Bypass capacitor selection

[Vijetha H.N]

Hai,

For a generic board design i use 12V,2A supply voltage. The board has got many interfaces and the worst case current consumption may be around 1.5A. For this design what can be the bypass capacitor value in the power circuit? Please share the proper way of calculation of bypass capacitor value.

Regards,
Vijetha

 

[BradtheRad]

There is a formula but I forget how it goes.

Here's a crude method using simple calculations...

Assuming your supply is a conventional full-wave diode bridge, and your mains is 60 Hz...

The capacitor receives a burst of current 120 times a second.

For 1/120 of a second the capacitor powers a 1.5A load at 12V.

The load calculates as 8 ohms.

Suppose you permit 5 percent ripple voltage. Then the capacitor discharges by .6V in 1/120 sec.

Now we calculate the RC time constant. This works out to the time it takes a capacitor to lose 63 percent of its initial charge through a resistor.

If it can discharge by 5 percent in 1/120 second...

Then this calculates to 63 percent in 12.6 /120 of a second, or .105 sec. (For simplicity I am regarding the discharge graph as a straight line.)

The time constant can be .105 second. The time constant is defined as R x C.

So divide by 8 ohms. The capacitor value should be .013 F, or 13000 uF, if we allow 5 percent ripple.

This sounds like a big capacitor. I seem to remember my supply has a 4700 uF. Its transformer is 12V and the ripple is obvious when it is putting out a couple amps.