Pulse Control Unit

Build a Pulse Control Unit for better slow running of locos

Parts list for the Pulse unit
1. BC108 x 4 transistors (Small signal tansistors)
2. 2N3055 or Equivelent Power transistor.
3. 5000 mf Electrolitic capacitor
4. 0.47 mf x 2 Capacitors (see note)
5. 8k2 x 2 resistors
6. 2k2 x 2 resistors
7. 560R resistor
8. 180R x 2 resistors
9. 47k linear Potentiometer.
10. Double pole double throw toggle switch (centre off)
11. Copper strip board (to build on)
12. Case to hold the unit (approx 4 inches or more square)
13 Knob to fit the 47k potentiometer.
14. Mains to 12v DC transformer.
15. Bridge rectifier.

Circuit diagram of the controller
Circuit diagram of the controller.
Assembly of the device.
The circuit above is built up on a copper strip board. A piece about 2 inches by 3 inches will be sufficient for the main circuit and transistors 1 to 4.
The 5th transistor needs to be mounted on a piece of metal to dissipate the heat that may be produced during operation. You could use a small piece of aluminium of about 3 inches square or the case itself if you are using a metal one.

How it works
Transistors TR1 and TR2 operate as a multivibrator circuit the mark - space ratio of which is controlled by the position of the 47k potentiometer. At mid setting the on and off times of TR2 will be equal resulting in pulses of 12volts appearing at the output terminals of the unit. These pulses will give full power to your loco for a very brief period, (about 1/50th of a second) followed by the same period of no current.
At the high setting TR2 will be switched hard on (ie no multi vibrating)and full power from the unit. At the lowest position of the potentiometer TR2 will be locked hard off resulting in no output from the unit.
TR3 is an isolating and amplifying transistor which also inverts the output from TR2.
TR4 and TR5 act as a Darlinton pair and follow the output voltage of TR3.
So as you start from zero and slowly increase the power, the mark - space ratio (on off times) of the output voltage follows this with the on times of the supply getting longer and longer until at full power the output is a constant 12 volts.

This circuit gives excellent control of your locomotives especially at low speeds. In fact because the power is applied in small doses of full voltage you can have a steam loco up against the buffers on the track and have the wheels turning at about one revolution every minute.

Note:- I have been asked on several occasions if there is and short circuit protection for the unit and although none is shown on the drawing I in fact feed the output from the 12v transformer and bridge rectifier through both filements in parallel of a car stop and tail lamp bulb. Solder one wire to the case of the bulb and the other wire to both of the filament output studs. This will limit the current to about 2.2 Amps and also gives a visual indication of any short circuit.

John Essex Heywood Model Railway Group


This page updated 30th October 2016