DCC Friendly Turnout
- Main article: Turnout
Digital Command Control: Turnouts
A commercial "DCC Friendly" turnout or switch is a turnout that is manufactured or wired for use with Digital Command Control. Commercial switches are often labelled DCC Friendly or For DC and DCC Use on the packaging.
The most obvious feature is the gaps that electrically separate the frog from the point rails. The frog is often unpowered. There may be additional gaps to prevent a short in the frog. Others may use a plastic frog, or a hybrid of metal rails and plastic.
- 1 The Issue with Digital Command Control and Turnouts
- 2 The Ideal Turnout
- 3 Terminology
- 4 Power Routing Turnouts
- 5 DCC Friendly
- 6 Rail Gaps at Frog
- 7 Unfriendly Turnouts
- 8 Drawings
- 9 NMRA Standards Relating to Trackwork
- 10 Videos on YouTube Explaining Switches and the NMRA Standard
The Issue with Digital Command Control and Turnouts
- When track work and wheelsets are properly gauged, there should be no momentary short circuits occurring at turnouts. The backside of the wheel will not graze the open point and will not cause a short between it and the stock rail.
Many Digital Command Control Boosters are very sensitive to sudden changes in current flow. Just the wheel grazing the point rail and allowing current to flow can be enough to trip their protection circuit and cut power. Which is annoying.
Older technologies employed by analog power supplies were not a sensitive to a momentary short. Often they didn't deliver enough current to do a lot of damage either. DCC Boosters can deliver three times or even more current than was typical for analog operations.
The solution is the NMRA Track Gauge. Checking turnouts with the gauge will show if there may be problems or not. Many commercial offerings have compromised the clearances required. Verifying the correct gauge of the wheel sets on your rolling stock will also identify offenders which may cause an issue. The gauge will measure points, flangeways, track and wheel gauge. Handlaid track usually has no issues with DCC as the NMRA Gauge is an integral part of making a turnout.
Also use the gauge to verify your wheel sets are in gauge. Many track manufacturers will recommend the use of NMRA RP-25 S4 profile wheel sets for the best results.
The Ideal Turnout
The Ideal Turnout for use with Digital Command Control has the following features:
- Points connected to nearest stock rail
- Frog electrically isolated and powered by switch contacts (Metal frogs preferred)
- All rails after the frog are powered
It is also important that the turnout is gauged properly using an NMRA guage, as well as the wheelsets. Shorts between the stock and point rails are a source of problems.
Point Rail: Used to direct the wheels in the direction the train is supposed to go in. These are the moving parts of the switch.
Frog: This is the assembly that allows the wheels to cross from one track to another.
Stock rails: These are the rails on the outside of the switch
Diverging Rails: Those that follow the frog, allowing the two routes to converge as they enter the frog
Throwbar: The device the point rails are attached to, and allows them to be positioned. The switch stand or motor attaches here.
Also see this video from Fast Tracks: How A Railway Frog Point Works. See below for more links.
Power Routing Turnouts
Turnouts which are advertised as "power routing" are wired so that the point rails are electrically connected to themselves, the frog, and the departure rails beyond the frog. Power is routed to them depending on how they are aligned, often by the mechanical contact between a stock rail and the point rail. The throwbar can also function as a simple electrical switch to control the flow of electricity to the point rails.
It is important if the frog is powered that the switch controlling power routing disconnects before making the new connection (Break before Make).
Power routing turnouts are not DCC Friendly.
There is no advantage to power routing with DCC operation as there is no need to control the power flowing into sidings. We want all the track to be powered all the time.
The problem with power routing is that the back of a metal wheel may make contact with the point rail, and cause a short to the stock rail. DCC systems are very quick to react, unlike analog power supplies.
- Note: when building a crossover, power routing turnouts that are non-isolating can cause a short where the frogs are connected together. The frog rails must be gapped (e.g. by using insulating rail joiners).
A turnout labelled as DCC Friendly is one designed for use with DCC. The point rails are not connected together, instead they share the polarity of the stock rail nearest to them.
The frog is often unpowered, and gaps sever the electrical connection between it and the point rails.
DCC Friendly turnouts can be Live Frog (All Live) or Dead Frog. With live frogs, the power must be fed to frog by some mechanism that switches the polarity to match the route set. This can be a electronic device called a Frog Juicer, the contacts on a Tortoise slow motion switch machine, or the contacts on solenoid or motor driven switch machines. It is also possible to power frogs using some manual throws with contacts.
Frogs can be powered if needed, often because some locomotives will stall on them. This is usually needed for short wheelbase locomotives, like switchers, with four axle trucks, or an 0-4-0 or 0-6-0. Between a dead frog and dirty rails, the loco can stall. Those with all wheel pickup are less likely to have problems than lower cost units with fewer pickups.
Rail Gaps at Frog
With analog, the practice was to gap the rails just past the fouling point (where two trains on adjacent tracks can collide).
For DCC, the gap should be much closer to the frog, to eliminate a potential short should a train run past the fouling point gap.
While these may not be the ideal turnouts, they can be adapted for use with DCC. Click on the link to see how.
PECO is discontinuing the Electrofrog and Insulfrog products:
New PECO Unifrog Design
The wiring of these new turnouts is a development of both the Insulfrog and Electrofrog designs. The stock-rails are wired to the centre-rails at the factory, which in turn are connected to the corresponding frog-rail. This means the turnout is completely live (except for the tip of the frog), no extra wiring required. If the turnout is being used as a switch to isolate a section of track then it is simple job to remove the wire that joins the centre-rail and stock-rail and it will work like a current Insulfrog.
The Peco Insulfrog turnouts have a very small insulated frog and on initial inspection they can appear to be live frog because the metal frog rails are so close together. Because of this, it is possible to have a short if a metal wheel bridges the two rails. Usually, this will only happen with old rolling stock wheels that have crude profiles.
The Peco Electrofrog turnouts are power routing right out of the box and therefore they are not DCC Friendly. However, they are designed to be easily be converted to DCC Friendly if needed.
Shinohara / Walthers
Shinohara of Japan made quality turnouts, Walthers would later distribute them under the Walthers/Shinohara name.
There are two types of Shinohara turnouts out there. Newer ones are DCC compatible, and state that fact on the package. Older ones are not, as the power is routed by the switch, and the point rails share the same connection.
For more details see the Shinohara page.
(Clicking on these images does not work, scroll down to see the turnout switched)
Images courtesy of Fast Tracks
Note that the point rails share the polarity of the stock rail adjacent to them. This makes them DCC Friendly, as a metal wheel's flange touching the point rail will not create a short between the point and stock rails.
The frog is being powered by contacts included in the ground throw mechanism. This can be done using a switch throw with contacts, or the contacts on a switch machine.
Images used with the permission of FastTracks for illustrating various track work concepts.
NMRA Standards Relating to Trackwork
These link to the the NMRA Website:
Videos on YouTube Explaining Switches and the NMRA Standard
Fasttracks (user name FastTracks Tools) has posted a number of videos showing how to make a turnout using their jigs.
Demystifying The NMRA Standards Demystifying The NMRA Standards - Part 1 of 4