock on tarmac, as one wheel on each axle must slip, so the slowing on curves is detectable even with a conventional controller.

Personally, I would not bother with this high-resistance mode on smaller scales. The effect is extreme on the Hornby OO-gauge Live Steam system, unpleasant. This controller is not so bad (though one could program it to be so). Nevertheless, I look at it as ideal for a small child playing on the G-scale system; it gives them something to work on. "See, you have to turn it up to kick off, then come down a bit so the passengers get a smooth ride". Our passengers were usually a collection of plastic animals, and some not so plastic.

The circuit diagram of the SuperClipper and the NRTC circuitboard are: