??? Okay. DCC works on a constant level of AC Square wave (voltage) present at the track at all times. Fair enough. Very simple concept. BUT! When I select a loco without a decoder, at address '00', that loco has no decoder. HOW in the heck does the DCC system regulate voltage from a steady state squarewave of say... 14V to a varying state for a loco with no decoder. Seems to me that electrically (V=IR), if that motor sees V = 14, it is gonna NASCAR around the track. Help me out here....
Basically, this all has to do with timing.... As you noted, the DCC signal is alternating between V+ and V-. The nominal time spent in each state is approximatly 1/8000th of a second. As long as the time spent in each state is the same, the locomotive doesn't move. (the motor actually DOES move, which is why you hear a buzzing from analog locomotives on a DCC powered track - this is the motor changing directions about 8000 times per second). Effectivly, the locomotive is seeing a net voltage of zero over time. What happens when you bring up address 0, and turn the throttle up is that the signal timings are changed. The signal still alternates between V+ and V-, but the time spent in each state is no longer the same. Which of the two values is allowed to be longer determines what direction the locomotive travels in. Incidentally, there are two different timing patterns in the DCC signal. one is designated pattern "0" and the other pattern "1". The extended time in the V+ or V- state is only allowed with the "0" pattern, so this feature is typically refered to as "Zero Stretching". I hope this helps, Paul
Wait a minute. I have been thinking about this. The explanation beautifully explains the directional component of my question. But... what about speed???? The value of V (or amplitude) has to ramp up and down for a non-decoder engine. (either + or - depending on direction). The varying amplitude of V is what correspondingly varies the speed. How can the booster/command station keep the voltage constant for one engine, say old #1524 (decoder equipped) but vary the amplitude for the engine running @ "00" ???
What actually varies is the average amplitude over time. The instantaneous amplutude is always the same. As long as the amplitude in the V+ and V- states are the same, then the average amplitude is 0. If you spend more time in one state than another, the average amplitude is something other than 0, so the locomotive moves. There is a nice analogy here to what happens when you drive a car on the highway (without cruse control). Let's say it takes you an hour to travel 60 miles. Clearly, you were traveling at 60mph (distance covered/time). But that only gives you a partial picture. Durring that 60 mile distance, you might have been traveling 70mph for part of the trip, and 50mph for another part of the trip, not to mention it takes time to accelerate and decelerate between the two speeds.... In any event, the length of time spent traveling at each speed is a factor in determining your average speed over a given period of time. (A little calculus and/or physics helps to understand this, but it's basically just algebra). Paul
Okay.. That makes sense. Actually, the calculus I've had. My degree and training is in electrical engineering, but I stressed and now practice in 3 phase power. I took digital electronics in college...., but must admit that while in college... HATED IT! It was voodoo to me. Boolean math, reductions, and gate logic was like sandpaper on my pysche! But understanding now that emphasizing frequency on one side of the axis can control direction while varying the average over time affects speed.. it makes sense. Thanks again. You've answered my question.
If you know AC: For a sine wave the average voltage (not the RMS) is 0. The 0 axis is along the middle of the sine. If the axis is moved up or down, the average volts is non-zero (DC component on an AC waveform. Think the same for a square wave and that's how DC on DCC works. The DCC controls signals are sent by varying the AC frequency. (OK, someone will say those explanations are not be 100% accurate, but it's "good enough" for anyone who isn't actually manufacturing this stuff )
