DCC Basics: Power for Trains

As noted on the DCC Basics: Power Systems page, the function of a DCC system is to put voltage onto the rails in a way that enables it to be used (along with the control information also provided) by a DCC Decoder to operate the motors and accessories of model trains.

Note: for information about the power systems on my layout, see the Electrical Systems subsection under The Model Railroad.

Also see the DCC Basics: Power page for information on measuring voltage on a DCC layout.

Voltage and Trains

Why is voltage important? Because voltage is what controls the speed of a DC motor. At zero volts it’s not turning. At 6 volts DC a “12 volt” motor should be turning at something roughly half its nominal maximum speed. And at twelve volts DC, it should be at maximum. And that’s true for any (normal) model train, sort of. In the real world, nothing is that clean, and different scales have different conventions. Most HO systems are based around 16 volts (the normal maximum specified by the NMRA), most N-scale around 12 volts (the normal minimum), and some trains don’t follow these conventions at all. And then there are the power supply manufacturers, who seem to operate on a “more is better” philosophy, probably to avoid voltage getting too low due to losses in poor track wiring. A typical Kato N-scale pack puts out about 14 volts maximum, not 12, and power packs for the HO modeler often put out 18 or more (some popular MRC models put out 22 volts at max throttle). However, that’s not the full story.

The maximum voltage for a DC motor in a model train is codified in the NMRA’s Standard S-9 Electrical, which has been unchanged since 1984, before DCC was introduced. Actually, what the NMRA says is that the maximum voltage shall be “not less than” 12 volts DC at the rail. NMRA Recommended Practice RP-9 Electrical further recommends that DC power packs should produce a maximum between “12 and 16 volts [DC] while delivering rated current”.

While motor control in DCC is more complex than simply varying the voltage, the maximum available voltage is still the limit for the maximum available speed. In practice most trains don’t need even the “typical” maximum voltage to run at their highest prototypical speeds (high-speed passenger trains, aka bullet trains, are an exception).

All that makes perfect sense, but there’s one last bit: S-9.1, Electrical Standards for Digital Command Control, All Scales (PDF), says that “The RMS value of NMRA digital signal, measured at the track, shall not exceed by more than 2 volts the voltage specified in standard S9 for the applicable scale” and “The minimum peak value of the NMRA digital signal needed to provide power to the decoder shall be +/-7 volts measured at the track” (which implies an RMS voltage of 7 volts, not 12).

Note: when I describe voltage as “N volts DCC” I’m referring to the DCC RMS voltage, which is what a device like the RRampMeter will measure. The RRampMeter is an essential tool, and no operator of a DCC layout more complex than a dozen feet of track should be without one. I own two: one portable, one built into the layout’s power panel.

Now what’s interesting here is the reference to “for the applicable scale”, because NOWHERE does the NMRA actually require or recommend that there should be different voltages for different scales. The closest they come is in S-9.1 where the diagrams showing min/max acceptable voltages for power supplies and decoders have lines showing “typical” N, HO and O/G voltages (which appear to be 12, 14.5, and 18 volts respectively). But that’s clearly a reference to existing practice, not a recommendation. I’ve seen numerous online references to NMRA “requirements” that voltages be 12, 14.25 (that precise number from multiple sources, not 14.5), and 18 volts for those scales, from people who ought to know, and yet the standards don’t actually say that. The NMRA Conformance Testing requirements (PDF) will pass any command station or booster with an output between 7 and 22 volts RMS.

This suggests that S-9 (and the RP that goes with it), is a bit dated, and that the NMRA has subsequently been thinking about a wider range of acceptable voltages. This makes a bit of sense, as in 1984, when S-9 was last updated, HO and O dominated, and N was a niche scale. Today, there is substantially more N-scale use, and there is also Z scale which is even smaller. Electrical standards appropriate for HO aren’t necessarily correct for N or Z.

My reading of the NMRA standards is that a DCC signal having a minimum peak voltage of 7 volts is required “at the track”. That’s in conflict with the requirement that the DCC RMS voltage of that signal should not be less than 12 volts. That’s probably just sloppy writing, and what S-9.1 should have said was “at the track outputs of the power station”, with the difference between 12 volts and 7 volts being down to losses in bus wires, track, and locomotive pick ups.

A sensible interpretation would be that a command station for N-scale use should put out 14 volts DCC (12+2), while a command station for HO use should put out 18 volts (16+2) DCC. However, most actually put out between 14 and 15 volts DCC and systems that did put out 18 volts actually caused problems for some sound decoders.

So, How Many Volts?

Which doesn’t tell me what the voltage should be for an N-scale train. The NMRA only says that 12 volts DC is “typical”. My questions are: what is the “best” amount, and what are the safe limits (maximum and minimum)? The only real guidance is that most equipment was probably made to expect the older range (12-16 volts DC) from DC power packs, and so it should be safe to operate in that range. And track loss can be several volts, so voltage at the track as low as 10 is probably reasonable. But what’s the “right” voltage? That’s a harder question.

More isn’t necessarily better. The most common problem is having too much voltage, which causes engines to run too fast, and may damage lights and sensitive electronics. While the Digitrax Zephyr puts out 13.8 Volts DCC (measured with my RRampMeter), some entry-level systems are designed primarily for HO use, and put out substantially more, some 18 volts or more, which have caused Soundtraxx sound-decoders to malfunction. The NMRA maximum limit on “power station” output is 22 volts, but that’s really a safety limit, not something that’s desirable for a command station or booster to put out. The normal maximum output for a command station or booster intended for large-scale (O or G) use is 18 volts DCC RMS, which fits the NMRA recommended (RP-9) maximum of 16 volts DC plus the 2 volt allowance of S-9.1.

With passenger trains, lighting is also a potential issue. Lights (bulbs in particular) produce waste heat, which in an enclosed space can build up and melt plastic. If the lights are controlled by a decoder (like headlights usually are), they might be given a clean 12 volt DC signal by the decoder (that’s a big “might” though, and more likely they’ll get track voltage less about one volt lost in the decoder). And in any case, car lighting is often not controlled, and simply takes the voltage from the track. This means that extra track voltage goes straight to the lighting, potentially overdriving it.

Overdriving a bulb can shorten its lifespan, or even melt a model. Overdriving modern LED lighting is worse. It WILL shorten the life, often significantly, and could cause heat-related problems, although LEDs produce significantly less waste heat than bulbs. But the real problem is that LEDs have a limit, and going beyond this simply causes the LED to fail immediately (and possibly melt, which could damage the car it is in). LED lighting designed for DC use should be safe to at least 16 volts DC, since many DC power packs are rated for that voltage, and possibly to the NMRA 18 volt limit. But that may not be as true of Japanese trains, as the market there is largely N-scale. Also, what’s going to matter here is probably the “peak” voltage, which in practice will be a bit higher than the “DCC RMS” measure of track voltage due to overshoot.

There are other components involved as well. LEDs use resistors, and some LED-based lighting uses a rectifier (or diodes) to keep it constantly lit whether the train is in forward or reverse. Often these are made with tiny surface-mount components that can be overloaded themselves at voltages above those expected. Yet another reason to be sensitive to maximum voltage.

It is pretty clear that N-scale trains built for DC are generally intended to operate at maximum voltages of 12 volts DC (in fact that’s the rating of Kato’s power pack), although they may be safe to operate at higher voltages. I believe that derives from the original multi-vendor de-facto standards for N-scale trains, rather than from any NMRA standard. And since a DCC decoder can put out up to nearly the track voltage to the motor, that’s clearly a good limit to honor. Many DCC command stations and boosters have selectable output voltages by scale, with “N-scale” meaning 12 volts (DCC). So, despite the fact that there isn’t an NMRA standard (or any formal standard as far as I can tell) that spells out per-scale voltage requirements, I clearly want a command station and/or booster with a nominal track voltage of approximately 12 Volts RMS (i.e., “12 Volts DCC”) for my N-scale trains.

Kato markets the Digitrax Zephyr (which puts out a 13.8 volt DCC RMS signal), and they’ve told people that operating their car lighting on DCC without adding the DCC controller is okay (“okay” doesn’t mean it won’t shorten the LED life though). So a 14 Volt DCC RMS track voltage should be a safe maximum even for Kato’s trains, and perhaps for other Japanese manufacturers (although I have reason to suspect others are more sensitive). More might be safe, or might not.

One caution: light rail models (i.e., “trolleys” or “trams”) from Japan are often intended for use with lower voltages, because the models run slower than normal trains. This means that even 12 volts is high for such a model (9 would be more typical) and thus if these are converted to DCC, using a system with more than 12 Volts is probably a bad idea.

The other question is minimum voltage. That’s harder, as there are no specs. Various DCC systems I’ve seen are rated to work properly at the NMRA minimum voltage of 7 volts DCC RMS, but not all, and that’s clearly lower than you want to be. I’ve seen reports that Kato locomotives will work okay with just 10 volts, and that makes sense to me, since they’re intended for track with lots of Unijoiners, which is going to experience loss, and Kato’s power pack puts out 12 volts maximum. I don’t know that that’s true of all N-scale trains I’d run, but it seems reasonable. So that’s my target range: 10-14 volts DCC RMS at the rail, with a desire to try to stick close to the center of that range at 12 Volts DCC RMS to keep peak voltage as low as reasonably possible while still allowing for dirty track and Unijoiners.

Current and Trains

If voltage controls the speed, it’s current that provides the power to maintain that speed under load. The more work a motor is doing, the more current it will draw. An individual N-scale train may use 0.5 - 1.0 Amps (500 - 1000 mA), particularly if equipped with older bulb lighting, and one with a sound system could use even more. However, a modern motor draws less than 100mA except when stalled, and likely under 250mA at stall current (decoders need to be sized based on stall current). In typical use, even a long passenger train with LED lighting isn’t likely to draw more than 200mA under normal use, and it can easily be half that.

The size of a power supply depends on the number of trains and their typical power needs, plus a margin. You need to have at least an Amp between the setting of the circuit breaker(s), which is the most your trains can draw, and the rated limit of the command station. Thus a 3 Amp command station can probably power ten typical trains ( (3,000 mA - 1,000 mA)/ 200 mA), and a five Amp station 20 trains. This can vary a lot in practice however.

With circuit breakers used to divide the layout into electrically isolated sections, each section will contain at most a few trains. In addition, Unitrack (which I’m using) is rated for a maximum current of 3 Amps. So that’s a good setting for the circuit-breaker trip point, meaning the zone protected by a single circuit breaker shouldn’t contain more than 20 modern N-scale trains at any time, and you probably want to ensure it’s always fewer to avoid any spurious circuit-breaker trips.

But that doesn’t say how much current the command station (really the power station contained in it, or any booster) should supply. One of the rules of thumb is that for N-scale, 5 Amp supplies are a good maximum. But that’s mainly to reduce the risk of damage in a short, and with circuit breakers that isn’t really a requirement since they’ll provide the protection at a lower value, regardless of what the command station can do. I chose to use a 5 Amp command station, but I could probably have safely used a larger one.

Of course, in the worst case, the power that can be unleashed in a short is limited by the size of the power station. More power, means more chance of damage. That’s a good argument for using multiple smaller power systems (e.g., several 3 Amp boosters instead of a couple of 5 Amp boosters or a 10 Amp command station). However that can cost more, and 3 Amp boosters aren’t exactly common, so most people have settled on using 5 Amp systems.

Note that older trains, and larger-scale trains like HO, have larger requirements than what I’ve described above, which is more specific to my N-scale modeling.

DCC Power Supply

The Digitrax command station I settled on using, the DCS 100, includes a built-in DCC “power station” to provide up to 5 Amps of track power. With six tracks (not counting the helix or storage tracks), and the likelihood of having at least two trains on each of the commuter tracks (three if I want to get clever), it’s possible that even a 5 Amp command station could be inadequate. However, the more I look at it, the more it seems that a typical train is going to use less than 200 mA (0.2 Amps), so one 5 Amp supply should cover my needs. But just in case, even though I’m starting with just the command station, I’m planning for subdividing things and adding a booster later if I want.

My command station, and any future booster stations (add-on power stations), needs a 5 Amp 12 Volt DC power supply. Digitrax makes the PS2012 supply, which can drive up to four such stations. A “scale” switch on it controls the output voltage: set for N-scale, it outputs 13.8 volts DC (which is “close enough” to 12 volts, per Digitrax’s manuals) at up to 20 Amps total. A special cable, part YC52, splits one of the supply’s two outputs into two, with inline 5 Amp circuit breakers. Without these, some separate fusing would be required to protect the command station input line. With a pair of these cables, and the supply set for N-scale, I have four 5-Amp, 13.8 volt DC power supplies. I only need two, so the other two will be used for DCC accessories.

But honestly, from what I know now this is seriously overkill. I could probably have run the entire layout just fine using my old Zephyr command station. It’s only likely limit was the “10 simultaneous trains” limit, and I could have replaced it with the current Zephyr Xtra and added a small booster like the Railstars RailBooster to address that if I’d wanted. It might be a bit more expensive than what I did, but my current thinking is that more, smaller, systems are the best approach for N-scale. For my “next” layout, whenever I get to that, I’m strongly considering use of multiple 3 Amp boosters instead of one big central 5 Amp supply.