Technically, it’s not a square or rectangular wave, it’s a pulse train. In a square wave, the positive and negative portion of the wave are equal in duration, rectangular wave, they’re not, but they do have a fast rise and fall time as compared to a sine wave. With the DCC signal, the pulse width isn’t constant, the zero bit (reference) is longer than the one bit. This is the signal placed on the tracks, the output to the motor from the decoder is standard DC, the motors are the same for DC or DCC. For DCC, the motor has to be electrically isolated from the frame because the decoder will determine the voltage level and the polarity applied to the motor.
Tom I agree to what you are saying, but that is the coded signal going to the rails, for the decoder to process.
Has anyone seen the signal from the motor leads on a scope?
I haven’t seen them, but I have seen the signal on Industrial machines, and it is a Square wave Pulse. The Pulses have electronic control to the motor, doesn’t give any voltage when the motor isn’t running, but as soon as it is supposed to start, Square wave pulses (of full voltage) are sent to the motor and they vary in width, according to how fast it is suppose to run. The faster, the wider the pulse. Positive pulses for one direction and Negative pulses for the opposite direction.
I have addjusted the min. volts on locomotives with the shell off, and watch the flywheels to see when they turn, and I have seen them actually take 8 to ten steps for one revelution. This is the way I set the Min. volts on my locos.
Since they started making the silent decoders it is harder to do, because the pulses are shorter and faster.
I have not seen them, but I believe your description is correct, but the terminology is a hair off. A square wave would have equal high and low portions. In this case (I believe) the total width of the high plus low would stay the same, and the ratio of the high to the low would determine the speed. Hence the reference to pulse width modulation (though this isn’t really that, since at this point the only signal is the drive to the motor, but that’s what it looks like.
Hmmm after using the term for 30+ years, as I was reading this post I just realized that “square wave” is an oxymoron. We always draw a connecting line between the two pulses but in reality it is an instantanious switch from one to the other, no transistion.
Actually, there is a transition, but it is quick. That’s in the spec, too. The quicker you try to get the edge the more ringing you get at the other end. And nothing switches ‘instantaneously’.
I stand corrected, the signal going to the motor, are not square waves, they are pulses.
The circuit (industrial) has the motor in a Bridge Circuit. On paper it looks like an H, with the motor as the Horizontal bar of the H and the vertical sides have 2 switches in each side, A,B,C&D. The control circuit is operating at a predetermined frequency, for discussion I will assume our 60 Hz house current. or 60 cycles per second. Full wave rectified we can get 120 pulses per second. (It is possible to hear this frequency as a whine or buzz).
When the controller selects the direction for the motor, it just determines which switches it is going to turn on, A&D or B&C. No current going to the Motor, Yet. See Diagram below.
When the speed control is increased, the controller sends a signal to close A & D, which gives the motor a pulse of current through the motor from x to y, at the rate of 120 pluses per Second and the voltage is the full volts available from + to - ,the motor starts to turn, the speed is set by how wide the pulse is timewise. As the speed control is increased, the pluses get wider (or longer time wise) and the motor starts running faster, until the speed control is set to max, at which time the motor has almost pure DC on it. One advantage of this type of control (PWM) is that at slow speeds of the motor, it is getting the full rated voltage across the motor and it effectively has more HP or torque than straight dc would give it at the same voltage for the corresponding RPM.