The objective of this project is to get a stabilised output voltage of 12v and a current of 100mA.

As Mauritius have a 230v AC voltage distribution with a frequency of 50Hz.

The VN30.15/10525 transformer fit with a frequency range of 50hz to 60hz and a 230v input.With a 15v Output.The 2,3 volt higher output are necessary for the other components like the power diode and the resistors.

We then start making a bridge “full wave rectifier” which turns the AC to a DC voltage.

To prevent the diode to burn with a charge from the transformer and at the same time from the capacitor=15v*squareroot2=approx. 22volt *2 we need a PIV of 44v or higher.

The bridge composed of 4 diodes ,only 2 work at the same time depending if the ac current coming is positive or negative.As each power diode use 0.36v and only 2 are active at the same time so 0.36*2=0.72v.The transformer give an output of 15 v so 15-0.72=14.28v.Which means 14.28v remains.

We then need a capacitor:

(Load current*time of one cycle)/ripple voltage

(120ma*10ms)/1v=1200 microfarad

We make 15v *squareroot 2 to get the peak value which is approx 22 volt.

So 22*2 we need a capacitor of 44 voltage minimum.

We assume we want a ripple voltage of 1 volt.

As told earlier we have 14.28v remaining.

12 volt is needed in the Zener diode so the remaining 2.28v need to go on the resistance.

As 20ma is used by the Zener diode and 100ma by the output.We have an input of 120ma and we need a resistance which can take 2.28v.0.27w power dissipation also is required because 0.12a*2.28v=0.2736w

R=V/I

2.28v/0.12a=19ohm

So we use a A-2642 18ohm resistor with its 5% tolerance value it reach 18.9ohm and it has also a 0.5w power dissipation

Can someone tell me if it is correct if there is something wrong?
Also is there a difference with load and without load?

 

Is this a homework project or are you actually trying to build it?
I don't follow your logic in the 14.28V remaining. I think you forgot to multiply x 1.414 again.
Ignore the voltage drop of the diodes to work out the DC voltage as your transformer 15V will vary with mains volts anyway so the precision of you calculations is a bit misplaced.
So, simply 15 x 1.414 = 21V.
Use 35V rated cap.
Current = 120mA
You need to drop (21-12) = 9V across the resistor.
power = 9 x .12 = 1Watt. Use a 2 watt or higher resistor.
resistance = 9/.12 = 75 ohms.
Under no load, all the current will go through the zener.
so 12 x .12 = 1.44 watts. Use a 5 watt zener.
In zener regulated supplies, the load on the transformer is constant as long as you do not try to draw more than the designed load. In this case, a constant 120mA will be the DC current drawn. With no load, all the current will flow through the zenner.

As it is about 4:30AM here and I should be asleep, these calcs I have shown are pretty rough but can work for a start. As I mentioned, for one thing, the 15V from the transformer probably will be different.
Try it and see how you go.

 

Use a Lm7812 ...job done.