- Main article: Crossing
This article deals with wiring track power for crossings and in particular how to switch the frog polarity. There are other articles discussing wiring in general, the various types of crossing and how to operate them remotely like turnouts.
Here are some recommendations for wiring crossings on a DCC layout...
- Feed track power to all metal rails in the turnout.
- Do not rely to on metal rail joiners to feed power to or through crossings.
- Use live frog crossings if possible.
- Feed track power to live frogs using frog polarity switcher.
Dead Frog Diamond Crossing
This diagram shows where to feed track power to a typical dead frog crossing. Basically, feed power everywhere an arrow is shown and do not rely on the metal rail joiners, they don't always make reliable electrical joints.
In particular, try to feed power to all of the separate metal rail components in the crossing (except the guard rails). If two components are already electrically bonded with with internal wires (usually on the underside) then you only need to feed one of them.
The diagram above shows a diamond crossing with isolated routes (which is fairly typical). This means the two routes are totally electrically isolated and you are free to wire them up how you want:
- Same parallel power district. This is the most common case where the two routes meet again (e.g. at a turnout) without any reversing loop involved. This would make the red and magenta rails common and the green and blue rails common.
- Same power district but reversed. This is what would happen if there was a loop at one or other end of the crossing. This would make the red and blue rails common and the green and magenta rails common. This in itself doesn't make a reversing loop but if it forms part of a reversing loop then an auto reverser would be required to prevent a short where they eventually meet at a turnout.
- Different power districts.
Live Frog Diamond Crossing
These can be a bit tricky to wire up because the frogs will only be correct for one route and there are no moving parts to which to attach mechanical frog polarity switches.
- Use a DCC Auto Reverser which is dedicated to just the frogs of one crossing. One output goes to one frog, the other output goes to the other frog.
- Use two electronic DCC Frog Juicers. Don't be tempted to double-up one juicer, it won't work, the frogs always need to be opposite polarity.
- Use the turnout motor accessory switches of closely related turnouts to infer the frogs' polarities from the selected route.
For the last case you can also feed the frog power through high wattage resistors or 12V automotive light bulbs to reduce the short circuit current if you accidentally drive a train against the route setting. This will prevent your booster from tripping.
Don't be tempted to only use automotive light bulbs or other Frog Current Limiter techniques for live frog crossings. One route will always be wrong and no better than had you used a dead frog crossing - remember, you were counting on the live frog providing power, now for one route you will get no power from the frog.
An exception to all the above is when one end of the crossing loops back on itself (e.g. in a figure of eight configuration). In this case the live frogs can be hard-wired and will always be correct because the frog rails always match.
Dead Frog Slips
Dead frog slips can be wired up essentially the same as dead frog diamond crossings except that all routes should be in the same power district.
If a reversing loop is involved (one end of the crossing loops back around on itself) then a DCC Auto Reverser will be required for the loop and insulating rail joiners or gapped rails must be used.
Live Frog Slips
Live frog slips can be wired up in a similar manner to live frog crossings as follows:
- Use a DCC Auto Reverser or two DCC Frog Juicers as described above for diamond crossings.
- For double-slips use the crossing's turnout motors' accessory switches to switch the frog polarities as described below (see #Single and Double Slips).
- Single-slips are a little more tricky to wire using accessory switches because there is one setting of the points which makes it equivalent to a diamond crossing (see #Single and Double Slips).
Regardless of whether the crossing includes a single or double slip, a DCC Auto Reverser can be used to switch the polarity of the frogs.
The diagram below shows how to wire up a live frog crossing (e.g. a Peco Electrofrog Diamond Crossing) with an auto reverser feeding the frogs. The lightning bolts represent a train arriving on the other route and causing a short circuit. This auto reverser detects this short and flips the polarity to allow the train to proceed.
Note that you want an auto reverser module, not an auto reversing booster. Whilst in theory you could use a booster, it would be a waste to use an entire booster just to power one crossing's frogs.
Regardless of whether the crossing includes a single or double slip, two DCC electronic Frog Juicers can be used to switch the polarity of the frogs.
The diagram below shows how to wire up a live frog crossing (e.g. a Peco Electrofrog Diamond Crossing) with two juicers feeding the frogs. The lightning bolts represent a train arriving on the other route and causing a short circuit, the first short occurs with the entry frog and then a subsequent short occurs on the exit frog.
Turnout Motor Accessory Switches
This section will assume that the mechanical method of switching frogs uses one or two SPDT switches attached or integrated into the turnout motor. There are other methods (such as manual throws with built-in switches) but these are effectively the same for the purposes of this discussion.
This solution has to be divided into two categories; slips (which have points) and diamond crossings or scissor crossings (which don't).
Single and Double Slips
Slips are the slightly more straightforward case because the turnout motors that switch the points can also be used to switch the frogs. The key thing to remember is that each frog should be fed from the opposite turnout motor accessory switch.
Single slips have one case (both sets of points closed, not thrown) that causes a bit of a problem. In this case both routes are available and each route will have one frog polarity wrong if the simple wiring described above is used. There are a couple of solutions to this:
- Use automatic turnout routing to prevent this case from happening. The 3 remaining cases cover all possible routes.
- Use a more elaborate circuit that combines the accessory switches of this single slip and a related turnout to infer the route in this case.
The diagram below shows a double slip crossing operated by two turnout motors each with one SPDT accessory switch.
Diamond and Scissor Crossings
Scissor crossings can definitely use this method whereas diamond crossings can only use it if there is a related turnout that can be used to infer the selected route. For example, if the crossing is due to a junction on a double track main line and the turnout is set to the diverging route then the only safe route through the crossing is the diverging route. The main line that it crosses should be signalled to reflect this.
The diagram below shows an example junction where the down line to branch has to cross the up line. The crossing's frog polarities are set by the down line turnout. Note that this junction is simplified for clarity, in reality there would either be another turnout on the up line for trains returning from the branch line or the crossing would be a single-slip.
This diagram below shows the frog wiring for a scissor crossing operated by 4 Tortoise turnout motors.
This image was used with the kind permission of Fast Tracks.