Just like last week’s post, this week’s is a how-to regarding DCC electronics.  In particular it’s about DCC auto reversers, and how to quickly test they are working.

Why test them?  Well if you have one installed and the system isn’t working it’s a good idea to test the auto reverser away from the layout as it may not be the part at fault. In that case changing it won’t help.

Before I show you how I test them, I will explain how an auto reverser works, and why we need them as I often get asked this.

Imagine you have a straight railway and at one end you have a single point/turnout.  The first track coming out of the point leads to the other via a loop or balloon as in the image below.

This allows you to turn a train around but it also creates a problem in that the rails will short each other out.  If you look at the image again you see that the upper rail in the straight section runs around the loop and connects to the lower rail.  This is called a dead short.

To stop this, both rails need to be isolated in two places and in this situation it makes sense to do it just after the point, as shown below.

The power to the loop can then be supplied separately allowing the polarity to be switched.  This allows a train to run into the loop and stop.  Then a switch is thrown reversing the polarity of the loop power.  The point/turnout is changed and the train can then proceed back onto the line facing the other way without shorting.

But what if the switch was set the wrong way upon entering the loop?  The train will short.  The same applies if you forget to stop the train; it will short when it tries to leave the loop.

This is where the auto reverser comes in.  This device automatically reverses the polarity of the track it’s supplying when it detects a short. Normally when a DCC command station detects a short it shuts the power off so nothing gets damaged.  (Shorting electrical circuits cause lots of heat which is not a good combination with model trains!)  However, the auto reverser detects the shorts and changes the polarity faster than the command station can react, therefore avoiding a shutdown.  This allows a DCC train to run around the loop, assuming the point/turnout is changed, and back out onto the line without stopping or stuttering.

Wiring in an auto reverser is fairly simple, but sometimes you may need to test it to see if it’s working without connecting it up to the layout.  Below is a typical auto reverser.

Normally they have an input side and a switched output side.  For this one the input is the two red wires.  For the test I have simply used a section of track which is connected to my DCC command station at my work bench.  I quickly soldered the two red wires to the rails, it doesn’t matter which wire goes to which rail.

Then starting with one of the yellow wires, I touch it to one of the rails.  At the heart of the auto reverser is a relay and you can normally hear it trigger when one of the wires touches a rail of the opposite polarity.  At this stage if the command station beeps or shuts down due to a short, the auto reverser is faulty.  Assuming the command station did not shut down, do the same with the other yellow wire.  Then do both together, but always on different rails as the yellow wires should not touch each other. If that works well then the auto reverser is working properly and is ready to be installed into your layout.

If you wanted to test your auto reverser because it wasn’t working when installed, but it passes the test above, then here are some other things to check.

Are both rails in your reversing section isolated from the rest of the layout? This may cause the loop to work in one direction but not the other.

Is the auto reverser installed the right way round?  The DCC supply needs to be connected to the input side.

Is the reversing section too short?  It needs to be longer than your longest train otherwise the front will try and trigger the auto reverser and the rear will try and put it back.

Is the section you are reversing drawing too much current?  For example: is the section you are reversing a whole staging yard with lots of locomotives in?  Most auto reversers are only designed for about 1 amp and several sound locos could be drawing a lot more.

Is the auto reverser close to the section you are reversing?  The further away it is the longer the wires and the higher the chance of resistance in the wire affecting the short detection circuitry.

Are the DCC wires and track feed wires bunched together?  Having the positive and negative wires directly next to each other can cause problems with the DCC signal due to inductance.  This is the magnetic field generated by each wire affecting its neighbour.

Hopefully this will help in diagnosing any problems you may be having with your auto reversing sections.

Next week I plan to share with you my designs for another 3D printed project, this one should be the first of several I have on the drawing board.

This week’s post is a ‘how-to’ about DCC decoders and identifying which ones you have.

As DCC (Digital Command Control) is becoming evermore popular, second-hand locomotives already fitted with DCC decoders are becoming more frequent on websites such as eBay.  It’s easy to grab a bargain but what have you actually bought?  Normally they are just listed as DCC-fitted, leaving it fairly ambiguous as to what’s inside.  Sometimes you can remove your locomotive shell and then see the make and model on the decoder.  But if it was a tricky or custom install it may not be that simple to see.

So why does it matter what the decoder is?  Just about all DCC decoders have the same basic functions, such as motor control and lights, but some are far more advanced than others and have hidden talents.  That being said the one primary thing people change is the address of the decoder, normally to match the number on the locomotive: most DCC systems have that ability no matter what the make or model of the decoder. But if you want to alter any other functions or features which the decoder may have, you will need to know what decoder you’ve got.

This can be done fairly simply by reading the value of CV8 from your decoder.  Now that isn’t as daunting as it sounds and I’ll explain how to read your decoder in a moment.  CV stands for Configuration Variable and it’s these which allow decoders to be set up how you want them.  Different CVs are allocated for different things, for example, CV3 is for Acceleration Rate or Acceleration Momentum.  The higher the value stored in this CV the slower the locomotive will accelerate.  But how do you know which CV is for what?  Well the NMRA (National Model Railroad Association) foresaw this could be a problem and have developed a set of standards for manufactures to follow.  You will notice on all reparable DCC products it will say ‘NMRA compatible’.  This means that all the systems will work with each other so you can use different make decoders and locomotives all on the same layout.

All these CVs can be read using most DCC command stations. This normally involves placing your locomotive on your programming track and selecting the CV Read function.  Depending on which DCC command system you have will depend on how this is done so you will need to look at your instruction manual.  It will ask you which CV you want to read and by selecting 8 it will return a three digit number.  This number will be the unique manufacturer number of your chip.

Again the NMRA control this. They have a list, which can be found here, which has all the manufactures of NMRA complaint DCC products listed by name and by identification number.

So for example if the number was 101 then you have a Bachmann Trains decoder.

Manufactures tend to keep the features and functions the same across their product ranges, so downloading a manual from the manufacture’s website should give you the information you need to set up your decoder.

But what if the decoder in your locomotive is an older model? And companies usually make more than one type of decoder.  To help with this the NMRA have allowed CV7 to be used as a version number by the manufacturers.

So, for example, if your decoder has a CV8 = 129 and CV7 = 49 then you have a Digitrax decoder from the ‘Series 3 with FX3’ family.  There are lots of different decoders in this family but the controls are the same so any instruction manual for a ‘Series 3 with FX3’ will give you what you need.

The NMRA do not list the contents of CV7, as this is down to the manufactures, which can make it a bit tricky to find the information.  But once you know the manufacturer, from CV8, you could email their help desk with the CV7 number and they will be able to help.

Checking the decoder type in your second-hand locos is a good idea because you may be missing out on some great features you never knew you had.