This isn't quite right. The voltage is not regulated at the decoder(I will say that it is possible for there to be some decoders that do regulate the voltage, but if there are, that is not the common way to do it and I have never seen one that does). The output from the decoder to the motor is ratiometric, which means it is relative to the supply voltage. If you have 12 volts on the rails, you'll have about 10.5 volts going to the decoder after going thorough the rectifer diodes. If you apply 50% throttle, you would have 5.25 volts going to the motor. If the supply voltage drops to 10 volts, you would have about 8.5 volts going to the decoder and 4.25 volts going to the motor at the same 50% throttle setting. There are two things that affect the speed of the DC loco: ohm's law governing the voltage drop in the wiring(V=IR) and the poor regulation of the power supply. With DCC, you usually have better regulation of the power supply, but you still have to deal with the voltage drop of the wiring because it does affect the voltage going to the motor(see above). It usually is not as bad as it is with DC because DCC layouts are usually wired better. The better wiring results in less resistance(R) which results in less voltage drop from V=IR, but that voltage drop is still there and still increases when you add locos.
Yes. No. Maybe! We're actually quickly treading on a lot of vendor differences. Depending on the vendor, if it has BEMF, how they implement speed steps and speed curves, and what scale, you're right and wrong. This is an area where the simple description isn't too accurate. However, your general concept is right, they all use a variant on pulse power to drive the motor... It would be interesting to post oscilloscope pictures of the motor output from different decoders in different conditions.
Regulate (as it has a specific meaning to electronics heads) may be the wrong word here. I did an experiment a few years ago. I used an Atlas DCC loco (so Lenz deocoder) at the factory settings other than address. At the first show my module was where the booster plugged in, and the second it was ~50-60 feet away. In both tests I ran the loco at 50/100 on my Digitrax throttle, and measured the speed with my module. I have a set of opto-sensors on my module hooked to a TrainSpeed. The loco, by itself, went exactly the same speed in both tests. Now, it may have been a fluke, but speed does not seem dependant on the voltage drop from the booster. I'm not sure if this is traditional regulation, or some interaction between voltage, BEMF circuity, pulse modulation, etc. It may be that it is just reduced by 1-2 orders of magnitude and thus becomes undetectable....
I believe that is your answer. As an example, if you use the voltage drop calculator posted previously and plug in 12v, .2 amps, 14 awg, and 60 feet, you get a voltage drop of 0.6%. At a scale 50 mph a change of speed of 0.6% would be a change of 0.3 mph. I believe the TrainSpeed measures to 1 mph, so it would not be suprising for the TrainSpeed to not show any difference.
Doesn't "duty cycle" have something to do with the reason that electronic components used in power supplies use a pulsed output? Martin Myers
I did explain it more fully before, but you asked for a formula. That is the formula. The only way to 'change' it is to use subscripts which are a short version of what Robert (I think) showed, like Vd = IRt . You then need to qualify what the subscript means (in this case d=drop, t=track). I was a little puzzled at your request for a formula and initially typed more explanation, but then decided maybe you wanted it simple and pruned it back. Sorry. Pulsed outputs exist for two reasons in two devices. In DC controllers it is common for the unit to introduce pulsing at lower voltages as this is generally considered to give better control at lower speeds. This is partly because the higher voltage of the pulse may cut through (eg) dirt on the track better and (I think mainly) because the pulsing helps to keep the motor and mechanism free of static friction. The pulse is usually added by mixing a bit of the AC power into the DC which is why you see it at 60 Hz (it's 50 in the UK). PWM (Pulse Width Modulation) as mentioned above, was originally used for heat reasons, though it has become useful in other ways too these days. If you turn a transistor partially on, say so it passes 6 Volts from a 12 V supply, it is acting like a resistor and has to dissipate the same heat. At 1 Amp that will be (W=VI) 6 Watts. If instead you turn it full on 50% of the time and full off 50%, the average output voltage (subject to certain other conditions/assumptions) will still be 6 V. The heat generated will however only be down to the 'silicon' volt drop (about 0.7 V) for the 50% of the cycle when it is full on, which is W=0.7x2x0.5 = 0.7 Watts. (It is 2 Amps because you are applying 12V which will drive twice the current that 6V does; but only for 0.5 of the time, so the average is still 1 Amp.) This is the system used in most (maybe all) DCC decoders - see posts previous - because dissipating several Watts is just not an option
Mike, I was asking for someone to explain how to apply the formula. My apologies though, your initial explanation was pretty okay. I just think I needed it repeated to "get it", as someone who has never thought about this particular thing before. It's perfectly clear to me now.
OK, I'm with you now I think; seems I needed to hear your end twice too, before I got it. When learning something new I find it helps to hear it a couple of different ways - I think of it as giving my brain a stereo image that it can build a better model from
Right on, I appreciate all the time you and others have spent posting in this thread. I've learned something, and I don't think I'll forget it!
Yup. You can send some of the power all of the time (DC control), or all of the power some of the time (PWM). Two ways of doing the same thing, but the former requires you to dissapate power lost in the voltage drop as heat. The PWM that decoders use on their motor outputs is much more efficient in this regard.
Rails, I have been away from the forum since beginning this thread due to personal problems. I just want to say thank you to all of you for the some 29 posts on the subject. When I get the chance I plan to read and try to absorb the great info here. It's good to know it'll always be here,too !
Rails, I have been away from the forum since starting this thread due to personal problems. I just want to say thanks to all of you for the some 29 posts on the subject. It's good to know they'll still be here when I have the time to try to absorb it all.
