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I had designed a rectifier circuit, in that I don't know how to choose capacitor. for example if I'm get in input supply as 9 V, now I want change it for 10 V, then how should I choose capacitor.

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  • \$\begingroup\$ Are you asking just for the voltage rate of the capacitor? \$\endgroup\$ – GR Tech Dec 12 '14 at 8:34
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10VRMS is about 14VP, hence you should find a capacitor with a minimum rating of 20V; 16V won't be high enough since capacitors lose a lot of capacitance as you approach their working voltage.

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The voltage rating of a smoothing capacitor (which is what we assume you are asking about) is a critical parameter and was addressed in the prevoius answer by Ignacio. However you also need to consider the capacitance value. This value will determine how much ripple you will be left with on the smoothed DC output. So how much is enough? The answer like so much else in this business is 'it depends'....

In particular the level of ripple depends on the smoothing capacitor value, current being drawn from the supply and the frequency of the AC supply.

In general you can take it that a smaller value of capacitor will give you more ripple for a given load current than a larger value. This being the case it is tempting to think 'well I'll just stick on a very large value here & be done with it'. Thats not a great idea & heres why:

At the moment of switch on, the voltage across the capacitor (assuming it is discharged - it might not be) should be 0V. The input voltage will come up more or less immediately, but the capacitor needs time to charge. This leads to a large inrush current (large compared to the operating current) and is one reason why psus are fitted with time delay or slow blow fuses. The larger the capacitor, the longer this condition persists as larger capacitance takes longer to charge in the came circuit than smaller values. Large inrush currents are bad news in my view and can be easily avoided by careful design.

First, determine how much ripple is acceptable to you. This will depend on the application. For digital systems you get away with more ripple perhaps, and you will normally be regulating your supply after rectification & smoothing as well. For analog applications you might want to minimise ripple. Notice I'm giving no numbers here, just rules of thumb :).

Having decided on an acceptable ripple for the maximum load current needed & frequency in use, just plug the numbers into the formula below & you get the minimum capacitance value needed in Farads. Naturally you'll come up with a value you can't buy in the shops, just get the next value up from that.

C=(Iload*t)/dV

Where Iload = the maximum load current, t = the time between voltage peaks in the rectified voltage and dV is the acceptable ripple voltage.

Hope this helps.

PS I should add that in applications where very large smoothing capacitors are used, you will often see some mechanism used to limit the inrush current. In audio amps for example, this will often take the form of power resistors in series with the capacitors. These limit the inrush current while the capacitors charge and are short circuited under control of a timer or a circuit which measures the voltage across the caps. In this way tou get the benifits of using large caps, without the large inrush. The tradeoff being the time taken to reach operation & the extra cost involved. I've only ever seen this used in audio amps or power supply units where the voltage being smoothed is > say 50VDC & the output currents are expected to be significant.

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