There is a tremendous amount of confusion in the hobby about the nature of the electricity found in model train tracks. This is aimed and explaining the characteristics of the different types..
Before we get into actual trains let's get the terminology straight first.
Alternating current (AC)
Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC)
The usual waveform of alternating current in most electric power circuits is a sine wave, whose positive half-period corresponds with positive direction of the current and vice versa. In certain applications different waveforms are used, such as triangular waves or square waves. These currents typically alternate at higher frequencies than those used in power transmission.
Direct Current (DC)
Direct current (DC) is one-directional flow of electric charge. The electric current flows in a constant direction, distinguishing it from alternating current (AC)
A pulse wave or pulse train is a kind of non-sinusoidal waveform that includes square waves (duty cycle of 50%) and similarly periodic but asymmetrical waves (duty cycles other than 50%).
Digital Cab Control (DCC )
This is the predominant standard digital protocol, and is maintained by the NMRA. The electrical characteristics are defined by NMRA: S-9.1 which defines the signal on the rails as:
The NMRA baseline digital command control signal consists of a stream of transitions between two equal voltage levels that have opposite polarity.
The command station/booster quickly alternates the polarity on the rails, resulting in a modulated pulse wave.
Now, let's look at some model train history. There are two ways of supplying current via the tracks to electrical model trains, three-rail and two rail.
- Three-rail systems have an electrical contact between the two outer rails and power is picked up by a slider that runs along the third rail and returns power through the wheels to the two running rails which were electrically connected.
- Two rail systems use both running rails to provide the two electrical poles.
The upshot of this nomenclature is that many people now think that any 3-rail system is AC and that 2-rail systems are DC. That was not much of a problem until digital control came along.
As can be seen by the definitions of AC and the NMRA DCC specification, there is no other logical conclusion other than the fact that DCC is a type of alternating current. The current alternates polarity rapidly, which is what defines AC current. The DCC signal is not sinusoidal, but as is clear from the Wikipedia definition, AC current is not limited to being sinusoidal.
Here are three different AC waveforms:
Now that all describes the nature of the current in the rails. When a digital locomotive gets a digital signal from the track, it feeds that into a digital decoder. The decoder decodes the information in the voltage polarity oscillations into data that it uses to decide if it needs to do anything (such as go faster/ stop/ turn on lights etc.) and it also converts (rectifies) that alternating current into a DC current. In addition to powering the decoder itself, the DC current is also fed to lights, and the motor with the polarity matching the desired direction of travel. The speed of the loco is governed by pulsing the DC current on and off at different ratios. This is called Pulse Width Modulation (PWM).
Modern decoders prefer using DC to drive the motor for two main reasons:
- It is backward compatible with DC motors found in analog locomotives as well as universal AC motors.
- It allows load control using back EMF - essentially it is able to get feedback from a DC motor to determine how fast the motor is turning.
Other digital systems
...it was decided during the design phase of the Märklin Digital system to limit it to only two "conditions", namely "positive voltage" (approx. +20 volts) and "negative voltage" (approx. - 20 volts)
The c 80 locomotive decoder receives information sent by the Central Unit or Central Control. This information is first checked for frequency. The decoder is able to differentiate conventional operation (50 Hz), information for solenoid accessories (approx. 10 kHz) and information for locomotives (approx. 5 kHz).
The "digital current" is rectified at the entry point to the decoder so that a continuous current for controlling the locomotive motor through the track is available independent of the flow of data. The power supply for the auxiliary function, by comparison, comes only from the negative part of the third rail potential.
If DC current is present in the track, the locomotive will function only when there is a positive potential in the third rail. If the potential is negative, this corresponds to the condition "turn on" in the Digital control and results in a quasi "stand by": the locomotive remains at a halt and the auxiliary function is shut off. The information for speed and auxiliary function remain stored, however. As soon as the third rail potential becomes positive again, these stored commands are carried out.
- Amazingly it is possible for both MM and DCC to coexist in the same track. This is achieved by controllers such as the Intellibox, allowing both MM and DCC decoders to run on the same track.
- Many decoders can handle multiple protocols and even analog, switching to whatever style of current they detect.
- If you are still not convinced that digital track signal is a form of AC, place an LED with a 1K Ohm resistor across the two track poles. LEDs can only tolerate power in one direction. They will burn out if AC current is applied or DC is applied in the incorrect polarity. If the LED lights up and stays on, then you have DC or pulsed DC current. This can be confirmed by reversing the LED, switching the polarity. If it then burns out, it confirms that you have DC or pulsed DC power. If an LED burns out no matter what the polarity, then you have AC power.