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# Clasic DC PSU calculations help

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#### t_maggot

##### Member level 3
Problem:

DATA:
I want to make a simple AC->TRANS->BRIDGE.RECT.->CAP->DC PSU.
I want in output ~10% ripple, when Vpeak DC = 12V, and when Irms (on load)=20A.

Questions:
1.What must be the size (VA) of the transformer and what must be the Vrms secondary output?
2.What is the desirable capacitor value?

Please answer both or none. Any other help appreciated.

You can calculate the ripple using this equation.
Vp is the peak voltage, RL is the load resistance and C is the capacitance in farads
Dt is the time between peaks i.e if 50Hz supply and full wave rectified peaks will be at 1/100 of a second
Vripple =(Vp/RL C)× Dt

The power rating of the transformer will need to be at LEAST 20x12 VA. The secondary output V needs to be 12V rms
You do not say why you need this. The above calculation is approximate. If you are actually going to use this circuit you will need to take into account that the voltage will rise as the current decreases and choose the voltage ating of your capacitors accordingly. If the power supply is being run continuously you will need to buy a transformer that is rated for continuous use.
Hope this is of some help

### t_maggot

Points: 2
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Thanks for the answer and the equation you give me.
But if the Vrms on secondary of transformer is 12V rms (~16,9Vpeak) and we have a capacitor large enough to have a ripple of ~10% then the Vrms has to be at least 15V. For example if the cap is infinite the Vrms out is going to be 12•√2≈16,9V. I want 12Vrms DC.
Anyway I simulate the circuit with spice, in witch the results is more close to reality (and I dont need to make calculations).Realy usefull program.

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