how to calculate capacitor value and its voltage used in full wave bridge rectifier

full wave bridge rectifier with capacitor filter calculation

Hi friends today we are discuss about, how to calculate capacitor value and its voltage, used in full wave bridge rectifier.

The circuit is above ,The transformer is there (230 /12 VAC). There are 4 rectifier diodes. The electrolytic capacitor and 5 volt regulator 7805 Output we will get 5 volts. We will calculate the value of the electrolytic capacitor, for 5 volt and 1 ampere load. Other two are 0.1 µF polyester capacitor.

Here we have 1 transformer And primary voltage is 230 volt. But there is always variation in input voltage. So we assume, that it can go up to +10 % And lower side it can come down to - 20%. The 12 volt is the secondary voltage when input is 230V. When voltage is less, This also will be less.

Now to calculate the capacitor value, We take lower value of input voltage. When voltage at input is less, -20%, That time this 12V will become (12 - 20%,12-2.4=9.6v). So, we will get 9.6 volt AC, when input voltage is less.

how to calculate capacitor value for bridge rectifier

Now we will calculate the maximum voltage. Here we were getting 9.6 volt. Then 1st we will calculate the peak here. That will come

9.6 *√2. (√2 =1.414 volt). Then from result of this peak voltage, we reduce 2 times of diode drop. Two diodes will be coming in series of the capacitor. So double drop. So when we calculate this, we get 12.2 volt.

9.6 *√2-(2*0.7)= 12.2

So at this point we will get 12.2 volt DC.

Now we will calculate, how much the capacitor will discharge, When we connect a load. Just now we calculated, maximum voltage at the rectifier output point is 12.2 V. Now if you see the catalogue of the regulator, Then it is written that minimum voltage should be 7 volt. This means this capacitor can be allowed to discharge, up-to 7 volt because of the load. So here voltage was 12.2 volt peak. When you connect a load, capacitor will start discharging. It can go minimum 7 volt. So net voltage drop will be 12.2 volt - 7 volt= 5.2 volt.

how to calculate capacitor value for 12v power supply

Now we will calculate value of this C. Formula is,

C=I*t/V C =1*10m/5.2=1923 µF

Here, I is load current (output current), I have written 1 ampere. t is half cycle time. For 50 Hz half cycle time will be 10 mSec And V is voltage across capacitor. What we calculated , 12.2 - 7 = 5.2 V. So value will come in micro farad, it is 1923 µF. So this is the calculated value.

But what happens, When you go to market & buy the capacitor, Capacitor will have tolerance. So the value written will not be exactly there. It may be little less. 2nd thing is, The transformer will have some resistance, Because of that drop, we may not get 12.2 volt here. 3rd thing is, lot of spikes may come. Because of these reasons, We select higher value, at least 50% more. So that value will come about 3000 µF. We will not get 3000, we will get 3300 µF. So value of C will become 3300 µF or higher. You can take 4700 µF also.

power supply capacitor size

Now we will calculate the voltage of the capacitor. For calculating value (of C) we have taken -20%. But for calculation of voltage we will take +10%. When input voltage is 230 volt, Here we get 12 volt in secondary. When it is +10% at input, Secondary voltage will be,

12 volt + 10% = 13.2 Volt.

Now to get peak voltage, again what we will do? We will multiply 13.2 with √2 to get peak voltage.

13.2 * √2-(2*0.7)= 17.3 v

This total value will come 17.3 volt. So capacitor voltage should be at least 17.3 volt.

Here I have explained you method of calculation of capacitor voltage and value.

Now we will see the final value. Earlier we have calculated capacitor value 1923 µF. But we have changed it to during selection process 3300 µF. Similarly, voltage we have calculated 17.3 volt. We will take little higher. may be 20 - 30% more. So higher value may come 21 or 22 volt. We will not get that value. We will get next higher value 25 volt. So rating of the capacitor is 3300 µF, 25 volts. You can take higher value also, This is for one ampere load. For other loads it may be different. You can always select higher value.

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