Reversing Loop

Introduction One of the mysteries that some Railway Modellers face is the "Reverse loop". This is where you have a a section of the track that forms a complete loop so that the train will go round it and come back in the opposite direction on the same rails. The major problem when trying to achieve this is the fact that the loop puts a short circuit on the track. See the diagram below.

If you follow say the positive rail and go round anti clockwise you can see that it actually goes round and meets the negative rail, and no matter which way the point is switched there is always a short circuit between the two tracks. This could be prevented by putting a break in each of the rails but at some stage you will finish up with the loco wheels bridging the gap and you are back to the short circuit situation again. So how do we get round this problem? The answer lies in the use of the humble diode, or to be more precise four diodes in the form of a "bridge rectifier". If we isolate the loop section of the track at each end where it leaves the point and then feed this isolated section via the "Bridge Rectifier" then we can drive round the loop and simply throw the direction switch from forward to back when the train is in the loop. See the drawing below.

How does it work? The above drawing shows the same loop but now with tracks cut at the point outlets and the loop section is fed via the four diodes that are wired as a "Bridge Rectifier" (you can actually buy a complete bridge rectifier as a single component with the four wires or four tags for connecting to ) Let us imagine that we are driving the loco along the straight towards the point which is set to the right, so we will be making an anti-clockwise circuit around the loop. The power convention for model railways is that in the direction of travel, the rail on the right is always the positive one. So in our case the bottom rail is the positive one and the top rail is negative. Power to the loop section is fed via the diodes and since they only let current flow in one direction (in the direction of the arrow formed by our diode drawing) then you will see that the outside track of our loop is positive and the loco still moves in a forward direction. Now when the loco is in the loop section we can reverse the controller direction switch so that the top rail is now the positive one ready for the loco to move away to the left when it comes out of the loop. Again follow the positive current through the diodes and you will find that the outside rail of the loop section is still positive and even with us reversing the controller the loco still travels anti-clockwise. All we need to do now is to throw the point ready for the loco to emerge from the loop when it has gone right round. Remember that for this to work as shown above you will always have to go round the loop anti-clockwise. If you want to always go round clockwise then simply reverse the two wires that feed the loop from the diodes. Diodes suitable to make the "Bridge Rectifier" are type 1N4002 available from "Maplins" at about 7 pence each. John Essex Heywood Model Railway Group

This page created 12th February 2003