3 Way Point and Diodes

This article is to help with the use of the 3 way point and to show you how to use a simple Diode Matrix to make operating the point so much easier.

About The Point
The 3 way point is really 2 points one within the other and requires 2 point motors to make it work properly. It can be very confusing to work out which point setting are needed to get to any given track out of the point.
By using a simple matrix the selection of the correct 2 moving blades can be done by the push of one button or one touch of a probe to a stud on your control panel.
The drawing below shows the layout of the point and the 4 motor coils. This layout is looking down from above the point.
The outlet tracks from the point are labeled L (left track), C (centre track) and R (right track)
3 way point
The Coils
The 4 motor coils have been labeled A,B,C, and D.
Now let us work out what combination of coils need to be operated to get to each track. If you have a 3 way point to hand you can verify this by actually moving the blades.
To get to the left hand track we need to move all the blades to the right which would call for coils B and D to be energized.
To get to the centre track we need one blade left and the other right which calls for coils B and C to be energized.
Finally to get to the right hand track requires both point blades moved to the left needing coils A and C to be energized.
Diode Matrix
The Diodes
Having now sorted out which coils need to operated we can look at how to do this by feeding current to just one connection for each option and operating the 2 motors needed each time.
The circuit diagram above shows the layout of the 6 diodes required to to do this action. There are 3 input wires labeled L (left),C (centre), and R (right) and 4 output wires labeled A,B,C, and D which are connected to the 4 point motor coils. A diode will only let current flow in one direction and forms a total block to current trying to get back through it.
Taking conventional current flow theory where current is assumed to flow from +ve to -ve (it is actually the other way in reality but don't worry about that) then if we applied +ve current to connection L above then it can flow through the 2 diodes to B and D but cannot get back through the other diode connected to output B. Similarly if you were to apply current to the other inputs it can only get to the coils that are required.

Wiring and Power supply.
The matrix can be simply laid out with the diodes soldered in position on a short bit of tag strip which can then be placed either in your control panel or under the baseboard near the point. The advantage of having it near to the point is that you will only need 3 wires from the panel instead of 4 if you placed it in the panel.
All of the return wires from the 4 motors can be connected together at the point itself and then wired to your common return or brought back to your panel with another single wire.
Since diodes only let current flow one way you really need to use a DC supply to drive the point motors coils, and because you are needing to operate more than one motor each time then a "Capacitor Discharge" unit will be most beneficial here. These units are fed from the AC feed off your power unit and give out DC with an extra burst of current to get the points moving.

Note:- It has been posted in the forum that coils A and D do not in fact need a diode at all as they only have one connection to each of them. However I have included them and would recommend that they are used for the following reasons.

1. A diode causes just over half a volt drop so those coils that do need them (B and C)
would always get less voltage to them than A and D. This in most cases would not cause a problem but if the power supply was a little on the low side or you had long cable runs to the point, then Coils B and C could be sluggish or not work due to being robbed of current by A and D.

2. The above possible problem can be totally avoided by fitting the 2 extra diodes and at a cost of less than 7 pence each you can make certain that all the coils receive the same current and behave as exactly alike as possible.

John Essex Heywood Model Railway Group

This page updated 6th February 2003