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Model Rail Power Supply

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Most layouts require three forms of Power Source. The first is a variable 12v supply to control the speed of the train, the second is a fixed 12v supply to power all ancillary equipment such as points, signals, etc, and the third is 16vAC for some types of Points and signals.
 
How much power do you need.
Electrical power is a function of voltage and current (literally: Power in Watts is equal to the current in Amps multiplied by the voltage in Volts, or P=IV). Power for model trains and accessories is typically measured in thousandths of an Amp, or milliAmps (mA) Note: one milliAmp of current at 12 volts provides 12 milliWatts of electrical power.
What really matters in a model train is the torque produced by the motor (which varies with current) and the power to move a train (“power in rotational motion”, which is at it’s maximum at about half the full unloaded speed). Torque is used to overcome friction, and is particularly important when starting a stationary train, this we will cover in the chapter Control panels, but you need the power available to run the Speed Controller.
 
The Power Source.
There are many types of Power Supplies on the Market today. What you are looking for is a 12vDC supply with a current rating of between 2 to 5 Amps. The current rating required will depend on the size of your layout and how many Locomotives you are running at the same time, for very large layouts you could go up to 10 Amps or higher  You will also need a 16v AC supply if you are using certain types of Point Motors and Signals

















      A is a Power Supply delivering 12 volts DC and rated at 3 Amps.
      B is a PSU delivering 12 volts and rated at 5 Amps
      C is a PSU delivering 9v to 15v and rated at 10 Amps. This unit has a potentiometer for you to set the voltage you require
      D is a 16v AC power unit with a separate circuit to convert the 16vAC to 12v DC at 1.1 Amp, if required

Which ever one you decide to use you will now have to distribute the power to certain equipment on your layout, the below diagram gives an idea of how this could be done. You must however bare in mind the current capabilities of the PSU. The fourth example would appear to be the logical one as you have the 16v AC and the 12v DC all in the one unit. However the 12v DC is only rated at 1.1 Amp so will be limited to what it can power.
The circuit diagram for Type D is as follows.:
























A better Power Supply system is as follows.
Here we use the 16v AC Power Pack just for the AC supply, and introduce a high current Power Unit for everything else. Now you have enough power to run a number of Speed Controllers and all the other 12v DC equipment.


















In all drawings Blue is shown as AC, Red is Positive (+) DC, and Black is Negative (0) DC.

Now that the Power is sorted out, we need to look at the Speed Controller.
 
Speed Controllers.
Most Propriety manufactures make their own Speed Controller, here are a few examples. They all supply a variable voltage to the tracks and most have a Forward/Reverse switch to change the polarity on the tracks. Some speed controllers will be supplied with the Power Unit built in so you may not require a Power Supply Unit. However these will be limited to the amount of current they can deliver, so at some stage you may have to invest in a separate PSU.

















We now have the basis for the Power System for the Layout.

The wiring of these is straight forward and all will follow the circuits below.


















In all drawings Blue is shown as AC, Red is Positive (+) DC, and Black is Negative (0) DC.
 
As your system becomes larger and you put more equipment on the layout you may find that some of these items only require 5v DC. This is ideal for powering LED’s and as you fit more it would be a good idea to have a separate Power Supply Unit for them, this will be a 5v DC and is available as a plug in unit.




















While looking for a Speed Controller you will come across the phrase ‘Pulse Width Modulation’
 
Pulse Wide Modulation.
PWM is a modulation technique used to convert a voltage into a pulsing signal. This modulation allows the control of the power supplied to electrical devices, especially to loads such as motors. The pulse width modulation speed control works by driving the motor with a series of “ON-OFF” pulses and varying the duty cycle, the fraction of time that the output voltage is “ON” compared to when it is “OFF”, of the pulses while keeping the frequency constant.
The power applied to the motor can be controlled by varying the width of these applied pulses and thereby varying the average DC voltage applied to the motors terminals. By changing or modulating the timing of these pulses the speed of the motor can be controlled, ie, the longer the pulse is “ON”, the faster the motor will rotate and likewise, the shorter the pulse is “ON” the slower the motor will rotate.
Also the amplitude of the motor voltage remains constant so the motor is always at full strength. The result is that the motor can be rotated much more slowly without it stalling.
 
Add an Ammeter
As mentioned before the voltage is not the problem with a Layout it is the Current drawn. As your layout grows so does the demand for power whether it be your 12vDC, 5vDC, or 16vAC supply. The current you draw may need to be monitored if you are expanding. A panel mounted 10 Amp meter is readily available and is connected into the Positive wire as near to the Power Supply Unit as possible.  Where the positive wire goes into the Terminal Block, remove it and connect to one of the terminals on the back of the Ammeter. Fit another wire from the second Terminal to the Terminal block. The circuit below shows the position from each PSU where you can break into the supply and fit an Ammeter.






















You could also use your Multimeter or purchase a Clamp Ammeter all will do the job. The Clamp Ammeter is the only one that does not require any rewiring.
 
Build your Own PSU.
You may wish to build your own Power unit and speed controller from scratch.
The circuit below shows a typical Power Supply circuit where the Track Speed Controller does not have a Transformer included.
The transformer would be a 12 volt AC output at 4 Amp, The Circuit Breaker should be rated initially at 3.0 Amp to protect your circuits and equipment from short circuits. The Rectifier converts AC to DC and must be capable of carrying at least what the transformer is delivering 4 amp or above.
The speed controller can be any transformer-less unit from any proprietary manufacturer.
















     Note: A)  All wiring should be no less than 16/0.2 Equipment Wire.
     Note: B) The DC power supply above is unregulated and not stabilised and will work fine for most Railway systems, and will run ancillary        
                     equipment fine. It however is not recommended for use on sophisticated electronic circuits. The following circuit will do this job.
 
Connection to the Household Mains
All this needs to be connected to a domestic socket where ever your layout is.
A few points to remember.
Start with an RCD in the domestic socket.
What is an RCD.
A residual-current device (RCD), is a device that instantly breaks an electric circuit to prevent serious harm from an ongoing electric shock. RCD’s are designed to quickly and automatically disconnect a circuit when it detects that the electric current is not balanced between the energized (line) conductor(s) and the return (neutral) conductor. Under normal circumstances, these two wires are expected to carry matching currents, and any difference can indicate a short circuit or other electrical anomaly is present, such as leakage. Leakage can indicate a shock hazard (or shock in progress) which is a potential danger to a person.
A Multiway Connector. There are many types and styles on the market, some even come with built in RCD’s. Keep the cable from the Household socket – RCD – Multiway Connector as short as possible. Do not use a coiled extension lead, or if you do, uncoil the lead before you use it.
Plug all your Power Units into the multiway and keep the wires to the control Station as short as possible.
Why uncoil extension cables.
A powered up extension cable is the making of an electromagnet, albeit very crude, however the coil can heat up, this could then cause a fire. I must stress this is very unlikely with the currents you will be using, but it is always better to be safe than sorry.

FORWARD REVERSE SWITCH
A forward/Reverse switch can be added to your layout by incorporating the following circuit. The Switch can be added in a number of places shown on the above circuits as 'A', 'B', or 'C'
Please Note: this switch should only be used when the Locomotive has been stopped, as it will instantly change the polarity of the tracks putting the locomotive in Reverse at any speed.

























This circuit uses a 3 pole changeover switch. The Positive and Negative (RED & BLACK) are connected to the switch as shown (where they cross each other on the back of the switch ensure they are insulated, as they will create a dead short if they touch each other). The LED indicators are connected as shown with the 1K resistor on the Negative line. If using 12v Filament bulbs the resistor is not required.
The output to the track is shown in Blue as the polarity is now 'either or' depending on the position of the switch. The numbers on this drawing represent the numbers on the back of the switch.

 













Note: A) All wiring should be no less than 16/0.2 Equipment Wire, except to the LED's which is fine in 7/0.2 or 1/0.6 Equipment Wire.
 
Parts List.
Miniature Toggle Switch                     SW315
Red LED                                                 SL100
Green LED                                             SL102
1K Resistor                                            RE137