Old fashioned car battery charger technology

Well, not that way round we wouldn't...not 240v to 12v!

Yes...I had some of those. I remember a Practical Mechanics article (must have been in the 60s) where someone described his BEBOP - Battery Energised Boiler Operation Plant. When the mains failed, it would run the programmer and pump (little in the boiler itself I guess aparet from solenoids). He then did a Mark 2 which only cut in when the thermostat demanded heat.

Frank Erskine laid this down on his screen :

I remember that with the standard rheostat type controllers, there was always a problem at low speeds with the voltage being so low, it was unable to overcome the contact resistances between track and train wheels/motor brushes etc..

Your electronic version applied the full 12v, but in short pulses - thus overcoming many of these problems. I remember reading about an improvement on that idea, which superimposed an even higher voltage to overcome high resistance contacts, but sourced from a high resistance.

The modern way is to just supply the voltage straight to the track and have the speed/direction controller next to the motor, each digitally addressable so that a number of engines can occupy the same bit of track and each separately controllable.

Fascinating stuff.

Bob Eager used his keyboard to write :

I said 240v >FROM< 12v :')

I know. But that wasn't what you appeared to be replying to...we used a rotary converter for the opposite operation...where an inverter might not be so sensible.

I meant selenium, sorry for that.

And full wave rectification does not mean 4 diodes. as you pointed out.

Technically, with two diodes it is bi-phase half wave, which gives a very similar output waveform to full wave bridge.