The cost of a plug pack (Wall Wort) on eBay is less than making the power supply yourself and is guaranteed to isolated and insulated from the mains.
They simply [plug into the power-outlet and deliver the required voltage and current specified on the case.
You can get an old-style with a transformer or one of the new designs using switch-mode technology.
The switch-mode units are smaller, lighter and deliver a higher current with much better regulation.
The voltage remains constant at 12v.
In technical terms, a plug pack is called a A POWER SUPPLY.
It converts the mains (110v or 240v) to a lower voltage and outputs this voltage as AC or DC.
This is the end of the term: "power supply."
Along with the tem: "Power Supply" we have the terms: Current Rating, Voltage Stability, Reliability and Overload Protection.
Some of these terms refer to a BENCH POWER SUPPLY costing over $100 but we are interested in producing the cheapest power supply for out Model Railway projects.
Looking on eBay you will find a 12v Plug Pack capable of delivering 1 amp for less than $10.00 (posted) (many plug packs are $3.00 to $8.00) and you will need one or two of these for our projects.

If you deliver 12v to a locomotive it will move around your layout very quickly and travel faster than scale speed.
If you add a full set of carriages, the train will travel at a reduced speed and appear to be about realistic scale-speed.
If you want to operate any of your trains at this velocity, you will need a power supply of about 14v because about 2v is lost in the circuitry between the power supply and the track - in the controller.
That's why train transformers deliver about 14v DC to 18v DC, so you can set the throttle at less than maximum and appear to have more "driving power . . . up-your-sleeve."

12v Vs 14v
The point is this: Most 12v power supplies are just suitable as a MODEL RAILWAY POWER SUPPLY and you can say they are not quite suitable.
Many 12v power supplies are OVER RATED and cannot deliver the specified current.
On top of this, some of the controller projects contain a set of diodes in a bridge and 2v is dropped (lost) across them. This means you need 14v to get 12v to the track.
The explanation for this is quite complex but it all revolves around  . . .

One of our projects is a DCC CONTROLLER. It allows two locos to the operated at the same time on a layout.
This creates a problem for the power supply. It is called a CURRENT PROBLEM.
If the power supply cannot deliver the current required by both locos, the voltage reduces and the velocity of one train is affected by the other train.
That's why you must have sufficient current.
Here is the problem.
Most power supplies rated at 1 amp are "tricking you."
What really happens is this:
The output voltage at no-load is about 14v -16v and this reduces as the load increases. It drops to 12v. 
This problem is not noticed when a single loco is active on a layout, but if you have two locos in motion, the voltage fluctuation produced by one loco will affect the velocity of the other.
That's why we have to take a new term into account. It is called REGULATION.
This is the measurement of the output voltage at NO-LOAD and the voltage at FULL-LOAD.
You cannot find these values anywhere. They are never described.
That's why you have to connect a power supply to the project and see how the locos are affected. It is only evident when you are CURRENT SHARING and this occurs when two locos are active.
Generally, a 1-amp power supply is really only suitable for 700mA as the rating of 1-amp is a maximum for a very short period of time.
And a 2 amp power supply is really only capable of delivering 1.5 amp.
We are referring to the old-style heavy plug pack containing a transformer.
The new design supplies using Switch-Mode electronics are much more reliable and will deliver the rated current.

Here is the cheapest 2-amp power supply. It is really a 1-amp Plug Pack (Wall Wort) charging a 12v battery with a trickle charge.
The battery will only last a short period of time and will need many hours to recharge but it will produce a very reliable 12v. It will deliver more than 2 amps and can be called a
10 amp power supply.

The Plug Pack can be 100mA to 500mA and the battery should be slowly charged over a 14 hour period. You need to remove the plug pack when the battery is charged.

If you don't want the annoyance of the battery going flat after a few hours of use, you will have to use a 2-amp power supply.


Talking Electronics has a number of pieces of TEST EQUIPMENT to help in the design and testing of projects.
Of course you can use a multimeter for most of the testing but some of the "tricky" faults need a special piece of equipment.
You may only need a LOGIC PROBE once a month, but the project you are designing will come to a stand-still if you can't locate a problem.
We designed all these projects because we needed them ourselves.
Add one of them to each order you place with Talking Electronics and eventually you will have the whole range.
27MHz Field Strength Meter
Tests 27MHz transmitters
$6.50 plus $4.00 post
Tests LEDs.
$1.50 plus $4.00 post

(buy a number of kits and pay
only one postage)
Only responds to resistance less than 50 ohms.
Ideal for digital projects as it tests connections very quickly.
$12.50 plus $6.50 post

(buy a number of kits and pay
only one postage)
- slimline
Detects HIGH and LOW signals on both TTL and CMOS circuits.
The piezo allows you to hear low frequency signals and the signal injector (Pulser) will over-ride clock signals to make a circuit operate at a reduced frequency.
$8.00 plus $6.50 post
20 different functions.
See article for the complete list of functions.
$18.00 plus $6.50 post
Tests transistors and shows the gain of the transistor.
Also has Signal Injector probe.
$21.50 plus $6.50 post
Simple Transistor and LED Tester - 3
Tests PNP and NPN transistors and LEDs.
Also teaches the amazing property of an air-cored coil in producing a high fly-back voltage.
$4.00 plus $3.00 postage.

(buy a number of kits and pay
only one postage)
Detects 240v AC mains hidden in walls etc.
Will also pick up RF signals from a keyboard to show you where Electromagnetic Radiation is coming from and giving you a headache.
$10.00 plus $4.50 post
Traces cables when the power is OFF.
Uses an FM radio to pickup beeps.
$10.00 plus $4.50 postage. 

(buy a number of kits and pay
only one postage)

Teaches how an op-amp works by using pots to control the voltages on the two inputs.
$24.50 plus $6.50 post
(comes with instructions)
Learn to program PIC chips.
Comes with a pre-programmed PIC12F629 chip with 3 routines.
$12.00 plus $6.50 post
model railway CDU-Inline
The cheapest CDU project you can get.
$8.50 plus $6.50 postt