I just read the DCC primer on tonystrains.com and was surprised to learn that with a DCC setup you’re actually putting AC power across the tracks instead of DC. Of course, now I have more questions that I hope someone can answer:
AC power: Is household 120VAC present across the tracks, or is stepped down to a lower voltage?
IF its running 120VAC across the rails, what fusing and short circuit protection do most people employ?
Locomotives: Are DCC locomotives equipped with AC motors, or are they just running DC motors and a rectifier circuit to get DC from the AC on the rails?
No. There’s usually a power supply that knocks the voltage down to around 16VAC (at least on the Bachmann E-Z Command), or else the decoder would get fried.
Most DCC systems have their own built-in fault protection.
No. The decoder converts the AC current to DC in order to run the locomotive. Otherwise, you would never be able to convert an older locomotive over to DCC without first changing out the motor. On some older locomotives, you sometimes do have to isolate the motor from the frame BEFORE installing a decoder.
Tim, unlike DC with a potentiometer to regulate the amount or percentage of voltage going to the locomotive engine; with DCC, the tracks are at 100% AC power, ALL the time - no matter what the throttle knob position is at. The Command station actually “pulses” a (binary) signal of ones and zeros at a very fast pace. (Like quickly turning your locomoitve on and off hundreds of times a second.) When the signal is one, the locomotive moves. When the signal is zero, the locomotive doesn’t move.
To look at it another way: At 10% power, the locomotive motor is on 10% of the time, and off 90% of the time. At 50% power, the locomotive motor is 50% on and 50% off, and so on. The more the motor is on then off, the faster the locomotive moves.
Wow I’d have to build lots little signs BEWARE DO NOT CROSS THE TRACKS!! or you’ll get fried!! Just Joking Tim LOL, Welcome to the fantastic world of DCC, it won’t kill you trust us.
tstage, I think you’ve cleared up most of my questions! So we’re talking about a 16VAC system, with the DCC commands transmitted using the AC power as the carrier signal, and then the DCC decoder at the engine rectifies the power to provide a constant DC voltage to the engine. To control power, we’re just Pulse Width Modulating the DC voltage.
With regards to fault protection, I was thinking more from the supply side, not at the decoder. Of course, if you’re only running 16VAC across the tracks its not AS bad as 120VAC, but still enough to give you a perm!
I’ve seen some postings regarding switches/turnouts and power them. On my 20 year old DC set, those were all DC. Do you still use DC-powered switches in DCC setups, where the DC power is wired directly to the switch motor/solenoid/actuator, on a separate circuit from the rails? Or is there some sweet way of powering them off the rails themselves and controlling them via a decoder, thus eliminating the need to run wires to the switch?
The voltage will not hurt you and probably wont even give you a perm; however the initial cost of decoders may put a sizeable dent in your wallet. You can put decoders onto your switch machines for turnout control. One caution, some of the older turnouts are not DCC friendly and may require some rework to either isolate and/or power the frogs correctly. There are several threads here on the forums addressing this.
Welcome back to the hobby and the wonders of twenty first century modelling!
Tim, I think you understand this better than you think. Most DCC controllers will allow you to run one “non-DCC” (or DC) locomotive. From what I understand, the pulse widths are lengthened to mimick DC. The 16VAC is what the Bachmann E-Z Command system uses. I’m not totally sure if that particular number is the “norm” for ALL DCC systems. Some may be slightly higher; others lower.
No. The fault protection is actually in the command station, not the decoder. If you lay a screwdriver across both tracks, the command station will sense the short and either shut down or “chatter” (like a squirrel) till the troublesome culprit is removed. The decoder has no fault protection other than that which is in the command station. You can fry a decoder fairly easily if you set your mind to do it.
I’ll see your “No” and I’ll raise you one! [:D][:D][:D]
Actually, it’s square-wave AC, as opposed to sine-wave AC like your house current. The width of those square waves is what creates the ones and zeros that make up the DCC packet.
The decoder rectifies that same square-wave AC and supplies pulsed DC to the motor. Some cheap decoders don’t filter it well, and those are the ones that “buzz”.
Also, control of a DC loco on those DCC systems that support it, is done by stretching either the positive or negative side of the zero bit. Stretching the positive side gives you one direction, and stretching the negative side gives you the other direction. And again, since it’s actually AC (but with a bias towards negative or positive), it will make that DC loco buzz or sing.
Awwww, I was hoping to come in and write a long post to save the day, but looks like tstage has beaten me to it, and done it better than I would have anyway. There is simply a diode (kinda like a check valve for electricity) in the decoder that converts the square sin AC into DC. Having the motor pulse on, off, on, on, off gives you full pulling power even at 1% speed. Also, I haven’t seen anyone mention the fact that you can run multiple locomotives or turnouts at one time, and they can all do something different, or if they’re programmed the same, they can all do exactly the same thing, like you’d do with DC.
This actually makes perfect sense. The basic idea of it has been used in telecommunications for years. AC carrier signal (AC Power on the rail) with a digital signal overlayed on it (the DCC packets) to carry information. The only difference here is that the frequency of the carrier is 60 Hz, the transfer medium is Atlas Code 83 [:)] instead of the air, we’re sending train controls instead of Lynard Skynard [:D]
Not EXACTLY. In DCC, the signal IS the carrier, and vice-versa. The pattern of the square wave is the data that is being transmitted. Unlike older carrier control systems like CTC16 and it’s varients in which a low amplitude signal was superimposed on a steady state carrier (DC int he case of CTC16). Or closer to the AC idea, the X10 home control stuff which superimposes a signal on top of the normal 60Hz AC power line voltage.
This is part of why DCC works as well as it does - the actual signal has a relatively high amplitude and is easily received at the decoder end and interpreted. The older analog systems had a relatively small signal amplitude which made it easier to interfere with.
Just to either clear things up or add to the confusion, a decoder will controll the speed of the motor by adjusting the number of signal pulses it allows through to it. For example, a 28-step decoder at step one forward will let one out of every 28 positive pulses go to the motor, at step 5 it will be 5 out of each 28. To change direction it will let the negative pulses through. Since the full positive or negative component is actually applied, slow speed control is better than with analog DC.
With address 00, the positive or negative zero bits are streched to control the DC motors. DC motors can get damaged if they are left still on powered DCC track for long periods of time. Even though the motor is not turning, there is still full current going through it ( causing the high pitch whine) and heating it up. Without the rotation to cool it, the motor can burn up. So even if the noise doesn’t bother you, turn off the track power or remove your analog locomotives from the track if you will be stopped for a while.
DCC uses the very same principles that PWM does (in fact it is PWM for all intents and purposes). The DCC command station changes the timing of the PWM, these small changes in timing create 1’s and 0’s and eventually make up a complete packet of data much like on an ethernet computer network. The decoders in the locomotives listen for these packets, and then control the motor and lights in the locomotive as per these instructions. This principal has been uses for many years in other products, and works quite well with model railroading as well.
Randy made an interesting point about X10. Although there are some similarities, I think the X10 signal is actually a 16 bit packet sent out like white noise during the zero crossing of the 60 Hz. phase rather than an altered timing of the phase itself. If an X10 controller were to modify the 60 Hz. sine wave for timing purposes (which obviously it can’t, but let’s assume it could), many strange things would happen to the other appliances on the homes wiring infrastructure. Remember many of the moden appliances use the 60 Hz. frequency for keeping accurate time… [;)]
[?] soooooooooooooo, there’s not just an "x"volts AC signal with a digital signal superimposed? Man, I’ve got to get my hands on that “Big Book of DCC” from Digitrax [:D]
hehe actually my point was that DCC is NOT like X10 in that it is NOT a superimposed signal on a fixed carrier. But you are right about X10, they use the zero crossing. Amazing thing is that stuff has been around in the consumer side nearly 30 years now, commercially even longer!
Anyway, what’s with people unsubscribing to the forums after they get their answer? It WAS a legitimate question, not some crazy troll. Stick around, there’s lots of information to be had here.
Hey, don’t worry. I’m back! I simply realized that my screen name was, well, kinda bland and gave out more info than I wanted. Sorry for the confusion.