Capacitor Value

Hi,-- something that is bothering me for quite some time now--Most of the regulator datasheets(78XX for eg) say that an input capacitor to the regulator is required to be connected if the dc unregul. voltage source is at a far away distance from the regulator. Is this true ??
If it is then what are the calculations that go into it?

I thought that the most imp. factor while choosing the cap. value would be the ripple factor (assuming known load current,o/p voltage, freq )

There's nothing to calculate - the 78xx regulators require capacitors close to the pins, on BOTH input and output - it's to stop them oscillating. You can use 0.1uF, 1uF, 10uF - it's not critical. I usually use a couple of small electrolytics, I don't even check the values! - they are going to be either 1uF, 2.2uF, 4,7uF or 10uF - all of which work fine.

The ripple factor depends on the load resistance. Higher the resistance, lower the ripple factor.

I normally use a 1000uF or a 2200uF shunt capacitor type filter. I have used the 7800 series and LM317 and you need to put the extra capacitor only if the IC is 6inches or further away from the filter capacitor.

I think you can put the filter capacitor as close to the IC as possible, because there is no restriction on far the filter can be from the rectifier