DCC Tutorial (Power)
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Contents |
[edit] Intro to power
Let's start off by saying that you don't need to fully understand the technical details of how DCC works to make DCC work. If you don't understand anything between Some DCC Details and "DCC Power", don't worry. Come back in a few months and see if you understand it then. If not, don't worry.
[edit] All you need to know is:
- There is polarity, which can be handled by auto reversers so you don't need to flip switches.
- There is a digital signal on the power lines, which is sent from the command station to the train.
- There is full power on the track at all times while the system is turned on. Voltage is not varied to control loco speed.
- The polarity of the power on the rail does NOT control the direction of the locomotive.
[edit] Some DCC Details
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| An AC waveform | The DCC waveform: note it is two mirror image waves |
- The power on the track is not simple analog (DC), but a special form of AC.
- The DCC signal does not look like the AC sinewave you would normally think of such as found in your AC wall outlet or on a transformer. Technically it is a bipolar squarewave modulated by the digital control signal. The DCC signal switches very quickly from 12v to -12v between the rails - and also varies the time at which it switches (the modulation) to convey information to trains on the track. See the illustrations for the difference between an AC and a DCC signal.
- The DCC track power can be seen to be:
- DC power to power the motors/lights/sound/animation.
- A digital control signal (the alterable time between DC polarity changes) to tell decoders what to do.
[edit] Intro to DCC Signals
The DCC waveform is created by a DCC controller (command station). It is sent to a booster (or multiple boosters for large layouts) where the DCC Commands are superimposed (modulated) onto the track power. That resulting waveform is sent from the booster to the rails. The DCC decoder in the locomotive picks up the waveform from the rails, rectifies the modulated DC to pure DC, and applies power to the motor(s), according to the information encoded in the waveform. Each decoder has an address, and will only respond to commands addressed to it.
Wireless DCC controllers have an RF transmitter in the hand (throttle), and an RF receiver that controls the DCC controller (command station). But the DCC Signal and power still goes through the booster and the rails.
[edit] DCC Power
The DCC booster puts a fixed voltage on the track. It's 'fixed' in the sense that it is continuous and doesn't change in magnitude. This means that all locomotives have power to their wheels, all the time. Instead of the voltage (and current) controlling the trains, a receiver (decoder) inside each locomotive listens for the commands sent out over the rails from the command station. These commands tell the decoder to make the train for go forward, reverse, fast, slow, or turn on/off lights or sounds.
With this setup, you control the trains and not (like with analog) the track. Because of this, and the whole point of DCC as a whole, it is possible to control multiple trains on the same track without having to deal with complex wiring to isolate each section of track to control each train.
For further details, you can read more about decoders, boosters, and command stations.
[edit] DCC Wiring
You need to be aware that, since all locomotive power comes from the track and more than one locomotive may be running at once, DCC boosters (and their power supplies) are designed to have current ratings of 2 to 10 amps. Because DCC locos and their current demand may be located anywhere on the layout, wiring to the track for DCC needs to be designed to handle a higher number of amps.
For all locomotives to run properly, you need to be sure that there are no voltage sags. That is, you need to be sure that all sections of track have sufficient power to handle the locomotives. The best way to do this is to scatter feeder wire connections around the layout. A second reason for a robust wiring system is to ensure that the over-current protective devices built into the DCC booster will operate correctly. This is necessary to protect your railroad equipment from damage caused by an accidental electrical problem, such as derailments.
With DCC, a booster can supply 12 volts (or as high as 18 volts for larger scales) at 2, 5 or even 10 amps into a track short-circuit without becoming overloaded. Such a short-circuit perhaps represents as much as 120 watts. This power could quickly melt trucks, engines, etc. To help prevent this, DCC systems have short-circuit protection built into the booster. When a short-circuit is detected, the booster will shut down. Once the short-circuit has been removed from the track, the booster will automatically turn the power back on.
For this to work, any point on the track needs to be wired so that the full load of the booster can pass through it so that the booster's protection can function properly. To test this, simply place a metal coin, or some other metal object, across the rails. The booster should shut off the power automatically, and turn the power back on once the object has been removed. This arguably means that heavier, larger wire, better track connections, and more track feeders are needed as compared to a traditional analog (DC) powered electrical circuit. (However a conventional analog system should have good wiring anyway - otherwise trains would slow down noticably in poorly wired areas).
- For more comparisons, read the DCC advantage over DC page.
- See the track wiring page for more details.
[edit] What's next
Getting started with a Starter Set.
