Model Railway Electronics - Train Controllers (AC,DC,DCC,PWD) What's the difference

Intro: There seems to be some misconceptions about these different concepts so I thought I'd write this quick and grubby guide to clear things up a little. I hope it isn't too wordy, I hate reading things on-line.

AC stands for Alternating Current.
Alternating current is the type of electricity delivered to your home from a power station and consists of a voltage sine wave. That is the voltage varies from plus through zero to minus over time as represented in this graph.

You'll notice this wave alternates from 0 to +16V back through 0 to -16V and back up to 0 in 20ms. So this wave has a cycle or frequency of 1/20milliseconds or 50Hz, i.e. it alternates 50 times per second. Why? Because the generator producing the electricity revolves fifty times per second. In the USA it would be 60Hz

Why is AC used? Well the wonderful thing about AC is that if you make a transformer with 240 turns of wire on one side and 12 turns of wire on the other side, when you stick 240VAC at 1amp into the first side you get 12VAC and 24amps out of the other side. This means that power companies can use a transformer to change 240V and 1000 Amps into 240000V and 1amp. 1amp can travel much further through a power line without heating it up than 1000amps can. And you can plug a transformer into a 240V wall socket and get a safe 12-16V out the other side to run trains with. It's used to run your train controllers (see below) and other lights and accessories like point motors, but if you use it on point motors they will buzz at 50Hz!

DC stands for Direct Current.
Direct current is the type of power obtained from a battery, a DC power supply or from a train controller. A battery supplies a nice steady constant voltage (until it runs out). A DC power supply uses something called a rectifier to convert AC into DC and a capacitor to smooth it out. A rectifier simply takes all the bumps that were under the zero line in the above graph and inverts them so that they are above the line. The capacitor stores volts and fills the dips between the bumps making a smooth voltage. A train controller is similar to a DC power supply but often it doesn't include a capacitor to smooth it out and you can vary the voltage manually from 0 to + or – approximately 16V
The following graph represents the different outputs.

Now there's nothing wrong with the train controller having a messy output like that because the pulsing voltage gives your trains motor a bit of a kick to help it go. It's a bit like the difference between banging a nail in with a hammer and just trying to push it in. The motor is slow enough that you don't even detect the pulses, the train just glides smoothly along.

It's this characteristic of motors that makes PWM or Pulse Width Modulation a useful and effective method of controlling trains. PWM is used in feedback controllers and DCC decoders to control the motor in a train.
The output from a PWM controller (or a DCC decoder) looks something like this:

In much the same way that it is more efficient for your hot water service to turn on or off at varying intervals to maintain a constant water temperature it is far more efficient for the controller to turn the power to your loco fully on or off at varying intervals to maintain a constant speed. It is this efficiency that enables a tiny DCC decoder to run a locomotive without heating up and blowing itself to smithereens. The pulses happen much faster than your average DC controller, in the order of 10000 pulses per second. Note in DCC this signal is only present between the decoder and the motor. You won't see this signal on the track. So how does the DCC unit tell the decoder what to do?

DCC stands for Digital Command Control.
The DCC unit places a signal on the track that is a combination of constant power for running locos (and other things) and codes that tell each decoder what to do. The power component is similar to AC in that it varies around a mid point but it is different in that it is
a. A “square” wave
b. A very high frequency – 10KHz - and
c. Varying slightly between two frequencies. It is the varying in frequency that is used to deliver information. On our graph it looks like this:

Where the wave is narrow it is read as a 1.
Where the wave is wide it is read as a 0.

The decoders in the loco read in a series of 1s and 0s from the track. A typical message sent from the controller will start with a whole lot of 1s meaning “attention, message coming” followed by the address of the decoder that the message is intended for followed by the message itself, eg “start” “speed step 1” “reverse” “turn headlight on” etc etc.

When no messages are being sent the signal is just a lot of 0s (or wide squares) but this is a rare condition, like when you first switch on a brand new unit and haven't entered a loco address. If the unit owns a loco address, even if the loco isn't doing anything, the unit is constantly refreshing the commands on the track, just in case a loco was crossing a dead frog or was temporarily isolated or there was noise on the track and it didn't receive the last message. The same applies for point controllers and any other DCC decoder that's attached, if the unit knows about it, it's sending messages to it all the time. You probably didn't need to know that but I'm on a run.

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