charging and discharging time for a cap with a voltage regulator.

[qtommer]

in the case of capacitors used for ripple filtering in the input and output of a voltage regulator, the values used are usually given according to device datasheet ratings. Say for example a LM7805 needs 0.47uF at the input side and 100nF at the output side.

Is it possible to know the charging and discharging time? Say example for the input capacitor (0.47uF) , we know that tau=RC. and we have C.. what would be the equivalent R then?

Thanks

 

[MrAl]

Hi,

You'd have to know the input resistance which is due to the transformer and line resistance, but there's also some inductance to consider from the transformer so it's not too simple to figure out unless you know these things. It's simpler to measure with a scope and make a note.
There's also the forward resistance of the diodes used for rectification which may be a little higher than normal for a short time, and also the charging of the main filter cap which slows the whole process down. So there's quite a few unknowns usually.
It's going to charge much faster than the main filter caps however so you can most likely ignore this extra charge time.

 

[audioguru]

in the case of capacitors used for ripple filtering in the input and output of a voltage regulator, the values used are usually given according to device datasheet ratings. Say for example a LM7805 needs 0.47uF at the input side and 100nF at the output side.

No.
The tiny capacitors prevent the regulator from oscillating at a high frequency. In addition, you need thousands of uF at the input to filter out low frequency ripple.