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Pulsed power supply in models trains
Principle of "Pulsed Supply"
The classic analog supply (from transformer, or from more modern power supply), consists in modifying the continuous tension sent on rails.
This tension is generally variable between 0 and 12 volts: · A tension in 0 volts corresponds obviously to the stop.
· A 12 volt tension corresponds to the maximum speed.
The supply by PWM, "by chopped current", or "by pulsed current", consists in sending, instead of a continuous tension, a periodic tension, of fixed frequency.
The width of the crenel during which we send the maximum tension is variable, which allows to change the average value of the tension.
The tension varies between the maximum tension (12 volts) and the tension minimum (0 volts), with a frequency which is of the order of 40KHz
For the engine, it's as though we sent the average of this tension to crenel.
The following drawing shows, in each of four columns, the correspondence beetween continuous voltage (analog classic: at the top), and tension in crenels (below).
The following cases are represented:
· total Stop
· low Speed
· Average speed
· high Speed
What are the advantages of the tension in crenels with regard to the continuous tension?
· In the case of an approach by cantons, the electronics of control of every canton is simpler than in the case of an analog control, because it is simpler to controle a tension in "all or nothing", because a tension which can take any value.

· The functioning in " pulsed current " is much more flexible than continuously.
Continuously, when we go up slowly the tension from 0 volts, the loco starts suddenly from a certain value, but with a speed which is generally far from being unimportant.
In "pulsed" mode, on the contrary, this starting up is very progressive, and we can reach remarkable slow motions.

· The intensity of the lighting does not vary any more with the speed