Digital Command Control's Advantages Over Direct Current

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Thanks to Bill Barger for getting this topic started

Digital Command Control's Advantages over Analog Operations comes through the ability to control individual locomotives without any complicated switches and wiring. This advantage can come as early as your decision to purchase a second engine. Contrary to some schools of thought, serious consideration of future DCC operation needs to start at the very beginning stages of Model Railroading. Planning ahead and wiring the layout with Digital Command Control as a future option can save time and money later.

In the analog (Direct Current) environment, no matter how much you grow your power availability, all you ever control is the track. It either has power on it, or not. With DCC, you control the engines - i.e. you drive the train!! You control the speed, the direction, whether or not the lights are on and, potentially additional lighting or sound effects. You can put additional trains on the track without having to worry about extra wiring, switches, or power supplies. You do this with multiple engines on any track you want, even on the same track as other locomotives.

With the retail availability of locomotives with DCC decoders installed, we encourage all model railroaders to consider DCC at a very early point in your Model Railroading experience. By the time you have your monster layout built, and a stable of 50 or more locomotives, it's a daunting consideration to adapt all those engines to DCC. Not impossible, but certainly a big challenge and a big expense. In reality, a few good engines, DCC equipped, will cost less in the long run than the great accumulation of engines we frequently have sitting around, with no ability to run them.

Contents

Digital Command Control Mythology

Let us start with a few myths which surround Digital Command Control.

You Only Need Two Wires?

True, yet False.

In theory Digital Command Control is implemented with only two wires. But, Voltage Drop is not your friend. While you don't need block wiring, you should not try to run your entire layout with only two wires feeding the track.

With DCC, accepted practice is to feed the track at regular intervals using a Power Bus. This minimizes any electrical issues, with the added benefit of insuring your over current protection will work effectively. See the articles on Wiring for more information.

I Must Rewire My Layout to Eliminate Blocks

No. You can just set all the blocks to select the same cab, and install your DCC system in place of that cab.

Some features like reverse loops still require gaps in the rails, but the feature can be automated with a DCC Auto Reverse unit.

If you are converting from direct current (analog) to Digital Command Control, a thorough review of your wiring is in order. See the article on Converting to DCC.

My Switches are not Compatible with Digital Command Control.

There is no real issue with switches and DCC. All of your trackwork should have no issues with DCC. The special problem with switches is short circuits. DCC boosters will react much faster to a short than an old analog power pack would. Check your switches with an NMRA gauge for proper clearances, and make any modifications needed.

See the articles on switches for more information. There are also articles on the popular PECO Insulfrog and PECO Electrofrog switches and how to make modifications if needed, which also apply to other commercially available switches.

The Cost of Converting My Motive Power Fleet for Digital Command Control will be too Costly!

No one says you have to convert your entire fleet of locomotives to Digital Command Control immediately.

As you will read below, you only need to convert those you use regularly. It may not be economical to convert some older locomotives, others may have mechanical issues that need to be addressed first. You can expand the fleet of DCC locomotives as time and money allow.

You can also choose from a large number of well detailed, smooth running commercial offerings with DCC already on board, ready to run on your DCC layout. You can even get them with sound on board.

Sound is an Unnecessary Expense!

While some don't like sound and prefer silent running, this is one of those issues where many will say that once you have a sound locomotive, it changes everything. It is always a topic that generates a lot of discussion.

Digital Command Control Will be Obsolete Soon!

The purpose of the NMRA DCC Standard was to define a common protocol which would eliminate the chaos of the command control market. It won't be obsolete anytime soon. The standard is defined at the rails, not at the throttle or the decoder.

Much like Television was in the early days with competing standards, once that issue was settled the industry thrived. An NTSC colour set from the 1990s could be transported back in time to the 1950s and work. A black and white set with vacuum tubes from 1961 would have functioned until the day analog broadcasting ceased. (The experience outside of North America would have been different.)

The Internet is much the same. While it is constantly evolving with new services, the underlying protocols have not changed much in decades, allowing for innovation while maintaining compatibility with everyone's equipment and services.

Who Benefits From Using Digital Command Control?

Everyone!

Everybody can benefit from using Digital Command Control, over Direct Current, regardless of how large or how small their layout is. Here are some benefits:

  • First and foremost, the main reason for DCC is the realistic operation attained by greater control over your train. No longer do you control the track, making sure other locomotives are not in the same block, or flipping switches to direct power around the track. The realistic operations comes from the ability to slow your train to a crawl, and keep it there. There is also smooth acceleration throughout the range of locomotive speeds.
  • Next, but not any less important, your chosen brand of DCC system will not become obsolete if the manufacturer goes out of business. Nor will a new product from your chosen brand obsolete your current equipment. This is because DCC is an NMRA standard, and as such, you can use other brands of equipment (primarily boosters and decoders). You can use any DCC command station with any decoder, and everything will work just fine.
  • As mentioned earlier, a major benefit of DCC is having greater control of your trains. All systems have a basic 14 speed step control, that is, your can control your speed in 14 steps. More common in this day and age, most DCC systems offer 28 and 128 steps. Many of them have a modifiable speed curve as well as programmable momentum and braking effects. But, if that isn't adequate, the newer systems have 28 speed steps with the same programmability.
    • With 128 speed steps, you'll be able to make your locomotive crawl at a snail's pace - you won't even know they are moving unless you are watching closely.
  • Turnouts can be controlled from your hand-held throttle. So, no more need for multiple switches all over the place, you can access everything from a single device. Alternatively, you can connect a computer for control, if you wish. And there are currently several companies coming out with incredible sound systems. Because they are DCC-compatible, it will make no difference which DCC-compatible system you have, you will be able to choose from any of the companies that produce these sound systems.
  • DCC also has built in directional constant lighting with the use of decoder functions. You can also control ditch lights, cab lights, Mars lights, or anything else you can imagine…even animation. Of course, there are different advantages for different scales, but they all have those we've mentioned so far - and much more.

Small Layouts

On smaller layouts, it generally too difficult to operate two trains at once since the electrical blocks have to be really small. You end up having to toggle switches every five to ten feet; which is simply no fun.

However, with DCC there are no required electrical blocks, except if you want detection blocks for automation and animation. If your layout has a mainline, switch yard, industrial section, and/or bypass, DCC will allow you to run one train while a friend runs another; both on the same electrical block. Imagine how easy this becomes - no more cab control. As a side benefit, you'll get to enjoy your trains with a friend - the two best things in the world, in our humble opinion.


Large Layouts

Large layouts benefit from simpler wiring. A lot of extra wires, and switches, not to mention other things needed for smooth operation, are eliminated with DCC. The DCC version of blocks, are in this instance is called a power district, which aid in routing power and minimizing interruptions caused by short circuits.

A Power District can be as large as the entire layout, or the layout can be split into several power districts using boosters and/or power management devices.

The advantage comes from simple implementation, which reduces the cost and time needed to wire everything. Another advantage is that the simpler wiring scheme reduces the chance of mistakes, and speeds up troubleshooting.

A typical DC layout uses a number of small, low current power supplies located around the layout. The larger the layout, the larger the number of power supplies. A DCC system could eliminate all of them with one command station/booster, with a few additional boosters as needed. This is where planning ahead for DCC becomes important: The wiring for a DC operated layout using multiple blocks and power supplies is often inadequate for the needs of DCC. Reliable operation with DCC needs heavier wire to distribute power around the layout, whereas a small DC power supply serving a small area of the layout and one or two locomotives doesn't have the current demands that need heavy wire.

Comparing Analog to Digital Command Control

HMContoller.jpg

Before we get too deeply into DCC, we should discuss the differences between analog and DCC. In brief, DCC controls your trains, not your track. DCC allows you to control multiple trains at the same time, while avoiding complex wiring and block assignments needed for analog operation.

Analog

For standard analog controls, the ‘throttle’ or 'power pack' puts a variable direct current voltage on the track. The locomotive's motors and headlights are directly connected to the track through the wheels. This is how the speed and direction of the locomotive is controlled - more voltage, more speed. Reverse the DC polarity and the locomotive changes direction. The brightness of the headlight depends on the voltage, but regular lights don't care about polarity - only LED's do.

Don't confuse amperes (amps) with voltage (volts). Here is a simple analogy comparing water to power. The water pressure, or how fast the water comes out of a faucet, could be compared to voltage. The more pressure, voltage, the faster water comes out. If you have a small water pipe, you cannot get a high rate of water flow, or current. The wider the water pipe, or electrical wire, the more water, or current (amperes), can go through.

What does this have to do with trains? The typical control device is a rheostat, a large variable resistor that controls the amount of voltage applied to the rails. Electrical current flows out of the analog 'throttle', to the track feeder wires, which are connected to the rails. The current then courses through the train's wheels, lights the headlight, turns the motor, and the train moves. As the voltage is increased, the speed of the train increases as well.

Command Control

Command Control is a term for a method of controlling locomotives on the track independent of each other. There are, and have been, a number of command control systems made over the years. Many were analog (sometimes called Analog Command Control) in nature, with a few digital systems.

Command Control systems use a variety of voltages applied to the track, usually a constant direct current voltage. Riding on top of the voltage is a small alternating current component, which provides the control aspect. Using various frequencies, ranging from audio to RF, commands are sent to a receiver installed in the locomotive, which in turn controls the speed and direction.

Digital Command Control

With Digital Command Control, the booster puts a constant voltage on the track. This is not an analog signal, it is a pure digital waveform.

This means all locomotives have power to their wheels, all the time. Instead of the voltage level controlling the trains, a receiver (decoder) inside each locomotive listens for 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. Each locomotive responds only to commands specifically addressed to it, and ignores all others.

With this setup, you control the trains, and not the track as with analog. Because of this, DCC is able 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 throttles, command stations, boosters, and decoders.

Wiring Differences

If you have an existing layout and are converting from analog to Digital Command Control, and if you are NOT using common rail wiring, you can simply install a DCC system in place of one of the cab throttles and set all blocks to be controlled by that cab, so that all track power is connected to the DCC output. This allows you to use your pre-existing wires to setup a DCC system. The power load is shared among the smaller feeder wires from the preexisting system. You MUST be using a wiring plan that gaps both rails of the track, which requires feed wires to both rails from the block selector switches.

Note: Check the wiring first, and upgrade if needed. Many direct current setups were wired with wire sizes that are inadequate for DCC. Be sure to do the 'quarter test' to make sure the short protection will work, and upgrade the wiring as needed.

For a new layout, you need to be aware that all locomotive power comes from the track and more than one locomotive may be running in any given area at once. DCC boosters (and their power supplies) are designed to have current ratings of 2.5 to 10 amps. Traditional analog power packs are meant to run only one train at a time so they are designed to supply only 1 or 2 amps. However, because DCC locomotives and their current demand may be located anywhere on the layout, the wiring to the track for DCC needs to be designed to handle a higher amperage. DCC layouts also tend to have more locomotives on the layout at once, and to run more locomotives at a time than analog layouts. Why? Because it's possible!

For all the locomotives to run properly, you need to be sure that there are no voltage drops. That is, all sections of track must have adequate wiring to handle the highest anticipated load, and the track voltage doesn't drop below 10 volts, the point that DCC signals may become unstable. The best way to do this is to add feeder wire connections at regular intervals around the layout - for outdoor railways, about every six to nine feet is recommended.

A second reason for more robust wiring is to ensure that the over current protection 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 or going against a switch. With analog control the small power packs are 12 volts at 1 or 2 amps, or about 12 to 24 watts of power. Since you are probably operating only one train at a time, it is relatively easy to observe that the train stops when a derailment causes an electrical problem, and turn off the throttle while fixing it.

With DCC, a booster can supply from 12 to 18 volts (for larger scales) at up to 10 amps into a short circuit without becoming overloaded. That represents perhaps 60+ continuous watts. At five amps, it doesn't take long to weld a wheel to the rail or melt a sideframe. This current can quickly damage trucks, engines, or let the "Magic Smoke" out of decoders. To prevent this, DCC systems have short circuit protection built into the booster. When it detects an unexpected event, the booster will cut track power for a short period of time. Once the short has been removed from the track, the booster will automatically turn the power back on.

For this to work properly, every point on the track needs to be wired so that the full power of the booster can pass through it. This usually means heavier wire, better track connections, and more track feeders. To test the performance of the layout wiring, simply place a coin or other conductive metal object across the rails. The booster should shut off the track power automatically and turn it back on once the short has been removed.

See the track wiring page for more details.

Starter Sets & Power Supplies

A typical starter train set comes with a tiny, low cost analog power supply -- literally a throw away item due to it's rudimentary controls, limited power availability, etc. As this initial power supply is only good for only powering one engine on a small loop of track, any advancement past this starter stage requires a more advanced and expensive power supply--and provides an ideal opportunity to investigate the advantages of DCC. More advanced power supplies can certainly be purchased, and they will cost less than an entry level DCC system. However continued growth of the layout will require more power supplies, more electrical switches, more wiring, etc.

DCC System Considerations

  • Capacity, expressed in amps (A), to supply the current required for the number of locomotives you desire to operate. For many applications, 5A is adequate.
  • Engine Addresses or slots -- system limitations on how many addresses are available.
    • As each engine will occupy an address or slot, this has a direct relationship to how many engines the system can operate at one time.
  • Expandability--railroads grow, the number of engines GROW--can your system GROW ALSO?
  • Analog Operation Capable? Some DCC systems have the ability to run analog engines (engines without a decoder) on DCC powered rails using address "zero". This can be a great option, affording you the chance to run newly purchased engines, or visiting engines, without having to first install a decoder. Be aware that some motors may make a lot of noise when fed a digital signal, and coreless motors will be destroyed by the digital signal. Decoderless locomotives should not be left on the track as they may overheat.

Cost: Digital Command Control versus Analog (Direct Current)

A common argument against Digital Command Control is cost. Some claim that DCC is very expensive, and then back it up with a large figure they claim represents the cost of going to DCC.

But the truth is a little more complex.

Both Direct Current and DCC will cost about the same in the end. The difference is when those costs come into play. DCC typically has a large front end cost in terms of the equipment. Once the initial investment is made, the cost goes down.

Whereas with Direct Current, the cost is spread over time. But all those toggle switches and extra wire, that isn't free either. Just not as noticeable.

  • Starter Sets for DCC range from systems with basic DCC features that cost no more than a decent analog power pack/throttle, to expensive feature laden sets. The advantage lies with the fact that you can decide how simple or complex system you want to begin with, and there may even be an upgrade path. But it is easy to use, because you don't have the overhead needed by DC for complex operations.
  • With Direct Current, the additional wiring and switches add complexity, from an operational and technical standpoint. Operators unfamiliar with your system will have trouble operating a train. The additional wiring and switches also introduce problems, more complexity, more problems.
  • It is not necessary to convert your entire motive power roster to DCC immediately. Conversion of a few favourites or those often used during an operating session is a good start. Some locomotives may not be suitable or capable of being converted to DCC. Others may have mechanical issues that require attention first, or are not economical to correct. Those can be sold or traded. Just like the prototype, the fleet can be upgraded over time. (This is often the source of astronomical cost figures or a "reason" not to go with DCC. You don't have to convert everything immediately.)
  • The Future: It is easier to start out and build your empire in DCC, than it would be to invest substantial time and effort into upgrading the layout and motive power to DCC. If you don't see yourself running multiple sound equipped locomotives in the future, Direct Current may be adequate. Be warned, once you have seen sound locomotives in operation, it won't be the same. They also demand more current, which may be a limitation for a DCC system too. DCC opens a whole new world of operational possibilities, which you should not miss out on.

Ultimately, it is your choice. But don't let people throwing around big numbers scare you. The question is, what do you want to do? Analog may be appropriate in one case, but a limitation in another. Act accordingly. Be sure to do your research before buying a DCC system, so you don't end up with a limited system with expensive upgrade options. Or one that lacks features you feel you will need in the future.

See also

  1. DCC versus Radio Control - See comparion of R/C (non-DCC) to DCC control
  2. DCC_Tutorial_(Basic_System) for more details. Includes picture of a cab control panel and it's wiring.
  3. Decoder_installation
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