Stacks Image 4241
Short Circuits No.2 - Thinking Inside the Box

Almost everyone who has a model railway has a mains powered electrical device to make their trains run. They may refer to this device as a transformer, a controller, a line controller, a throttle or even a power unit. Whilst all these terms are generally accepted, there is really a little bit more to it than that.

These devices consist fundamentally of five essential and discrete components within the box. These components are, a transformer, a rectifier, a user variable speed setting device, a direction switch and an overload protection device. There are two principal types of unit, ones with
variable current (fig 1) control and ones with variable voltage (fig 2) control. Both types are in common usage today.

So how do you tell the difference then? Connect the unit as usual making sure the controller is at stop and there are no trains on the track. Connect a test meter set to measure about 30 volts DC across the track. The reading should be zero. Turn the controller just off stop and observe the voltage reading. If the reading goes negative, reverse the test leads. If it immediately goes to about 20 or so volts, it is a variable current unit. If the volt meter reading increases proportionally with the speed setting, it is a variable voltage unit.

Variable current devices have been more common in the past and these rely on the value of the series connected variable resistance being compatible with the impedance or resistance of the motor in the train. Some units have a high/low resistance switch, some have a full/half wave switch and some even have both and if set correctly, performance will be noticeably improved.
Stacks Image 4244
Variable voltage units give a more responsive performance on a much greater range of motors from early Hornby Dublo and Tri-ang types up to the more efficient low current motors of today. The two diagrams illustrate the principal of the fundamental difference between the two types of units. Some voltage controllers have a variable multi tapping on the secondary windings of the transformer whilst both may use semiconductor circuitry to regulate the speed.

Which type is best? The answer to that is really down to what you think it is worth spending on probably the most essential accessory item you will ever purchase for your train set, the quality of the control that you require and also what the unit will deliver. It is essential that you try any new unit before you buy. A good new unit will cost you at least the price of yet another new locomotive. The choice is all yours but remember, without a quality control unit, none of you trains will run at their optimum performance.

Connecting to set track type track requires two wires and a power clip specified for analogue (not DCC) use. These power clips are fitted with a radio and TV suppressor or capacitor which helps to reduce reception interference on these devices. Power clips designated for DCC do not have the suppressor as it corrupts the digital control signals.

Next time you operate the train speed control you will know just what is happening within the box.