Full wave center tapped transformer output voltage
Figure 1. Centre-tapped transformer with full-wave rectification.

Because there is no smoothing capacitor you just need a 15-0-15 V transformer. (You will lose about 0.7 V because of the diode drops so your actual output voltage will be 14.3 V DC.)

If you want smoothing you will be adding capacitors which will increase the average DC output voltage. There are many, many articles explaining this on the web.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 2. A more usual way of drawing the same circuit with the power-supply on the left and the load on the right.



From the comments:


  I want to understand it and what about the value of resistance between the two diodes.


It should be clearer in Figure 2 that the resistor is the load (whatever is being powered by the power-supply). It could, for example, be a light bulb or a motor.

One thing to watch is that the transformer will have a maximum power rating, usually in VA (volt-amps). In a real circuit you need to make sure that the current drawn by the load (R) is not greater than the transformer can supply without overheating.

See Electronics Tutorials for a good introduction.