Simple DC Controller
Many cheap controllers on the market merely consist of a mains transformer, bridge rectifier and a power rheostat. These usually give an unsatisfactory result, with “jackrabbit” starts, frequent stalling and poor slow speed control. The controller described here gives a performance which belies the simplicity of its circuitry, and is a big improvement on most of the controllers supplied with train sets, etc. In fact the serious modeller will find its performance more than satisfactory. The maximum output current available is 1 amp.
How it Works. The 14-18V a.c. input to the circuit is derived from either the accessory output of an existing controller, or a separate dedicated mains transformer.
Ignoring the circuit breaker and its attendant components for the moment, the a.c. is rectified by the bridge formed by D3-D6, and the resulting pulsing d.c. is applied across speed control VR1. The required proportion of the d.c. voltage is tapped off by this component, and fed to the “Darlington pair” TR1 and TR2. This transistor formation provides a low output impedance, which is necessary to achieve optimum current transfer to the loco motor. D8 removes inductive overshoot produced by the loco motor, and S1 is the direction change switch. D9 ensures that the output from the controller is at zero volts when the speed pot is set to zero. LED D7 acts as a track power indicator, glowing brighter with increasing track voltage.
Should a short circuit occur across the output terminals, or the output current capacity of the unit be exceeded, the circuit breaker will open, allowing current to flow through D1, the overload indicator LED. (Diode D2 allows this LED to run on a.c. voltage, and R1 limits its operating current to a few milliamps). Because the circuit breaker is open, the current drawn through the output transistor will be negligible.
Construction and Testing. Layout is in no way critical when building this circuit. Direction change switch S1 should preferably be a high quality toggle type. A centre-off switch is recommended, as it is then possible to cut the power to the tracks in an instant by returning the switch to the central position. Note that TR2 must have a heatsink; it should be a finned type of 40 x 20 x 30mm minimum dimensions. If the controller is to be housed in a metal case, the transistor can be bolted to this, but it MUST be insulated with a mica washer and insulating bush.
To test, turn the speed control to minimum, and apply power. Advancing the speed control should result in the POWER LED glowing, if not, switch off and check the wiring. The OVERLOAD LED should only come on when a short circuit is applied to the output. It should extinguish a few seconds after removing the short. Should the speed control operate “the wrong way round”, simply reverse the wires to the outer two terminals of the potentiometer. If all is well, connect the unit to your track, and start running trains!