I have a full-wave center-tapped transformer circuit with input voltage of 220 V.
How do I get an output voltage of 15 V? What equations should I use?
Here is the circuit:
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.
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.
How about you do your KVL homework on sinewaves, source voltage, turns ratio and peak to full-wave conversion charts to proof and I'll show you the answer.
I used 240Vrms 50Hz because I assumed you were not in Canada. Correct me if I was wrong.
My diodes conducted 0.73Vpk into 100 Ohms. But you can assume 0.7V
My simulation using arbitrary load R of 100 Ohms and Vrms is computed on plots with Vmax and Vmin shown.