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DCC Friendly Switches/Turnouts

This section talks about the WHY and WHAT ARE DCC Friendly Switches and information on where to go to make DCC Friendly Switches/Turnouts.  
What you need to know about the physical modifications can be found here: http://www.wiringfordcc.com/switches.htm
But the REASONS WHY behind all of this are discussed in more detail below.

THIS SECTION COVERS

1) FACT: NO SUCH THING AS A DCC TURNOUT
2) WHY IS THE TERM "DCC FRIENDLY TURNOUT" USED?
3) BACKGROUND OF THE PROBLEM

4) WHERE IS THE PROBLEM?
5) WHAT IS A POWER ROUTING TURNOUT?
6) HOW DO I KNOW I HAVE A PROBLEM?
  (A test you can do)
7) WHY DO SOME PEOPLE HAVE PROBLEMS AND OTHER DO NOT?
8) DO YOU NEED POWER ROUTING WITH DCC?
9) WHAT CAN BE DONE TO PREVENT THIS FOR A NEW LAYOUT?
10) WHAT IS AN OPTIMIZED DCC FRIENDLY TURNOUT?
11) WHAT CAN BE DONE FOR AN EXISTING LAYOUT WITH INSTALLED TURNOUTS?


1) FACT: NO SUCH THING AS A DCC TURNOUT 

Any turnout that works with DC will work with DCC.  

What is the Question then?  The question is NOT will a given turnout work with DCC, the question is "how well it will work?".   There are a lot of variables at play such that not everyone will have the same experience even if they used the same type of turnout!


2) WHY IS THE TERM "DCC FRIENDLY TURNOUT" USED? 

The term is normally associated with turnout manufactures that have made the "Power Routing" turnouts before with metal frogs as their primary product.  The DCC Friendly version is a new version of the same turnout but it no longer does Power Routing eliminating the potential for short circuits.  It will still have the same metal frog.   The frog, however, is now un-powered and the point rails will not cause a short circuit. 

Any turnout that has a 100% plastic frog is DCC Friendly by default since Power Routing is impossible to implement correctly and hence impossible to create the conditions of a short circuit.

A DCC FRIENDLY turnout has:

a) everything to do with short circuit prevention that can causing a engine to stutter or stop
b) nothing to do with engines stalling due to loss of power pickup.

What is Power Routing?..read on...


3) BACKGROUND OF THE PROBLEM

DCC power is global to the layout it since it is running all of the trains at once.   Any short circuit no matter how brief can lead to total layout shutdown or at least a temporary one depending on how the layout is wired.   This means not only the given train you are running stops or stalls but all the other trains running may also stop/stall.    Bottom line: You may have a less than satisfactory operational experience not only for yourself but for other train operators too.

Why is DCC so sensitive to shorts circuits?   Two reasons:

1) To run multiple trains at the same time, DCC must puts out a lot more power (3 to 5 amps) than a DC PowerPack/Throttle would have.  Remember with DC, there is only one PowerPack per train.  The DC power only has to be enough to run that one train and hence the DC power (1 to 1.5 amps) is a much lower value.

2) DCC additional high current requirements must not be allowed to damage the trains they run.  To protect the trains, DCC systems have a very aggressive short circuit protection system that the DC PowerPacks do NOT have.  In other words, DCC is less forgiving of minor short circuits and will tend to bring out all the electrical flaws in the track and rolling stock that you have that were always there but not noticed before.

4) WHERE IS THE PROBLEM?

The problem only appears if the wheels are made of metal.  There are two areas.

1) THE POINT RAIL AREA: The power routing class of turnout introduce a POTENTIAL short circuit hazard in the electro-mechanical design of the turnout that some rolling stock (Cars and Engines) can trigger as the train rolls through the turnout. 

The most annoying and mystifying short circuit occurs in open point rail area with metal wheels.  The wheel temporarily bridges the air gap between the stock rail it is rolling on and the open point rail.  The short is between the back side of the metal wheel and the back side of the point rail.  See the picture on the left.

The back side or rim of the left wheel is touching the left point rail that is sitting in the open position.  The existence of the short will depend on how the turnout is electrically designed.  This short circuit is the annoying intermittent short that is unforseen and leaves one frustrated not sure if the problem is the turnout or the engine.  Typically the engine will ride through if it going fast enough but there is enough power loss to make is noticeable.

2) THE FROG AREA:  Again on power routing turnouts, the frog is metal and part of the power routing circuit.  If a train enter into the turnout from the frog end and the point rail are not thrown for that path, there will be a short circuit as soon as a metal wheel enters the frog rail area.  This type of short circuit is very obvious and you know it right away that the turnout is not thrown correctly for the train.  This type of short is often forgiven due to operator error but non the less the operational damage is done.


5) WHAT IS A POWER ROUTING TURNOUT?

There are two electrical types of turnouts:

A
) POWER ROUTING.

Power routing turnout has two FEATURES: 

1) Track power is determined by the track route that the point rails are set for.  With respect to the diverging track ends of the turnout, the "through track" path gets track power while the "blocked path" gets no power.   In the process of doing this "power path" routing, both the point rails and metal frog have electrical polarity changes.  Stated another way, a Power Routing turnout is any turnout with a live metal frog whose electrical polarity is determined by the physical position of the point rails.  

2) All power routing turnouts have a live frog which greatly improves the given locomotive electrical power pickup reliability as it goes over the frog.  Stated another ways, less stalling due due loss of power that are common on small or short wheel base locomotive like a 0-4-0 engine. 

"Power Routing" is popular with DC users for it offers an automatic way to kill power to a given spur (dead end) track to isolated a given DC locomotive/train by simply throwing the turnout against it.  This would allow a second DC train, to now move on the opposite track fed by the same turnout while still using the same DC throttle or PowerPack.

B) NON-POWER ROUTING.

Track power is not determined by the track route.  Both track routes are powered all the time.  These turnouts may or may not have a metal frog.  If it is metal, it is un-powered.  Non power routing turnouts are by definition DCC friendly since there is no chance for a short circuit.

To learn move about the turnout brands involved and what type of turnout they make, go here: http://www.wiringfordcc.com/switches.htm


6) HOW DO I KNOW I HAVE A PROBLEM?
 
Do your trains intermittently stall, stumble, or jerk on the turnout due to short circuits giving you an operational experience that is worse than what you experience with plain DC?   If yes, then you potentially have a problem.  

The following Test can be used to determine if you have a problem or not:

-----------------------------------------------
TEST SETUP:

Need two locomotives.

1) 1st DCC locomotive/train that is suspected of causing the problem with the turnout in question.
2) 2nd DCC Locomotive with NO SOUND or "Keep Alive" capacitor options (See Test Notes a & b)

Place the 2nd locomotive on track that shares power with the turnout in question. 
The 2nd locomotive shall only be a standing locomotive with it's headlight turned on. 
Make sure both engines and associated track are clean prior to doing the test.  You do not want false indications due to dirty track.


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TEST PROCEDURE: 

A) Start the suspected engine/train moving running through the turnout in question. 

B) Watch the standing locomotive's to see if it's headlight blinks or dims erratically as the engine roles through the suspected turnout. 

C) Did the 2nd locomotive headlight blink?

YES: Then you have the basis for the problem.   The next step is to verify the class of turnout you have.  Read on.....

NO: Then you do NOT have this problem.  Your locomotive stalled because it simply lost power due to poor electrical pickup issues.  You know this because you never lost power on the standing locomotive headlight (no blink.)

D) Repeat as required for the given locomotive.  You may need to try different train speeds to completely verify.  Test all your locomotives.  Why?  See all of the variables involved in the next section's question.

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TEST NOTES:

a) Make sure the locomotive does not have any "Keep Alive" or "NeverStall" option.  (Large hold up capacitors).  The headlight must be 100% dependent on track power to keep it alive.

b) Alternatives for 2nd Locomotive:

b1) Use a locomotive headlight to monitor the track power, you can use a 12V-14V bulb connected to the track.  A common 14V headlight lamp used in DC locomotives will work just fine.

b2) Use a lighted passenger car with no keep alive capacitor.  The light must be 100% dependent on track power to keep it alive.


7) WHY DO SOME PEOPLE HAVE PROBLEMS AND OTHER DO NOT?

There are a lot of variables involved in the appearance of the problem which is why two people can have the exact same turnouts and one has problems and the other does not.  Here are all the variables involved:

1) What brand and class of the turnouts are involved. 
2) What size of turnouts are involved (Turnout #).  Any turnout size can potentially have the problem but the problem increases dramatically with smaller turnouts.
3) The type of rolling stock (Engines and cars) running on the layout.  Large engines vs small engines, Passenger cars, Long versus short freight cars
4) The presence of metal wheels on rolling stock other than powered engines.
5) The maintenance of the rolling stock.  Are the wheels in gauge?


8) DO YOU NEED POWER ROUTING WITH DCC? 

No.  Under DCC it does not make any sense.  Trains are stopped and started under DCC command.  No need to kill track power to keep a train stopped on the track.


9) WHAT CAN BE DONE TO PREVENT THIS FOR A NEW LAYOUT?

The list below is in increasing reliability.

A) Buy turnouts with plastic frogs
B) Buy DCC Friendly Turnouts (non power routing with un-powered metal frogs)
C) Buy DCC Friendly Turnouts and power the metal frog.  (Optimized DCC friendly turnout)
 
Have an existing supply of PowerRouting turnouts you can use?  Turn them into option B or C before you install them.
Here is how: http://www.wiringfordcc.com/switches.htm


10) WHAT IS AN OPTIMIZED DCC FRIENDLY TURNOUT?

An optimized DCC friendly switch/turnout eliminates the Power Routing capability so the point rails do not change polarity but still offers a power routed frog (with external support) to minimize power pickup related stalling increasing the operational reliability of the turnout.  Power routing the frog means the frog polarity will now match the track path the point rails are set up for.   The problem with powering the frog is that it partially brings back power routing short circuit problem.  Partially because the problem depends on which way the train is entering from on the frog end of the turnout.  If the train is going AGAINST (wrong way) the point rail selected track path, then there will be a short circuit when the wheel bridges between the rail that is feeding into the frog and the frog itself.  Fortunately there are two easy solutions for this potential short circuit problem.

1) Current limit the frog power so there is no short circuit.  This is best done uses a light bulb taking full advantage of its unique electrical properties.   In this setup, a set of SPDT switch contacts on the turnout motor mechanism is used to power the frog and set it polarity but the light bulb is wired in series with the frog.  When the train enter the turnout the wrong way, all the happens is the light bulb lights up and no short circuit appears.  A good example for the need of this second requirement is a spring switch.  This current limiting function can easily be done by adding a small 0.5A (6-7 Watts) to 1A (12 to 14 Watts) 12V or 14V automotive bulb wired in series with the frog.  Low cost solution.

2) Use "Frog Juicers".  This is a commercial product that automatically and dynamically correct the frog polarity such that it eliminates the short.  It uses the same principle as a DCC Autoreverser device does for reverse loops.  It looks for the sudden onset of a short circuit and instantly correct the polarity of the frog to eliminate the short circuit.  This devices add a some cost to the complete turnout solution but it is a no-brainer to install.

NOTE: If you have any occupancy detection involving the turnout, then solution #1 is the best since it does not draw any power that can trip the occupancy detector.

11) WHAT CAN BE DONE FOR AN EXISTING LAYOUT WITH INSTALLED TURNOUTS?

A) Do not run long wheel base engines through narrow turnouts.  Although this is not going to be popular, often some people run engines that are TWO LARGE for the # turnout they are to run through.  This is especially true of long wheel base locomotives.  A big boy or a 2-10-2 running through a #4 or #5 turnout is asking for trouble.   Even if the engine successfully goes through the given turnout without derailing, that does not mean it will go through without a problem electrically.

B) If "A" above does not work, then you need to address the turnout itself to making it DCC Friendly.