Category Archives: DCC

WiThrottle and DCC++

One of the major design points for my shelf switcher layout is a compact way to power it.  Well, I pretty much have that ready now.

I put together a DCC++ Base Station using an Arduino MEGA board, an Ethernet shield and a Pololu MC33926 motor shield, then I added a TP-Link TP-WR802N pocket router for wireless connectivity.  I then worked on the software a bit, and added the ability for the DCC++ Base Station to communicate with the WiThrottle (or Engine Driver if you’re Android) app on my iPhone.

Now, when I install this in the shelf layout, I’ll be able to control the trains from my phone for a pretty low price…

Here’s a video of the gear in action…

Atlas MP15DC Decoder Install

Atlas MP15DC Decoder Install
I recently purchased this used Atlas MP15DC, and of course it needed a decoder. Since I’ve been more or less standardizing on Digitrax for my “run of the mill” decoders, I chose the Digitrax DN163A3, which is a “drop-in” version for this locomotive. TCS also makes a drop-in decoder, and of course a wired decoder can be used.

At low speed, this is one of the smoothest running locomotives in my fleet, even with the factory-default motor settings. There is an audible whine at higher speeds, but… this is a switcher. It’s not supposed to run at high speeds.

The install is so straightforward (and so typical of modern Atlas locos) that I won’t go into much detail here.

  1. Remove the shell
  2. Loosen the two frame shell screws
  3. Slide the light board out of the frame
  4. Place the decoder in the frame, with the large, wide end facing away from the cab end of the loco. Make sure to place it component side down, or LEDs up.
  5. Tighten the frame shell screws
  6. Replace the shell

The most important part of the install is to make sure the components face down when placing the decoder in the frame. The fit between the decoder tabs and the frame slots was quite tight, and required some force to get the frame to close up. If yours is loose, adding a little bubble of solder to the pads can take up the free space.

I also read in another install description that sometimes the motor tabs need to be trimmed or filed down just a bit to avoid incidental contact to the wrong tabs. I did not find this to be necessary on my loco, but be advised.

There was one thing that caught me off guard that I have not seen documented elsewhere, at least not with descriptive photos. There is a black piece of plastic inside the cab area of the shell that acts as part of a light guide for the rear headlamp. This piece is only press-fit, and seems to be able to fall out easily. Since I wasn’t expecting the part to fall out, I did not see where it fit originally, and it took some time to figure out where it went and how it fit into the loco.

To save you the trouble, here are a couple of photos:

The part is circled here.
Atlas MP15DC Decoder Install

This is where it goes… oriented upright like a chair, with the clear light guide poking through the hole in the black part.
Atlas MP15DC Decoder Install

One more tip… it’s easier to keep this light guide part in place if you flip the shell upside down and insert the frame into the shell, rather than placing the frame upright and putting the shell over it.

HTH!

Older Life-Like E8 Decoder Installation

I just inherited one of the older-model Life-Like E8 models.  It will need to be upgraded to DCC at some point.  After searching around, I found this guy’s tutorial, and I’m re-blogging it here so that when I’m ready I can find it easily.

This guy also has a lot of other tutorials on many locos, so if you are looking for something, be sure to check it out!

http://n-scale-dcc.blogspot.com/search/label/Loco%3A%20Life%20Like%20E8

I’ll post my own story of how the install goes when I get around to it.

Con-Cor / Rivarossi N Scale 4-6-2 “Pacific” Decoder Install (Part 2)

In Part 2 of this install, we’ll explore the trickiest problem involved in this install:  connecting power from the tender to the locomotive.

[youtube=http://youtu.be/-s0yAFzUSAc]

Older versions of the Rivarossi 4-6-2 used a plastic drawbar, and only the little spring wire pressing against the drawbar pin in the locomotive to transfer power.  For a DC locomotive, this turned out to be a fairly poor design, but for a DCC install, it makes isolating the tender from the motor pickups easy.

(Un)fortunately, this version of the model uses a metal drawbar, which is much better for ensuring good power transfer under DC, but makes it much trickier to isolate the tender from the motor.

A few options that either I considered or were suggested, which I rejected (and the reasons why):

  • Enlarge the hole in the loco end of the drawbar and put shrink wrap over the drawbar pin.  I was concerned about friction and wear on the shrink wrap, so no.
  • Replace the drawbar with a plastic one.  This would have been an excellent option if I could have disassembled the pin at the tender end.  I couldn’t figure out how without destroying it.
  • Insulate the axle wipers from the bar rivets and wire directly to them.  Again, I was concerned about wear, and I just didn’t have the parts.

In the end, the simplest solution seemed to be to cut the brass bar.  It would have been much easier if I could have removed it from the tender first, but it’s riveted in place, so no.  Instead, I used a drill press to drill up from the bottom where there’s an opening in the plastic, and then cut the rest of the way through the bar with a Dremel tool and a reamer bit.  I did a somewhat messy job of it, but it worked.

Here’s a view of the bottom of the tender before the cut. You can see the opening where I made the cut on the left.
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Once the cutting was done, I soldered a piece of wire to the bar in the middle of the tender and routed it through the hole to the front near the drawbar. At the drawbar end of the wire, I attached one pin of a NEM651 female connector and shrink-wrapped the connection.

Untitled

Finally, I finished the end of the wire coming from the loco with a single pin of a NEM651 male connector. At that point, we’re done except for buttoning things up and testing her out.

Untitled

I found when testing that with the female socket secured to the tender, it was very difficult to connect up the locomotive on the track. The quarters are very tight between the cab and tender, and I had cut the loco wire just a tad short. By leaving the plug loose, I was able to bring it out of the tender enough that the wires can be connected before the drawbar is hooked up. Much easier, and with everything being black it still looks OK. We’ll see what the customer thinks. I hope he likes it.

 

Con-Cor / Rivarossi N scale 4-6-2 “Pacific” Decoder Install (Part I)

I was recently asked by a friend to install a decoder in an N scale Con-Cor / Rivarossi 4-6-2 “Pacific” locomotive. It seems that install instructions for this model are scarce, so I will share how I managed.

DCC decoder install in a Con-Cor/Rivarossi 4-6-2
(shown with the steam-dome shell screw partially removed)

This locomotive model has an odd (really, just old) method of pickup. The fireman’s side (left, going forward) rail is picked up through the drivers, while the engineer’s side (right, going forward) is picked up through the tender. Power is transferred from the tender to the engine through the drawbar and a stiff bit of spring-wire that contacts a peg sticking down below the cab.

This is actually the trickiest part of the install. The drawbar peg on the engine is directly connected to the lower motor brush, and the drawbar itself is conductive. So in order to isolate the motor from the rails, you have to somehow insulate the drawbar and provide an alternate path for the tender pickup to the decoder. More on that later.

For this install we chose the Digitrax DZ126T as the best balance between cost and size. The TCS Z2 (also a good choice) is slightly smaller but more expensive. The Lenz LE077XF is only a little more expensive than the Digitrax, but is the largest of the three. Any of these would work, though, and would follow the same basic install process.

The first step is to locate a good spot to fit the decoder, and do any necessary milling or other modifications to make room. Another install page showed installing the Lenz decoder under the cab roof, after some milling to the inside of the roof and to the top of the frame. On looking at the frame design, I decided to fit the decoder into the boiler where the upper headlamp contact is. There is room for the DZ126T at that location without any frame modifications.

DCC decoder install in a Con-Cor/Rivarossi 4-6-2

Remove the shell by taking out the screw in the top of the steam dome on the boiler and then spreading the firebox sides slightly. It is a tight fit, but with some shaking the frame will drop out. Next, remove the two headlamp contacts from the center of the frame and disconnect the black wire running from the upper contact to the rear of the locomotive. Keep the lower contact. You will need it later. Also remove and keep the spade lug on the back end of the black wire. The upper contact and the wire itself can be discarded.

Next, I test-fit a pre-wired T1 white LED that I had handy. It fits nicely into the brass slug that previously held the headlamp. I went ahead and used this pre-wired model, but next time I would probably custom build a T1 LED and resistor to make it as compact as possible and make a little bit more room for the decoder. The shrink wrap on this LED assembly got in the way.
DCC decoder install in a Con-Cor/Rivarossi 4-6-2
Pre-wired T1 LED in the boiler

DCC decoder install in a Con-Cor/Rivarossi 4-6-2
Test fit of the pre-wired LED

After test fitting, I trimmed the LED assembly as short as I could and wired it to the decoder. The decoder Blue wire (common +) goes to the current limit resistor (connected to the LED’s anode) (red wire) and the white wire (forward headlight function) goes to the cathode.

DCC decoder install in a Con-Cor/Rivarossi 4-6-2

The upper brush contact of the motor contacts the frame through a brass spade lug. We must remove that lug, turn it around to face away from the frame, and attach the decoder grey wire to it.

To remove the motor, you must first remove the lower brush contact. Pull straight down on the white plastic ring around the tender pin until it comes loose. Be very careful. It is a tight fit, and when it comes loose it is likely to fly off, taking the spring and motor brush with it.

Next, pull the motor out of the frame by pulling (or pushing) straight back firmly but gently. Remove the spade lug tab from the top motor contact and solder it to the grey decoder wire. Solder the spade lug removed from the lighting tab wire to the orange decoder wire.

After measuring for proper length of the grey and orange wires (they’ll reach back from the decoder position over the top of the frame), I cut and soldered the wires to the spade lugs away from the loco to prevent overheating the motor brushes.

Aesthetically, it would be better to solder the grey wire to the bottom contact and the orange wire to the top contact, so the orange wire is hidden from view. This would make the motor run in reverse, but that can be corrected by programming the decoder.

I lined the inside of the back of the motor mount hole in the frame with Kapton tape to ensure no contact between the brushes and the frame. This might have been overkill but I wanted to be extra-safe.

Reinstall the motor in the frame and reinstall the lower brush contact and tender pin. Slip the spade lugs back onto the motor brush contacts with the tab pointing back away from the frame.

DCC decoder install in a Con-Cor/Rivarossi 4-6-2

DCC decoder install in a Con-Cor/Rivarossi 4-6-2

Once the motor is back in place, bend both spade lugs flush with the back of the motor.

Now, we’re almost done.

Cut and solder the red decoder wire to the lower headlamp tab and re-insert the tab into the frame. Secure the decoder to the top of the frame where the upper headlamp tab used to be. Tuck and secure all of the wires. The black wire should run back along the Engineer’s side of the frame where the black headlamp wire used to be, and will stick out the back of the cab for some length. The yellow wire can be cut short and tucked.

DCC decoder install in a Con-Cor/Rivarossi 4-6-2

Bend the red wire tab up a little past vertical. This will ensure the red wire is not sticking down where it can be seen when the shell is re-installed.

Slip the headlamp into the hole in the boiler slug, then carefully reinstall the shell. It is a tight fit, so you may have to do some “encouraging” to get everything in. It helps to take a small screwdriver and tuck the black wire under the lower edge of the boiler to hide it, much the way one would tuck a cable beneath the baseboard of a house wall. Be careful not to damage anything.

At this point, you should have a complete, running locomotive, except the black wire is sticking a few inches out the back of the cab. To test the locomotive, use an alligator-clip test lead to connect the black wire to the engineer’s side rail of your test track. The locomotive should power up, respond to DCC commands, and run (at least as far as the test lead will reach).

Now is the time to fix anything that is wrong.

In the next installment, we will see how to connect the black wire to the tender and complete the installation.

The CH&FR Goes Digital Part 7

[youtube=http://youtu.be/pGdrG5i_q1I]

Today we present the long-awaited (I hope!) Part 7 of my “The CH&FR Goes Digital” series.

This episode dives into block detection using the Digitrax BDL168 16-input block detector.  We cover track setup, wiring, connecting the BDl168 to your computer with JMRI, and provide a live demo.

Block detection is what we call “knowing where your trains are”, and it works very similarly to how the real railroads do it.  The layout is divided up into electrically separate segments, or “Blocks”.  Each Block is a section of track where you want to be able to tell whether there is a train on that track or not.  It might be a siding, or a length of mainline track, or (less likely) a track in a yard.  The Block is electrically isolated from all the other blocks, and the power feed to one rail is fed through a block detector like the BDL168.

When a locomotive sits on that section of track, even if it is not moving (under DCC) a small current flows through the locomotive’s decoder from one rail to the other, and the BDL168 can detect this current flow.  When it sees the locomotive’s current draw, it reports that section of track as “occupied”.  If there is no current flow, the BDL168 will report “unoccupied” for that track section.

Of course, on the prototype railroad, the trains provide their own power, but the block detectors are able to work very similarly.  By inducing a voltage between the rails, the detector can watch for the metal wheels of the train to short the rails together, indicating that the track is occupied.

On the model, without a little work, we can only detect the presence of locomotives.  Most model railroad cars (in N scale at least) come with plastic wheels which do not conduct.  On the few which come with metal wheels, the axles are insulated to prevent the car from shorting out the track.

By adding a small resistor to one wheelset on each car, the entire train can be detected, not just the locomotive.  One of my favorite videos on how to add resistive wheelsets is by Daryl Kruse who runs the UPRR Geneva Subdivision in N Scale.

For further reference, here are some links…

55″ Dispatcher Panel

55" Dispatcher Panel by BGTwinDad
55″ Dispatcher Panel, a photo by BGTwinDad on Flickr.

This is an early step in the “dream plan” for dispatching my layout(s).  The photo doesn’t really do the scale justice.  This is JMRI displaying my layout panels on our 55″ living room TV.  It’s big enough I can tell track occupancy from my kitchen thirty feet away.

For now, it’s just a novelty, because the current Glover’s Bend layout is small, but the ultimate goal, the “Chestnut Hill Sub” will be an around the room shelf layout big enough for multiple operators, and a panel this big will be easy for everyone to check when they want to see what’s coming.

Ultimately, all of this will also be web-accessible, enabling truly remote dispatching, for my friends who are too far away to visit in person.

Someday, when I win the lottery.

Technicals:  I just installed a Digitrax BDL168 block detector circuit under the layout, and it is monitoring 16 different blocks around the two main lines.  It’s feeding information back over LocoNet to my train room computer, which is generating the displays using the JMRI train control software.  My train room computer happens to have an HDMI display port, which makes it easy to convert my TV into a monitor.  Add a wireless keyboard and trackpad, and I can dispatch trains from my couch!  Easy-Peasey!

I’ll write an article or two about installing the BDL168 and setting up the panel once I get everything debugged and working properly.

 

Science Night!

Science Night by BGTwinDad
Science Night, a photo by BGTwinDad on Flickr.

Last week I was asked to provide a presentation on a science or engineering topic at my daughter’s school. Since most of the other presenters had the “work” and “research” angles well covered, I decided to show something fun.

To the left is a test loop of track that I built.  The loop is broken into 14 blocks that are wired for train detection through a Digitrax BLD168.    The front, back, and left/right curve halves are connected to Digitrax RX4 for transponding detection.  That may be a lot of technical “jargon” for some of you.  The BDL168 detects which of the 14 track blocks the locomotive is on.  The RX4 allows the locomotive to “check in” and report its road number to the computer.

Speaking of the computer, in front of the layout, left to right you see a Digitrax Zephyr Xtra command station, which actually runs the train.  The small box in the middle is a PR3 which is basically a USB connection for the Zephyr.  And then, of course, is the computer.

On the computer (an Apple MacBook Pro), the JMRI software program is running.  It’s displaying a map of the oval,with the train’s location highlighted.  At the same time, it’s running my Virtual Sound Decoder, playing the sound of the engine through the speakers at back, and of course the sound appears to be coming from the locomotive, which is the whole point!

The poster board to the right has some “slides” showing how various sciences (both “hard” and “soft”) can be applied to Model Railroading or any other hobby.

The whole thing was a big hit with the kids, I think.  Either that or they were very polite.  Of course, they were getting extra credit for it!

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.

[youtube=http://www.youtube.com/watch?v=F-oj_17ybLQ]

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.

Kato Genesis P42 Diesel Decoder Install

Kato P42
Amtrak #188 crosses the road, leading the Cardinal.

It’s been a while since I posted a decoder install how-to, so it’s about time I did so.  A few weeks ago, as a reward for some good news at work, I splurged on one of the Kato P42 Genesis Amtrak train “starter sets”.  The P42 Genesis is a really, really nice locomotive, one of Kato’s best in N scale, by many accounts.  It comes in the set as a DC model, but is very DCC friendly.  Digitrax, NCE, TCS and MRC all have drop-in decoders for the model (the MRC decoder includes sound!), and the install for all of them is quite similar.  In this how-to, we will be installing the Digitrax DN163K0A decoder, but you should be able to adapt this process easily to any of the other brands.

Here’s the locomotive in its packaging along with the rest of the train set (and my daughter’s “pet cow” Bessie).

The starter set

Step 1: Shell Removal

Shell Removed!

To remove the shell, pry the sides apart gently and insert some toothpicks to hold them out.  Pull down on the front trucks (gently but firmly – the trucks will pop out) and lift the shell off in a front-to-back motion.

Step 2: Unclip the motor tabs

Remove the grey clip

Near the center of the frame is a grey plastic clip holding two metal tabs (the motor contact tabs) down.  Gently pry up the clip and set it aside.  You’ll need it again later.  Then bend the two metal tabs up to free the light board.  You might want to take a sharpie and mark the spot where these metal tabs (used to) touch the copper pick-up strips that run under the light board.

Lift the motor tabs clear of the light board

(sorry about the focus on that one!)

Step 3: Insulate the pickup strips

Insulate the contact strips from the motor tabs

I’m sure there’s a more technical term for these, but those two long copper strips that run along the sides of the frame under the light board pick up power from the trucks and deliver it to the light board.  Remove the light board, and then gently lift these two strips out.  With a small piece of Kapton tape, insulate the spot where the motor tabs would touch these strips.  Be sure to insulate all the way around the strip, but don’t use more than one layer of tape.  The fit is fairly tight, and extra tape will make it hard to close things up.  Don’t use regular black electrical tape. it’s too thick, and doesn’t hold up well when heated.  Kapton is the way to go here.

Here’s what the insulated strips should look like when re-installed.

Replace the contact strips. Make sure they’re insulated from the motor tabs.

Step 4: Install the decoder

Install the decoder over the contact strips.

The decoder just drops into place where the light board used to be.  You’ll have to be careful not to dislodge the pick-up strips.

Step 5: Re-clip the motor tabs

Replace the grey clip. Press hard. Some extra tape might help.

Bend the motor tabs down so they contact the pads on the decoder.  Make sure they do not contact the pick-up strips!  Replace the grey plastic clip to hold the strips in place.  You have to press pretty hard to get the clip to snap in firmly.  If the clip is broken, or if you want to be extra-sure, you can solder the tabs to the decoder.  An extra strip of tape over the clip is also good insurance.

Step 6: Bend down the headlight

Bend the front LED down at an angle.

The last thing to do before re-installing the shell is to bend the front LED to about a 30-45˚ angle.  This helps make sure the LED fits into the light guide in the shell.   Once that’s done, slip the shell back in place, install the extra parts provided by Kato, and enjoy your locomotive!

Here’s a “wide shot” of the completed install:

And, of course, some video!

[youtube=http://www.youtube.com/embed/REWq5KtoqRM]