Simple regulated 12v 100ma power supply help

Thread Starter

Endless Paradox

Joined Mar 16, 2018
7
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.

We use a MBR140SFT1G with a PIV of 40V.To prevent the diode to burn with a charge from the transformer and at the same time from the capacitor=15v*2 we need a PIV of 30v 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 assume we want a ripple voltage of 1 volt.

So we use a 39D128G015FL4.

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 just check for me if it is good.
I also didn't understand well the PIV for the 4 power diode in the full wave rectifier you need always the double.The transformer only have an output of 15v why is a piv of 30v necessary?
I also saw that the capacitor have a voltage value is it important?
 

OBW0549

Joined Mar 2, 2015
2,987
Instead of trying to describe your circuit with words, please post a schematic diagram showing your circuit. Make sure it is complete with all components and connections, all resistor and capacitor values, and all semiconductor devices identified with their full part number. Do that, then we can discuss it.

One thing I noted: you say your transformer has a "15 volt output" but are apparently misinterpreting this to mean that the output voltage of the transformer has a peak value of 15 volts. That is incorrect. Transformers are specified as to their RMS output voltage, usually at full load current. The peak voltage output will therefore not be 15 volts, but (15 * 1.414) volts, or 21.2 volts.

Because of this, the rest of your circuit analysis is invalid.

I also didn't understand well the PIV for the 4 power diode in the full wave rectifier you need always the double.The transformer only have an output of 15v why is a piv of 30v necessary?
A diode used in a transformer-rectifier-filter circuit ALWAYS needs to be more than twice the peak voltage coming out of the transformer, because that is the maximum voltage that will appear across it when reverse biased. Onto this should be added a safety factor to account for the fact that when lightly loaded, the transformer output voltage will be higher than its rating, plus another safety factor to account for mains voltage variations, and still another safety factor just for good luck.

Bottom line: for this circuit, use a bridge rectifier rated at AT LEAST 100 volts.

I also saw that the capacitor have a voltage value is it important?
Yes. If you apply a voltage to a capacitor greater than that for which it is rated, it will go "BANG!!!"

Post a schematic.
 

MisterBill2

Joined Jan 23, 2018
3,884
What's wrong with a simple lm7812?
Post a circuit ...
It is very handy to be able to visualize a circuit based on a verbal description. For those with limited vision it is also vital. So I am suggesting that a bit of effort to discern the circuit based on the verbal description is in order. Some folks don't have access to the means to deliver a schematic diagram. Not all people have spent the money for a smart phone with a camera and viewing screen and 4000 no-essential features.
I found the verbal description quite clear, and most of the comments about the voltages to be reasonably correct. But the reasons given for requiring a higher voltage rating for devices were a bit lacking. So if the goal is to share knowledge, that usually requires more than just providing data.
 
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