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.

DCC Speed Tables

My plans to start installing decoders were somewhat upset when a large quantity of the ones I’d ordered turned out to be out-of-stock, and a box arrived containing only a couple of decoders and some wire. I actually have a number of decoders on hand, though not enough to do a full EMU the way I want, or all of the models I wanted to experiment with. So, while I could have made a start, instead I decided to spend some time working out my standard configuration settings for the Digitrax decoders. I’m going to have a number of these even if I don’t use the DZ125 wire-in decoders, since my Kato “DCC Friendly” models use Kato’s EM13, which is essentially a Digitrax FX3-Series decoder. And I have a few models with lightboard replacement Digitrax decoders.