Train Power - When DC is not Direct Current

This is the second (or third if you count the previous diversion into history) in a continuing series of posts on powering model trains. The first installment covered some basics of DC motor design, and if you haven’t read it, it’s worth at least a skim before reading today’s post, since it provides a lot of background information (or read the DC Train Motors page, which has been updated considerably from the original material). Today we’ll get into how modern “DC” power packs aren’t really DC, and what aspects of DC motor behavior make that a good thing.

Most modern “DC” power packs don’t really put out DC in the sense of a fixed voltage for a given throttle position. The reason for this is that DC motors don’t really run very well at slow speeds. They work best at a fairly high rotational speed. In the early days hobbyists had to be content with models that went from “stop” to “rather quick” with no intermediate speed. This is often referred to as a “jack rabbit start”, and it’s hardly prototypical. Real locomotives weigh several hundred tons. Nothing that heavy accelerates quickly, and if it’s pulling a train that can weigh upwards of 10,000 tons it’s even slower to get up to speed.

Passenger trains (multiple unit type) tend to weigh less, and often have more and distributed power so they can accelerate faster. But there are limits to that, as passengers tend to object to being hurled backwards. And “weigh less” still means that a train will weigh 100 tons or more without passengers, and a full load of those can easily add 50 tons. That’s a lot to accelerate quickly.

I’ll get into the details below, but the short version is that modelers discovered that “pulsing”, or varying the power in a repeating manner, allowed a train to start more gently. This wasn’t without its problems, and various different approaches were tried over the years, although the earliest technique proved quite effective and continued in use for over twenty-five years. In fact many systems today use a technique known in the 1950s, but not considered “the best” at the time. It’s still a compromise, but a popular one. Today all but entry-level DC power packs produce pulsed power in some form, and it’s so commonplace that most don’t even mention it. There are exceptions, of course.


A Short History of Transistor Throttles

My interest in the design of transistor-based DC throttles (aka Power Packs) for model railroading ended up causing me to pick up the DVD set of Model Railroader back issues (henceforth identified as MR). While US$200 seems like a lot, I think it was well worth it, if only to satisfy my curiosity. And it works out to less than US$0.30 per issue, so in a sense it’s a bargain. I also dug up a copy of Peter Thorne’s 1974 book Practical Electronic Projects for Model Railroaders (mine is the third edition of 1975), which has a number of throttle circuits, including one using an SCR. This book can go for rather high prices online, but I found mine at a train show last week for the cover price of US$3.50; quite the bargain.

Early-on electric model trains were run with car batteries (some early ones used AC motors with AC from a transformer instead), first apparently at 6 volts but by the 1930’s DC motors were apparently designed for 12 volts even before cars switched to the larger batteries, requiring two batteries placed in series (per MR August 1934 article on the use of DC power). DC at 12 volts was more than enough to run small motors, and early throttles were little more than a variable resistor (rheostat) to reduce voltage for slower speeds, and a Dual-Pole, Dual-Throw (DPDT) switch to reverse polarity for direction control. Often a “knife” switch would be used for the reverser, which could be left in a central “off” position to disconnect the throttle from the track.

But modelers weren’t very satisfied with these. DC didn’t allow for smooth low-speed operation, and “jackrabbit” starts with a minimum speed over 10 or even 20 scale miles per hour (16 - 32 kph) made for poor switching operations. Plus, modelers wanted to model the behavior of real trains, with simulated momentum and realistic braking action.

This led to designs for more sophisticated “throttles” and ever more complex designs as electronics technology improved. Some of the results did a fairly good job of replicating the real behavior of trains, right down to simulating the performance of air-brake systems similar to the one in the diagram at the top of this post. It’s possible some of this took place before the transistor was introduced; vacuum tubes could have been used for similar things. However, nobody appears to have published their experiences with these, so it seem likely that little or nothing was done until the transistor came along.

The development of the low-cost transistor in the late 1950’s made more complex throttles accessible to a hobbyist with a relatively minor amount of electronics skill and for a reasonable price, and the next decade was a time of rapid change, with evolution continuing into the 1970’s. By 1980, interests had shifted towards running multiple trains using command control systems (the precursors of DCC), although the roots of those went back further. And even in 1980 you could still buy rheostat throttles, although they were definitely behind the times by then. None of these technologies fully displaced the others. The transistor has in fact soldiered on into the era of digital controls, and you can still buy transistor throttles today that aren’t too different in principle from those designs of a half-century ago.


MRC Tech 7 780

DC “Power Packs” for model trains tend to be designed for HO scale trains, at least in North America. There are separate ones for G scale, and some multi-scale packs catering to both HO and larger scales. But those sold for N scale are typically just HO models, with the usual HO “16 volt” or higher output, usually called “Universal” power packs.

Anyone who’s been involved in model railroading for any length of time knows the Model Rectifier Corporation, more commonly just MRC. They’re probably the dominant supplier of DC power packs for HO and N-scale model railroading in North America, aside from those included in starter sets, although Kato’s Unitrack power pack likely gets a fair amount of N-scale market share. But I’ll admit I don’t have hard facts on market share for anyone.

My first power pack, after a cheap Tyco one that came in a starter set, was an MRC Throttlepack 501, circa 1972 (yeah, I’ve been doing this a while, although I took a long break after High School). That 501 still works today, although one of the switches has finally started having problems. But it survived 40 years of damp basements, being thrown in boxes for moves, and running a large variety of trains. That kind of quality leaves a lasting impression. I’d followed it with another MRC, a Control Master II, when I got back into model railroading and needed a second power pack (back around 1992). It too has served well, and still works. But both put out relatively high voltages (19V+), so I’d rather not use them with N-scale trains.

So when I started looking to see what was available in a modern design suited to N-scale, and discovered the MRC Tech 7 line had a model with a rated 14.5 V, 10VA output, it didn’t take me long to decide I wanted to give it a try. It’s a dual-cab power pack, with two independent throttles in one box. But I’m going to use it on a double-track line where I’m normally the only operator, so that’s fine.

Train Power - DC Motor Basics

’m going to kick off the new year with a series of posts on some basic topics related to model railroading. Hopefully that will give me something to think about while my work on the layout is stalled, and reinvigorate my interest. I’m going to start with several about motors and control systems. Much of this recaps material I’ve covered in the past, but hopefully in a more concise and readable form.

Also, since my present modeling is all N-scale, I’m looking at things from that perspective. Much of this is equally applicable to other scales, although Z-scale motors may be somewhat different (“coreless” motor designs are more common there, and I’m not going to get into that).

While many trains today are used on Digital Command Control (DCC) systems, I’m going to focus on non-DCC systems that you’d find in a typical off-the-shelf model or a basic set, similar to the locomotive and DC power pack shown above. The same motors will work with DCC, with some additional electronics (the “DCC decoder”), so this is a good foundation regardless of the type of layout used. Future posts will cover more complex topics, but for now let’s stick to the basics: simple voltage-controlled motors and DC power.

Update: I don’t normally modify Musings after they are posted, but I have made some edits to this one to clarify sections and add some minor details overlooked in the original. These don’t alter the original in a significant way, but may make it clearer to some readers and help provide better background for later posts.

Kato DC Power Pack

Several years ago I investigated several DC power packs, including one of my Kato packs (I have three of them). At the time I was reluctant to take one apart because the screws are hidden under the glued-on feet. But a question on the JNS Forum spurred me to investigate the circuitry more closely, and so I took one apart. And in the process, I discovered that some of my old information was wrong.

Arduino Controls and a Simple Throttle

Having covered the motor control and the sensors, my next step in creating the automated two tram controller was to deal with the very small number of controls I need to have. In my original design, the plan was to have just three pushbuttons:

- Run: when pressed, the trains would start to move.
- Park: when pressed, the trains would return to their starting locations so the system could be turned off.
- Emergency Stop: when pressed, the trains would come to an immediate stop until run was pressed again.

And all three were to be “on when pressed, off when released” pushbutton switches. I was already thinking this needed to be changed, and when I started playing with switches I became even more convinced.

There are two benefits to using toggles versus “on while held” button switches: first, I eliminate a switch, since I need two toggles rather than three pushbuttons. Second, I avoid using those pushbuttons, which have proven to be problematic in my testing. They tend to “bounce” for a long time, and they may remain “on” only for a fairly brief time, making it hard to avoid all bounces and still reliably tell that they were pressed in the first place.

The other control I wanted to add was a way to customize the top speed of a train (or trains, really), so the different models could be made to run at similar (and prototypical) speeds. I don’t want one train rocketing down the tracks while the other creeps along, even if they have very different motors and gear-trains. So I’m going to add a pair of potentiometers, used as “tram #1 max speed” and “tram #2 max speed” controls. And note that these are for the trains, not the motor shield A/B outputs. At various times each tram will be controlled by one or the other.

My first step was to create a simple throttle that used one pot, three switches, and one motor control to run a train on a simple test track. This gives me a proof-of-concept example that ensures I really understand what I think I understand (always a concern with me and electronics).

Arduino CPUs and Motor Shields

I’m still playing with Arduinos this week, but the little beasties are multiplying. That’s partly because I want to be able to test with the various CPU architectures, but also because each has unique strengths and weaknesses, and I’m still evaluating which of them is going to be the right choice for my tram controller. At the same time, it turns out that there are a number of options for motor shields, as I mentioned last time, and they too have strengths and weaknesses.

Revisiting Arduino

Two years ago I wrote about my plans to use an Arduino to control a pair of trams on a double-track line sharing single-track stations at each end, replicating the design of the Tōkyū Setagaya line. That’s briefly written up in that earlier post, and I also have a full page about the project that goes into more detail, with a sub-page about the current testing work.

Back then I bought an Arduino Mega 2560 and a bunch of other parts including a “shield” used to drive DC motors, loaded the basic test program that blinks a LED into it just to convince myself I could do it and the hardware worked, and then put it on a shelf for “later”. And forgot all about it while I worked on other things, only infrequently looking at the empty tram tracks and thinking “I really should figure out how to make that work.” It’s not like I dislike the work: I enjoy both programming and soldering simple circuits together. I just never could work myself up to starting what I expected to be a fairly major project. It was always: “I’ll work on that after I finish project ‘X’”, but there was always another “X” to occupy me.

Other Lightboards

Up until now I’ve been concentrating on my Kato models as far as DCC conversion goes, and at the same time I’ve only really paid attention to the interior lighting of those cars. Now that I’m working up to large-scale DCC conversion of my non-Kato stock, which is mostly MicroAce and Greenmax at this point, I need to think about lighting the interior of those cars.

And it turns out, they’re pretty much identical. Both MicroAce and Tomix make their own interior lighting kits, and they’re very similar. A third-party company, F&MOKEI also makes lightboards, and claims they work with both manufacturers’ cars. And Greenmax notes compatibility of its cars with Tomix lights (Tomix is far larger a company than MicroAce, so it makes sense they only mention one).

More Wires

After saying I was going to finish up the DCC electronics several times, I’ve finally made a start. There are two parts to this. The first is finishing up the remaining DCC power protection and occupancy detector panels. I did the first three tables last year, starting around January. The third was the first I’d done as a removable panel, which I wired up back in November. At the same time I’d begun the work of setting up a third accessory power bus, adding a switch and meter to the main panel. However at that point I’d stalled, with my attention off the layout over the holidays, and when I returned it was to work on buildings.

Bus Wiring and November 2011 Status

DCC is often said to simplify a model railroad because it requires “only two wires”. While that’s true to an extent, most real model railroads will require quite a bit more. Or maybe I just like to over-complicate things.

DC Signals I: Kato

This is the first of what will probably be two postings about simple trackside signals for DC layouts. This one will discuss the basics common to the Kato and Tomix signal systems, and then provide details about Kato. Since I’m partly looking at the Kato to see if it’s a good candidate for modification for my DCC layout, I’ll have a few comments on that as well. A future posting will cover Tomix’s richer offering, which I’m looking at for use on my DC Urban Tram Layout (which will probably use the Tomix signals), although the four-color signal heads might be something I could use for my DCC layout (nobody in the U.S. makes a four-lamp signal of this kind).

Miscellany and March 2011 Status

March was another of those “not much obvious happened” months. I did manage to get the layout back together, with two of the circuit-breaker/block-occupancy-detector systems wired up. And I installed some lighting in the Subway Station as a test. But I still don’t have the track back together and operational (I’m waiting on some more DCC electronics on order). In the meantime, I’ve amused myself with several things and some work on the website as I plan my next moves.

First, I’ve taken more photographs of the Overhead Transit Station (photo above) and the associated platforms I’m using on the Urban Station scene, and updated my pages for it and for the Unitrack platforms. The photos were also added to the Stations photo album. Once I get the Riverside Station track operational, I’m going to be turning my attention to the Urban Station for a time (and as noted last time I’ve added a page about the Urban Station itself). I have some new track (Kato’s new V15 20-874 set and 20-875 single-track concrete-tie track) on order for that, about which more after it arrives. I really like the combination of the Overhead Station (and expansion for a second platform), the new platforms, and the V15 set; this makes for a really nice modern-looking station.

I’ve also done some more testing of DC power packs, checking out the behavior of pulsed power on motor temperatures (no effect that I could measure) and examining yet another power pack. The notes on both have been added to the DC Power Pack page. Photos were added to the Electronics photo album.

And I built another of Don’s LOLBoosters, and ran some tests for him. Not much to say on that, but I added a couple of photos and some text to my page on it.

DC Motors, Poles, and Skewed Windings, Oh My!

So, what is a “5 pole” motor, and why should I care if my models have one? And do they? That question occurred to me a few weeks back, and I’ve been doing a bit of reading since then. What I discovered was a mixture of fact and Internet lore of uncertain origin. I also discovered that some models do have “5 pole” motors, while others have “3 pole” ones. But oddly, at least in the case of Kato (and possibly also Micro Ace), the “3 pole” motors are the better ones. Or at least the newer ones, although I think they’re better too.

Freight Trains, Electronics and October 2010 Status

Not much got done on the layout itself in October, mostly I’ve been running trains (as documented in an earlier post with a video) and doing a bit of electrical work (mostly the previously noted update to the power panel). I’ve spent a good bit of time on a couple of other things though.

Adding a DCC Meter, And Related Topics

Although I wrote about the power panel previously, there were a couple of things left undone. I was missing one of the ammeters for the lighting power (the store I get them from had run out), and I’d misplaced one of my LNRP LocoNet Repeater panels, so I’d substituted a UP5 panel I took off the old HO layout. Also, I’d realized that my portable RRampMeter DCC volt/amp meter was useful, but wiring it into the system when I wanted to check something was a real nuisance. Over the summer I bought or found the missing bits, and finally decided to order a new RRampMeter (the version without the nice case) to mount as a permanent part of the circuitry (I’m keeping my other meter--the one with the case--for workbench and other testing uses). So it’s a good time to summarize what the panel does, and talk about the new meter a bit.

Myths The Internet Told Me and August 2010 Status

The Internet is a wonderful invention. Without it, I wouldn’t be able to order trains from halfway around the globe and have them delivered in under a week, nor would I have any idea of the difference between an E233-1000 and an E233-2000 (both are commuter trains, but the 2000 is a narrow-bodied variant with a end door for emergency exits; this model runs through onto the Chiyoda subway and nobody makes a model of it yet, but I want one). I hear that some people have more serious uses for the Internet, as well.