Powering a Switch Frog

  • 01 of 03

    The Problem With Switch Frogs

    Hand in front of train components
    ®2010 Ryan C Kunkle, licensed to About.com, Inc.

    Most two-rail track systems deliver power to the model trains with a positive current (+) on one rail and a negative (-) on the other. If these currents come in contact with each other directly, a short circuit will result. This poses a problem on track sections where the two rails must cross, including switches (turnouts) and crossings.

    In a switch, the point where the left and right rails cross is called the frog, so named for its resemblance to a pair of frog's legs. On both prototype and model track, these frogs can take many forms. On two-rail model tracks, the rails must be insulated from each other at the frog in some manner. Often, the frog is a separate casting and is electrically insulated from both rails by plastic spacers.

    Since the frog can't be the right polarity only 50% of the time, it is usually completely insulated. This means there is no power in the frog, creating a dead spot on the railroad.

    Locomotives with limited electrical pickups may stall when the wheels cross the frog. Crossovers and yards where switches are lined in rows present an even greater opportunity for loss of current as multiple wheels cross frogs at once.

    The solution: Power can be added to most switch frogs. The current to the frog must be reversible however to match the polarity (+ or -) of the rail for which the switch is lined. This can be done with a manual switch or an electric relay. Many electric and some manual switch motors and machines have relays built-in for this purpose. A third option is to use extra contacts included in many stationary decoders designed for controlling switches through DCC. Controlling the frog polarity simultaneously with the routing of the turnout eliminates the problem of operator error.

    Continue to 2 of 3 below.
  • 02 of 03

    Understanding the Circuit

    An electric relay or switch
    ®2010 Ryan C Kunkle, licensed to About.com, Inc.

    The electrical hookup to power a frog is as simple as three wires and a Single-Pole Double-Throw (SPDT) switch or relay. An SPDT switch or relay will have three contacts and is very similar in function to the track turnout. One end of the relay has one contact, the other two. This can be wired to divert an incoming current to one of two paths or to select a single exit path for one of two inputs. The latter is the way it will be used for the frog.

    A wire from each rail is connected to the B end of the relay. The A end is connected to the frog. This means the frog will be powered by whichever input is selected and this will be determined by the setting of the track turnout itself.

    This same relay can be wired the other way to route power to the diverging tracks of a turnout in such a way that only the route aligned is powered. Power routing is a convenient way to prevent run-away trains in yards, sidings or staging areas while also reducing the current draw on the layout as a whole.

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  • 03 of 03

    Wiring the Frog

    Different ways to route power
    ®2010 Ryan C Kunkle, licensed to About.com, Inc.

    Attach a wire lead from each rail to the relay that will power the frog. If you are already using a separate power bus wire to power the tracks, you can tap directly off of this like any other track feeder. If you don't have a separate wire bus, you can solder leads directly to the rails. Keeping wires color-coded will make it much easier to trace problems later on.

    The third wire must be attached from the relay to the frog itself. There are several options for this depending on the particular switch track you're using.

    1. Solder: You can solder to some frogs like you would any other section of rail. Other frogs have special tabs attached that allow the solder connection to be made more conveniently or discretely.
    2. Screw: It can be difficult to solder onto some frogs due to their size, location or metallurgy. Some manufacturers cast a small tab onto the frog into which a screw can be inserted. Tap the hole, insert a screw and wrap the wire lead around it and tighten for secure contact.

    Once the three wires are attached, test the polarity of the frog with an ammeter or locomotive. If you find a short, you've probably reversed the track leads at the relay. Simply switch the two input contacts and try again.