Model Railways On-Line - Power feeding with Droppers

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Droppers are feed points implemented to provide electrical power to track in such a way that fishplates are not relied upon as current carrying devices.
A bus is usually run in parallel to the rails and droppers are used to connect the two together:

Bus Diagram

Typically, on large layouts, reliance on fishplates as current carrying devices results in voltage drops in remote locations from the controller.
Droppers are not a prerequisite of any control system, DC or DCC, however, if long term reliability is required, best practice recommends that droppers should be implemented on all layouts and connected to a bus.

There are several different methods of implementing droppers. This article will cover the method used on the 'Ashprington Road' layout and discuss the reasons behind the technical decisions made.
The author acknowledges that the materials described here are by no means the only ones which can be used.

How are droppers installed ?

There are several different ways of installing droppers, mostly involving drilling a hole in a baseboard, poking a wire through and soldering it to the side of a rail.
Droppers should be soldered to the undersides of rails prior to track being laid.
Please see this article for information on soldering wires to rails.
When the track is laid, a slot is cut in the cork trackbed to locate the dropper wire and run it to the side of the track bed and thread down through a hole in the basedboard surface:

Click to view in larger size:
Dropper Picture   Dropper Picture

There are several reasons behind this approach:
  • Conventionally, droppers are poked through from under baseboards. The method used on Ashprinton Road doesn't require under baseboard activity
  • Conventionally, droppers are threaded from under basboards, directly in line with rails and then soldered to the rail sides. This method results in the Solder Globule problem
  • Sometimes, conventional methods attach droppers to rails and while track is being laid, they are threaded through the basboard. This method restricts alterations in alignment being made during laying
  • The Ashprington Road method allows significant side ways movement of track alignment during laying, all work can be done from above board and because of the way wires approach the track, it has provision to allow for rail expansion whereas generally, conventional methods fix a rail in position and the solder bond breaks if there is any expansion movement

How often should droppers be used ?

Some people quote the installation of droppers in terms of '1 every x feet'. This really is not necessary and can result in more than one wire per rail. The purpose of droppers is to ensure that every rail gets a feed. It is not necessary to double feed it, although there is nothing wrong in doing so.
It is generally recommended that one dropper is installed for every length of rail.
However, a pragmatic approach needs to be taken because there will always be those odd situations where a dropper cannot easily be fitted or fed, in which case, it is acceptable to rely on fishplates provided they are tightened before fitting.

What sort of wire should be used for Droppers ?

To determine this, one needs to consider what the operation of a layout is going to be.
When 'Ashprington Road' was built, it was built as a DC layout. All rails had droppers installed.
Single core copper bell wire was used for all wiring including the buses. There was one bus for each of the controllers.
Because DC layouts control the track, they only control the loco(s) in motion and all other locos are isolated. Copper bell wire has good conductivity and is quite capable of handling the maximum 1amp which was likely to be required on a DC layout - UK trains generally never have more than two locos, unlike their US and Australian counterparts!
When Ashprinton Road was later converted to DCC, all three buses were combined into one and replaced with 10amp mains cable because a DCC layout powers all locos at the same time and can potentially feed up to 5amps.
Copper bell wire was retained for the droppers.

One needs to remember the principals of wiring parallel circuits with droppers. No one dropper can ever carry more current than the total number of locos which can be placed on the length of rail it feeds - this assumes the rail is isolated at both ends. If it is not isolated (is the rail uses metal fishplates to connect to other rails with dropper), then other droppers will be carrying current in parallel as well (assuming they have not failed), therefore, adding another dropper reduces the current in each to one half. Three droppers means current is reduced in each to one third and so on.
It is therefore not necessary to use heavy duty cable for droppers. Just ensure that the dropper wire is capable of carrying at least the current of the maximum number of loco likely to draw on it if it is an isolated rail.

The short situation also needs to be considered and this is where the 'coin test' comes in handy.
Purposely place a coin across the rails and observe whether the controller cuts out. A DC controller may take a second or so, but a DCC command station should cut out instantly. If the controller or command station does not cut out properly, it probably means that the wire being used for droppers and/or the bus has sufficient resistance to quench the increased current caused by a short situation such that the controller/command station sees it as a load and not a direct short. If this happens, it means that the wiring is of insufficient capacity and must be replaced because a short situation is not going to be properly protected for and may cause damage to the layout in the form of heated wires or damage to a controller.

Are fuses necessary in droppers ?

It is not necessary to install fuses in droppers - see previous topic on the wire used for droppers.

What position along a rail's length should droppers be attached ?

We recommend that droppers should be attached to the centre of a rail along its length.
The reason for this is that a dropper tends to act as a fixed point about which a rail cannot move. By attaching to the centre point, if rail expansion occurs, each end will only see half the expansion of the whole length of the rail. If a dropper is placed at one end of a rail, that one end will be anchored and the other end will see full expansion movement.
Please note that the method described in this article as used on Ashprington Road does permit a certain amount of movement and is therefore beneficial over conventional dropper attachment methods.

Graham Plowman



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