Posted by: bgtwindad | December 27, 2012

DC vs DCC Locos and Control

It’s about that time of year, when for some reason there seem to be a lot of newcomers to the hobby, and a lot of people considering significant upgrades to their layouts.  Hmm… must be Christmas or something.

One very frequent question I come across, and one which seems to get a lot of answers, some accurate, some not so much, is the question of DC vs DCC control of locomotives, and specifically whether DC locos can be run on DCC track, and DCC locos run on DC track.  I shall attempt, in my rambling and overly-wordy way, to dispel some of the confusion and explain in some detail what is going on and why certain things work and others don’t.

In order to get this done in a blog post instead of a Ph.D. dissertation, I’m going to have to assume you understand some basic electrical concepts like DC and AC voltages and currents.  If you don’t understand these, I would refer you to Wikipedia in the short term, but I do plan to add some blog posts on these basic ideas later on.

Also, I will try very hard not to debate the merits of DC vs. DCC in this blog post, but only to explain (basically) how they work and what happens when you try to mix them.  The debate has long since passed into the “well beaten dead horse” category, anyway.

DC Control

Under DC control, the motor in the locomotive is directly connected to the rails.  It’s a DC motor, so it’s expecting a steady, DC voltage in one direction (one polarity) or the other.  The polarity of the DC voltage on the rails determines which direction the motor (and thus the loco) runs, and the voltage amplitude (level) determines the motor’s speed.  It’s a very simple system, but it works.

The catch is that because the rail voltage is directly controlling the motor, every motor on that set of rails will respond in the same way.  There’s no way to have two different locos running at different speeds or in different directions on the same set of rails.  DC control folks solve this problem by breaking the layout up into several blocks, each of which is electrically isolated from the rest of the blocks.  They use a set of switches to tell which throttle (“cab”) controls which block(s), and make sure that only one train is in each block.

DCC Control

DCC Control is a bit more complex than DC at this level.  In DCC, we put a little tiny computer inside the locomotive.  The computer is responsible for controlling the motor in response from a series of commands from the user’s Throttle (by way of the Command Station).  If you look at what’s happening on the rails, instead of a DC voltage (and polarity) that controls what the motor does, you see something quite different.  What you see is instead, technically, an AC voltage, and it’s quite high – a voltage that would be “full throttle” if we were under DC control.

This is the first major difference between DCC and DC.  In DC, there is always only the voltage required to turn the motor at the required speed, so to make an engine “idle”, you have zero volts on the rails.  Under DCC, there is always full voltage on the rails, no matter what speed the loco is moving.  For N scale and smaller, the DCC rail voltage is usually +/- 12V, and for HO or larger, the voltage can be as high as +/-18V to deliver the extra power the larger scale motors require.

The second major difference is the AC vs. DC part.  Under DC control, for a given direction of train motion, the polarity of the rail voltage is constant.  Reverse the rail polarity, and the train suddenly goes the other direction.  In DCC, the polarity of the rail voltage is always changing, and quite rapidly at that — over eight thousand times a second, in fact.

So keep this in mind.  While the DC rail voltage is a nice steady signal that only goes one direction and for a non-rocket-like train speed probably has a fairly low voltage, the DCC signal is banging back and forth thousands of times a second all the way from a full +12 or +18 to -12 or -18 Volts all the time no matter  what the train is doing.

Why is the signal banging back and forth?  Well, there’s a pattern in that oscillation, and that tiny little computer is paying attention to and deciphering that pattern.  And in that pattern are the commands being sent from the throttle.  Commands like “Run the engine at 50% throttle” or “Turn on your front headlight”, or even “Program CV42 to value 27”.

I could go on at great length about how this all works, but I think this is enough to make my points later on, so let’s move on, in the interest of brevity.

DCC Loco on DC Control

What happens if I buy a brand new DCC-installed loco (or install a DCC decoder in a previously DC loco), and plop it down on my DC-controlled track?  Well, the designers of DCC realized that LOTS of people are going to want to do precisely that — and that they will be very frustrated customers if their new, expensive engines don’t run. So the NMRA DCC standard allows for, and lately decoder manufacturers have been making what are referred to as “Dual-Mode” decoders.  Now,  you may find some older, outdated decoders that don’t do this, but pretty much all current decoders being sold in 2012 support dual mode.

A dual mode decoder is smart enough to realize that it might not be running on DCC track.  So, when power is applied and it wakes up, the decoder looks for a DCC control signal.  If it doesn’t see one, it assumes that it’s on a DC track, and will start following the DC track voltage and polarity as though it were a simple DC motor.  The result is that most modern DCC locomotives will work just fine on DC control, with a few minor caveats.

Caveats you say?  Well, yes, there is one fairly big one.  That little computer inside the decoder needs a minimal voltage to run – usually about 5 volts.  So while your “pure DC” loco will begin to crawl along at 1-2V DC, your DCC-on-DC loco won’t even wake up until a much higher throttle setting.   This is the main caveat.  There may be some other minor ones, but they depend on the specific decoder so we won’t go into them here.

Flying Locomotive Syndrome

There’s one really annoying thing about dual-mode decoders, though, and it’s a big reason why most decoders also have a switch to turn it off.  Every once in a while when you put a locomotive with dual mode enabled on a DCC track, and you power up the layout (especially when recovering from a short circuit), the decoder will mistake the changing voltage on the rails caused by the DCC Booster “waking up” (my term) for a DC signal.  The decoder will then switch into DC mode, and when the Booster starts putting out the regular DCC signal, the loco will race off into the sunset at full speed.  Not a pretty sight.  For this reason, lots of “pure DCC” users like to disable dual mode.

So, if you put your supposedly “dual mode” locomotive on a DC track and it doesn’t work it may be because someone has disabled dual mode.  Just plop it back on the programming track and check bit 2 of CV29.

DC Loco on DCC Control

This is where things get dicey.  If you plop a DC loco down on DCC powered track without doing anything else, here’s what happens.  Remember, the DCC track signal is at full voltage (either +/-12V or +/-18V depending on the booster setting), and it’s changing polarity 8,000 times a second.

Well, your DC locomotive is going to sit there trying to go full speed, reversing direction 8,000 times a second.  It just happens that the nature of the DCC control signal is such that the average time spent at each polarity is about the same, so the DC motor will spend about the same amount of time trying to go both directions.  It won’t really go anywhere, but it will make an ugly buzzing noise while the motor heats up and eventually melts the shell, if the motor itself doesn’t burn out first.

Short answer:  It won’t work.

Digitrax (and others) Address 00 Control

(Note: After learning that Bachmann and Lenz — at least — also provide this feature, I had to re-word this section a bit.  Having a look with a rested set of eyes pointed out a few technical corrections as well.)

NMRA to the rescue — sort of.  In another attempt to allow “backward compatibility”, the NMRA DCC standard allows for a method of controlling a DC locomotive on DCC track.  To my knowledge, only Digitrax has actually implemented this apparently this feature is available from at least Digitrax, Bachmann, and Lenz, maybe others.

They make this work by a method called “zero stretching”.  If you put a DC locomotive on your DCC track, and set your throttle to address 00, you can (usually) control the train.  Here’s how it works.

As mentioned above, the DC motor sees the DCC signal as a DC “full throttle” with a rapidly reversing direction.  It’s a “feature” of the DCC signal that the variations in the signal average out such that the motor doesn’t actually move — that is, the average voltage of that rapidly oscillating DCC signal is zero, because the signal is spending the same amount of time at +12(18)V as at -12(18)V.

When you increase the throttle, the command station starts stretching out some of the “spaces” between commands* so that the signal spends a bit more time at one polarity than the other.  The DC motor will see this as an average voltage somewhat above (or below) zero, and will begin to move.  DCC decoders are programmed to expect — and ignore — this “zero stretching”.  The higher the throttle setting, the longer the stretching time, the higher the average voltage the DC motor sees.

So it works. Sort of.  But it is noisy, and you still have the problem of the idle engine potentially overheating.  So I really don’t recommend it as a regular way to run locos.

Here’s a brief video of a DC locomotive sitting on my Digitrax DCC layout.

By the way, I’m only about 1-for-4 on getting a DC loco to actually move under “address 00” control, but your mileage may vary.

*It’s actually stretching out the length of the “zero” bits within the data, but that’s a detail that really isn’t important at this level of discourse.

DC and DCC side by side

So what does a guy do if he wants to try out or transition to DCC, but he has dozens or hundreds of DC locos, and can’t fork up the $30 each or the conversion time to go whole-hog DCC?  Well, there are some options, but we have to be careful there as well.

DC/DCC Timesharing

One thing you can do is time-share.  Using a DPDT switch, you can wire your DCC booster side by side with one of your DC cabs (let’s say Cab A).  Then, when you want to run DCC locos, clear all the DC trains off the layout, throw all the block switches to Cab A, throw your DC/DCC switch to DCC and go to town.  When it’s time to run DC locos, just throw the DC/DCC switch back, and have fun.  This is really the most practical thing to do.  The only catch here is to be careful not to leave a DC loco idling somewhere on the layout while running DCC.

In short, here, your layout is all-DC today, all-DCC tomorrow, and so on.

DC/DCC Block-Sharing

Another option is to take advantage of the electrically isolated blocks in a DC layout to run DC and DCC trains side by side.  In this case, you wire the DCC booster into one of the cabs as above, and throw only some of the blocks to that cab, such that for example, one loop on the layout is DCC, while the rest are DC.  Some modular layout clubs have taken to doing this, often designating one or more of the loops on the layout as DC, and the others as DCC.

In short, here, the inner loop, or the upper deck, or the left side, or whatever is DCC while the outer loop or lower deck or right side or whatever else is DC.

BE CAREFUL!

There is a real danger here.  First, this will only work if your DC wiring system isolates both rails. You cannot properly isolate the DC from DCC if you have a common-rail DC wiring setup.

Second, you must never allow a locomotive to bridge the two regions.  DCC and DC power must never be connected to each other.

What happens?  Two things. First, the DC voltage from the DC throttle will add to the DCC signal (AC voltage) generated by the DCC Booster.  The resulting signal on the rails will look like a DCC signal, but shifted up (or down) by the amount of the DC throttle voltage.  Second, the DC throttle and DCC booster will see each other as “loads”, and will try to feed power into each other.

Precisely what the end result is will depend on exactly how the DCC Booster, the DC throttle, and your DCC decoders are designed.  If the voltage offset created by the short is high enough, it could damage the sensitive electronics in the decoders, “letting the magic smoke out”.  Likewise, if either the DCC Booster or the DC Throttle are unable to “sink” the current being delivered by the other power source, then the output stage of the weaker device will fail.

Most likely the loser in this fight will be the DCC Booster.  If you are lucky, the over-current protection circuitry will kick in and simply shut down the booster.  If you are less lucky, the output drive circuits will be fried (as in — again — “letting the magic smoke out”), and your expensive booster will become an expensive door-stop.

So, if you choose to “block-share” DC and DCC on your layout, I advise that you be extremely careful that there is no way for the two power sources to be connected to each other, including through a locomotive crossing from one region to the other.

Advertisements

Responses

  1. Bachmann EZ-Command has a feature for DC locos. Still has motor noise though.

    • You’re right. Apparently Lenz also provides this feature, which makes sense since they (a) built EZ-Command for Bachmann and (b) developed the DCC spec in the first place. Thanks for the catch! I’ve updated the post.

  2. Note: The DCC electrical signal is defined by NMRA Standard S9.1. Among other things, it specifies that a compliant N scale decoder must be able to withstand at least +/-24V DC at the rails. Larger scale decoders must be able to withstand +/-27V DC at the rails.

  3. I’m in my 60s and haven’t touched a model train since my old O gauge Lionels of 45 years ago. Smitten by the bug this past Christmas, I see both the wisdom of HO scale, and the relative complexity off modern model railroading. This article succinctly explains my current dilemma and is the clearest, most efficiently written piece on this subject on the Internet. Thank you and kind regards.


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Categories

%d bloggers like this: