Here the desired load is a car battery with rating of about 40AH. Since the charging current of a battery should be 10% of the battery rating, the required charging current would be around 4A.
Now the required transformer secondary current would be around 1.8*4, i.e. approx 8A current. Since required load voltage is 12V, we can settle for a transformer with 12V/8A rating. Now the required RMS value of AC voltage is around 12V, the peak voltage would be around 14.4V, i.e.15V.
Since here we are using a bridge rectifier, the PIV for each diode should be more than four times the peak AC voltage, i.e. more than 90V. Here we select diodes 1N4001 with PIV ratings of about 100V.
Since here we are also designing a regulated power supply, the maximum allowable ripple would be equal to the capacitor peak voltage minus the required minimum input voltage for the regulator. Here we are using a voltage regulator LM7812 to provide a regulated 5V supply to the relay and the 555 Timer. Thereby the ripple would be around 4V (Peak voltage of about 15V and input regulator voltage of around 8V). The filter capacitor value would thus be calculated to be around 10mF.
Here the desired load is a car battery with rating of about 40AH. Since the charging current of a battery should be 10% of the battery rating, the required charging current would be around 4A.
Now the required transformer secondary current would be around 1.8*4, i.e. approx 8A current. Since required load voltage is 12V, we can settle for a transformer with 12V/8A rating. Now the required RMS value of AC voltage is around 12V, the peak voltage would be around 14.4V, i.e.15V.
Since here we are using a bridge rectifier, the PIV for each diode should be more than four times the peak AC voltage, i.e. more than 90V. Here we select diodes 1N4001 with PIV ratings of about 100V.
Since here we are also designing a regulated power supply, the maximum allowable ripple would be equal to the capacitor peak voltage minus the required minimum input voltage for the regulator. Here we are using a voltage regulator LM7812 to provide a regulated 5V supply to the relay and the 555 Timer. Thereby the ripple would be around 4V (Peak voltage of about 15V and input regulator voltage of around 8V). The filter capacitor value would thus be calculated to be around 10mF.
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