Digital Command Control - Power

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Summary: Digital Command Control may seem very complex, but armed with the proper knowledge, you'll soon understand what makes model railroads operate with DCC.


The Digital Command Control Waveform

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Overview of NMRA Digital Command Control – Power

You don't need to fully understand the technical details of how DCC works to use DCC on your railroad. If you don't understand everything, don't worry.

NMRA Digital Command Control, unlike other analog and digital Command Control systems, puts a 100% digital signal onto the rails, delivering both power and data in the same signal. The digital information is encoded in the time domain by pulse width, not amplitude. It is not superimposed on a DC or AC waveform, nor does it use a high frequency carrier, unlike past and current command control systems. Since the rails are alternately energized or held to zero, there are no issues with polarity.

Since the signal is completely digital, the NMRA Digital Command Control waveform is a square wave. The nature of a square wave results in more demanding requirements for wiring to avoid voltage losses and signal distortion compared to analog control methods.

Too Long Didn't Read (TLDR)

There are many myths surrounding the DCC signal. Most of those myths are the result of applying analog ideas to a digital concept. Some come from a literal interpretation of the NMRA DCC Standards. Others have been promoted over the years due to misunderstandings or dislike for the DCC technology. Another source of confusion is the application of incorrect terminology.

To truly understand it, you must not attempt to apply analog (DC) concepts to Digital Command Control. Many misunderstandings and myths are a result of attempts to fit DCC into an Analog Direct Current framework.

  • The booster creates a binary ON or HIGH/OFF (LOW) signal which is applied to the power bus, which is sent to the track, then the train.
  • The binary value is determined by the period from one HIGH state to the next: 106µs (58 + 58) has a value of One, 200µS is a Zero.
  • The data is encoded in the time domain. Rise times and the amplitude of the signal carry no information.
  • There is full power on the rails at all times while the booster output is turned on. Voltage does not determine locomotive speed.
  • There is a signal phase present on the rails, where one rail is energized (HIGH/ON) while the other is LOW/OFF. One rail will always be in the opposite (inverted) state compared to the other.
  • There is no polarity as there are only two possible states: HIGH or LOW. There are no negative voltages present.
  • Current flows from HIGH to LOW.
  • The rails are identified as A and B, whose phase can be handled by auto reversers when A meets B, without need to flip toggle switches.
  • The phase of the rails does NOT control the direction of the locomotive.
  • The DCC Signal is in the audio range. There are no high frequency carriers in the Megahertz region nor does "rise time" of the pulse carry data.

The DCC Signal

The Booster

H-Bridge audio amplifier Class D, the speaker could be a motor or the track. The transistor pairs are arranged diagonally across from one another.

The booster is a digital amplifier. It receives a logic level waveform from the command station. Logic level signals have two states: On or Off. The data is contained within the period of the on/off sequence. This logic level voltage is amplified to the necessary track voltage determined by the scale of your model trains.

The amplification is performed by two pairs of transistors functioning as switches. When the digital input is low, one pair is switched on to create a path for current to flow (Source to Sink) though the power bus. When the digital signal changes to the high state, this pair turns off and the other pair turns on. For example, when the digital signal is HIGH or ON, pair A is switched on, delivering power to Rail A and facilitating its return to the power supply. When the input goes low or OFF, the inverter (lower right on schematic) causes the booster's circuitry to connect Rail B to the power supply via transistor pair B. This switching sequence creates the pulses on the rails which deliver both data and power to the multifunction decoders on the track. Most boosters are constructed to prevent any output in the absence of a digital input signal from the command station.

The booster's power supply determines the total power available, and must be selected accordingly. Refer to the instruction manuals for details on selecting the correct voltage and current capacity of the power supply for your DCC system, as many are not delivered with a power supply.

Power

Power is a measure of energy. Energy is required by the multifunction decoder to operate the motor and lighting functions, as well as any other features it has. This power is delivered to the track by the booster.

The amount of power a DCC system can deliver is determined by the capacity of the both the booster and the power supply. An undersized power supply will limit the amount of power available. For N scale, the amount of power needed is low, from 36 to 60 Watts. For HO it can range from 50 to 80W. With larger current boosters, HO can reach 125W, and with larger scales, 240W.

Analog control systems rely on multiple small power supplies delivering low power to the track, such as 12 to 15W. The potential for damage when a short circuit occurs is limited, unlike a DCC booster which can deliver 5 or more amps into a short. For proper operation DCC requires more robust wiring (the power bus) than analog direct current, both for delivery of energy to the decoder, and quick reaction to a short circuit condition.

Volts

The voltage output of the booster is dependent on your scale. N and Z Scales require much less voltage than O or Large Scales. Where N Scale may require 12V, a large-scale locomotive may require up to 24V for proper operation.

Common DCC Track Voltages by Scale
Z N HO S O G/Large Scale
10 − 12 12 − 14 14 − 16 22 − 24 *
Note: These voltages are based on which scale the booster is set for, not on which scale you're actually running. The booster has no way of knowing what gauge track it's connected to! The user must select the appropriate voltage for their application.
* The full NMRA specification of 24 V is recommended if you run anything but slower narrow gauge steam locomotives. Large scale diesels from Aristo-Craft for example cannot achieve prototype mainline running speeds on 22 volts, they need the full 24 volts to overcome the low gearing. Also speed matching requirements for consisting means you need some "headroom" in voltage.

Source: Digital Command Control

Current Requirements

The amount of current required is determined by two parameters: The maximum amount of current the booster itself can deliver before overheating, and the current the power supply can deliver. Ideally the power supply should at least match the booster's current handling capacity. A power supply with a lower current output will work.

Of those two parameters, the booster limits the maximum current can be delivered to the track. The booster will also have the capacity to disconnect its output from the power bus should a short circuit occur, to protect itself from damage from excessive heat.

Signals on the Power Bus

Oscilloscopes display signals as vectors, as they have both amplitude and direction.

ScopeTraces-1.png

The oscilloscope traces on the left show (in Purple) the sum in Volts DC of both Rail A and Rail B. Rail A is the trace in Yellow, Rail B is in Blue. The yellow and blue pulses show the booster's output transistors switching states as the follow the data signal from the command station. The numbers 1 and 2 at the left represent Zero VDC present on the rail. Both output channels connect to a common point (labelled "ground") while the probes are connected to the Rail outputs. The Purple trace is twice the amplitude, as it is the sum of both signals. The purple trace represents a Differential Signal with no Ground, as per the NMRA Standards. (Note 1)

The digital data is represented by the time (period) the rail is energized. There are no positive and negative voltages on the rails, the rail is either On or Off. The multifunction decoder only sees its input signal go high (On) and low (Off). A total period of 58µS High + 58µs Low (116µS in total) represents a binary value of 1, a total period of 200µs represents a 0 value.

This is not an AC signal, as the rail will have positive voltage on it, or none, at any point in time. What is happening is the current is changing direction as it moves from one rail to another. Measuring this with an oscilloscope will display a peak-to-peak signal, which many will claim supports their assertion that the signal is an AC waveform. Since there is no reference point, the trace only indicates which connection was more positive than the other at a point in time.

Mathematically, the signal would be expressed as X +jY + X − jY, where X is time and Y is the amplitude or voltage.

This method of transmitting data and power in the same signal results in a very robust signalling technique with a high signal to noise ratio while reducing the space charge around the rails and the electrostatic attraction of contaminants. A multifunction decoder can receive data regardless of the orientation of the locomotive, as either rail has data available.

Signal Quality

The ideal Digital Command Control waveform is a square wave. Yet, the influence of the power bus and track, as well as the design of the booster present a number of challenges, such as distortion of the waveform.

Distortion can include ringing and switching (zero crossing) issues. The NMRA Standards include the requirement that both booster and multifunction decoder minimize any issues which may arise, such as the waveform quality at the booster output to requiring the decoder to accept a predetermined amount of signal degradation.

Matching the impedance of the track and power bus to the booster is impossible given the multitude of factors present in layout wiring, so the engineers designing DCC boosters have to try their best to ensure a good quality signal within the tolerances specified. They optimize the time the output is off during the transition from one state to another, and how quickly the on/off sequence should occur during the bit transitions. At the same time, they are trying to minimize excess heat production in the output stage.

Increasing the dead time between switching from one pair of transistors to the other comes with the possibility of increased distortion. Decreasing the period between switching can introduce ringing (overshoot) in the waveform. Ringing can be controlled using a termination on the bus.

Track Polarity

Digital Command Control has no concept of polarity. Binary signals do not have the concept of negative. A negative voltage would be considered an Undefined Value.

The track voltage on an oscilloscope's display indicates the presence of a plus/minus signal. It "sees" a negative voltage when the booster flips the rails, as the reference point is floating. There is no common reference point held to a zero potential. Another way to look at this is how the current is flowing: It will flow from A to B, then B to A. The scope trace will display this.

Oscilloscope trace of a Digital Command Control waveform as seen on the track. (Probe is connected to one rail and ground clip to the other.)

The track voltage has phase, one rail is always the inverse of the other. Phase is an issue with reverse loops and crossovers. A short circuit occurs during a phase mismatch, as current can now directly return to the power source. As mentioned above, the booster output has two pairs of transistors, or switches, each pair consists of a switch which connects the load to the power source, the other allows the current to return to the power source, completing the circuit. The state of these pairs is constantly changing in step with the data signal from the command station.

Unlike analog where polarity of the track controls the direction of the train, the direction of the train is controlled by the multifunction decoder in the locomotive. The decoder receives instructions from the throttle via the command station and controls the speed and direction accordingly. Physically re-orienting the locomotive has no effect.

There are claims the DCC track signal has positive and negative values, yet this would only be possible if:

  • The power supply, with two wires, has an output voltage of at least twice that of the track voltage, for example 30VDC at 5A
  • The power supply has three wires supplying both positive and negative voltages with a common return/reference point (15VDC / 0 / –15VDC)
  1. Sure, the booster could have a voltage doubler incorporated into its design. That adds cost and complexity for no real advantage.

It is not an analog waveform, where frequency, phase or amplitude have meaning.

Summary
Simple On/Off (binary) signal, where one rail's state always the inverse of the other.

NMRA Definition of Positive Rail

Input circuit for a multifunction decoder.

The NMRA defines the positive rail as theright hand rail when the locomotive is facing forward. This is completely subjective, dependant on the direction the locomotive is facing. The actual direction of travel is determined by the multifunction decoder itself.

The input circuit for a multifunction decoder consisting of an opto-isolator, with a "freewheeling" diode to protect it from a reverse bias condition. This circuit allows the vehicle to face either direction while still providing a usable data signal to the multifunction decoder.

Digital Command Control Signals

Main article: Digital Packet
The DCC signal produced by the Command Station. This shows the binary On/Off nature of the signal, and each cycle consists of an equal On and Off period.

Advanced DCC Power Topics

The Advanced DCC Power Topics are provided for information purposes. It is not necessary to understand these topics to enjoy Digital Command Control

NMRA Digital Command Control Standards

The DCC protocol is the subject of two standards published by the NMRA: S-9.1 specifies the electrical standard, and S-9.2 specifies the communications standard. Several Recommended Practices documents are also available.

Notes:

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1: https://www.nmra.org/sites/default/files/standards/sandrp/pdf/s-9.1_electrical_standards_2020.pdf

Read the entire document and any notes. The standard is deliberately not specific, as it only describes what the result should look like, not the exact construction. Also note it does not define polarity, as that is a quality wherein the direction the locomotive defines the positive rail.

See Also

Square Waves and Fournier Series

All about how you can create square waves from a sine wave and its harmonics.