Power Supplies

Train Controllers, Diode Type

Written by David Head. Posted in Power Supplies

At EMRC we use a simple yet effective controller. It is a mechanical/electronic controller. They have been used since the club's inception back in 1980.

The controller is a development from kits originally purchased from the Melbourne Model Railway Society and were modified by removing the jug style of element from the circuit and replacing this with diodes connected in series. This allows for a smoother operation of locos by varying the voltage rather than the resistance. They have been always been cheap to construct.

Basically the controller has a mechanical lever that moved a wiper around a circle of bolts which have diodes attached. As you move the controller around the wiper increases the diodes in the circuit.

It relies on the electronic characteristic of a normal everyday diode. A diode will always drop the voltage by about .5 of a volt, independent of the current. The controller adds the diodes into the circuit thereby reducing the voltage down in discrete steps.

The controllers control voltage independent of the current required. Slow speed is performance is satisfactory. The controllers work well with all types of locomotives.

Click to enlarge

Note that the other side of the track is grounded.



We usually drill 26 holes into plastic or wood - why 26 ? Well with .5 V difference, we need 24 diodes to stop a train. But we also leave one hole for a stop that is not connected to the circuit. We also leave the first bolt the same to allow for a perfect 0V stop.




























Circuit Description

Click to Enlarge

As the controller uses a spit potential power supply the wiring is simpler. Refer to diagram above. This allows the  use of three wires in the controller lead.

It's really simple, but here's a explanation on how it works:

  1. First of all the Power comes via "A" or "A2" depending on the direction switch at "B".
  2. Next the power passes to the base of the controller's pivot "C" and through the arm. It will then transfer to the bolt and thus through all the diodes.
  3. If the arm is on "E" , there is no power, this the controller is off.
  4. If the arm is on "F" , there is power, but the power has to pass through all diodes, thus you re at the slowest speed. (Some ultra high motored locos , like some Portscap locos can move on this setting.
  5. If the arm is on "G" , there is power, but the power has to pass through all diodes - 1, there is one less diode to pass through and the voltage will be ever so much more ( .5V more)
  6. So this continues. At the point "H" we are about halfway around and at half throttle.
  7. When the arm is on "I" we are no longer going through any diode and thus are supplying full voltage to the track - full speed !
  8. You notice the 12 V lamp shown at "J". It will provide protection with shorts & overloads.
  9. Finally, "K" is the track.


Modification for Single Rail Power Supply 

Click to Enlarge The controller can be used on a normally wired system. One bonus is only a single diode is required on the bolts, not one each way.

It is quite simple to make. Use solder lugs that are bolted in to solder the diodes to. The pivot and arm is the biggest challenge. The wiper/contact is the most important. If it wears it may lose contact with the bolts, causing loss of power to the locomotive. They should be sprung, not too tightly so that the arm can be rotated.