Simple Throttle with Momentum ~ Saturday, March 26, 2011
A few weeks ago I posted about running trolleys and trains with a 9v battery and a home-made, minmalist transistor throttle.
The design of that throttle was like this (I redrew it in a way that makes it a little clearer how it all works, to me at least).
This design, like most basic train controllers, lets you start and stop as fast as you want. Part of the challenge of operating a train or a streetcar is that they don't stop on a dime, and I always thought running miniature ones might be more interesting if you had to take stopping distance into account. My main throttle, an MRC Tech II, has this feature, but seems to be designed for long trains on large layouts--if I turn it on, even pressing the "brake" switch, a streetcar will make it most of the way around my tiny layout before it stops!
So here's my effort to add some a slight momentum feature to the simple throttle design. There's no separate "drift" and "brake" modes; the idea is that when you turn down the speed knob, this represents applying the brakes in a normal, non-emergency, "not making all the passengers fall over" way that brings the car to a stop in a reasonable distance.
The key idea is to use a capacitor to keep some slowly-decreasing current flowing through the the "control" B->E circuit through the transistor even after the potentiometer turns off (or close to off) current flowing from the battery. After some experiments and getting some advice, I realized that the two diodes are necessary to keep the capacitor discharge from flowing in unintended directions that allow it all to dissipate at once (and, apparently, even burn out the potentiometer).
The size of the capacitor (in conjunction with the resistance in the discharge path) will determine how quickly the control current goes down. A 47μF cap I had on hand seemed about right for my tiny layout. Experiment and see what suits you.
You could use a much larger cap to simulate coasting, and add another switch (push-button, probably) leading to a lower resistance, faster discharge path, for braking.