Perhaps you are fond of your old locomotives and need not run them digital, or perhaps your magnetic inverter does not work properly, or you do not like them jumping or seeing how lights flash during inversion and change their brightness according to speed. There are, of course, comercial solutions, such as a decoder or Uhlenbrok 55700, but they are nor cheap.

That is why I would like to show this adaptation of a Märklin inverter on some locomotives, that uses common electronic pieces, easy to build provided some ability with electronic circuits.

I have built this circuit on a piece of mattrix board cut 22 mm long and 10 mm wide, that is 7 x 3 perforations, The total size is equivalent to the magnetic inverter, including an external relay, and may be reduced even more if you use a microminiature 12V latching relay such as 23026 or Omron G6E (at a higher cost).



It works as follows:

Current is rectified (by a small 1A bridge rectifier or four 4001 diodes), but this does nor affect the motor with inductive stator. Change of direction is achieved, as it is well known, by alternating one of the two stator coils, wound in opposite directions. A 12VDC latching relay provides for this change.

The first transistor does not conduct while normal driving, so that the positive pole brings the second transistor to conduction, by polarising its base through the relay winding. With the inverting pulse, over 24V, the Zener diode conducts, so that the base of the first transistor is properly polarised and closes the circuit of the relay coil making it work, while the base voltage of the second transistor is lowered under conduction state thus stopping the motor.

Voltage between both ends of the three diodes in series with one or both stator windings measures 2.2V and the LED lights conected to them glow with a constant intensity (around 8 mA). If you adjust to a mínimum the regulator at the transformer so that it supplies this voltage, the lights will be on without any current making the motor run. Of course, this lights improvement is an option, the three diodes and the LEDs are not necessary for the inverter (it is not shown in the pictures).