Electronic Train Control
It works quite well, but it requires AC, or at worst unfiltered dc input, and so is not useful with batteries, etc. It is also a very dated design; it is like the old SCR or TRIAC-based power tool speed regulators that appeared in the late 1970s, but has a little gain provided by the BJT to sharpen the control, and give better low-speed regulation.
Needless to say, these things sell for a whole lot more than it costs to make them, including getting PCBs.
I decided I wanted the same sharp speed control action, but I needed the contoller to work with battery supplies as well as AC. My design appears below. It has the advantage that it can be used with either AC, or unfiltered DC or filtered DC.
There are a number of advantages to this circuit.
- Runs on AC or rectified DC or batteries;
- Offers simulated inertia, even without filtered DC supply;
- Requires no large electrolytics, and none at all if you do not want inertia;
- Sharp low-speed regulation owing to high gain;
- Standard, low-tolerance components.
Here is a PCB layout for the controller circuit as shown above. The control pot and the reverse switch are not mounted to the PCB. The ‘fuse’ is not PCB-mounted, and in fact it can be a 12V/21W or similar light globe, conferring the advantage that it protects without destruction.
How It Works
D5–D8 rectify ac or correctly polarise dc input. D1 (a yellow LED) provides a roughly regulated level for the speed setting control, and indicates power applied. RV1 picks off a voltage from D1 to set the speed. U1b acts as a comparator, driving its output high whenever the speed setting voltage exceeds the voltage that represents the load voltage. R7 and R8 and C2 filter the load voltage to remove spikes. When the drive transistor Q1 turns off, there is a brief conduction of D4, the flywheel diode, while energy is removed from the inductance of rails and motor. After that moment, the back EMF of the engine charges C2. U1b compares this level with the set level. When U1b sees that the speed is inadequate, it drives its output high. This charges C1 until U1a, connected as a Schmitt trigger, switches on Q1. At once, the load voltage will increase and eventually U1b will send its output low. If the supply is not filtered, it may drop to zero before this happens, in which case D2 will discharge C1. In any case, Q1 is eventually switched off, and after the flyback from the load inductnce, the cycle repeats. R3 operates in conjuntion with the opamp input bias currents to ensure that the comparator is biassed off when the speed control potentiometer is at minimum. C3 provides inertia, holding charge even if the supply is not continuous, by virtue of R2 and R11 combining with C3 to give a long time constant. R4 must be trimmed to set the free-run frequency to suit the mains period if supply is unfiltered. C3 is trimmed to set the inertia time constant. A value of 3.3uF for C3 may be sufficient unless you desire to make driving the train realistically difficult.
This is an implementation of the controller. Made with Yellow Heart, Red Heart, and Wenge woods, with a fawcett-style throttle, knife reversing switch, and voltage and current monitors, this is the Rolls-Royce of feedback regulator controllers.
This is a small, hand-held version, like the equivalent Gaugemaster, but with a bigger knob, a wooden case, and the ability to run on battery or low-voltage AC supply.