some questions about voltage regulator

[miaomiaooh]

usually use voltage regulator to step down voltage, the pin 1 (Vin) and pin 2 (Vout) should place a capacitor,y should place it??
then how i know wat value capacitor i should to use??
wat effect of the capacity higher or lower for a voltage regulator??

 

[Analog]

If you are talking about a 3-terminal regulator, please include a part number. For these, check the data sheet for appropriate values. The caps are needed for stability purposes.

 

[miaomiaooh]

If you are talking about a 3-terminal regulator, please include a part number. For these, check the data sheet for appropriate values. The caps are needed for stability purposes.

the voltage regulator is 7808, i found some data sheet about the 78xx,pin 1 is place 0.33uF and pin 3 is place 0.1uF.
if i place both 15pF, wat different?

 

[audioguru]

If the datasheet says to use certain capacitors, then why change them to be 22,000 times smaller in value?? 15pF is almost nothing.

 

[Hero999]

15pf is too small for a three terminal regulator unless you're using it for phase margin compensation in a more exotic regulator but we don't really know since you haven't posted a schematic!

 

[miaomiaooh]

If the datasheet says to use certain capacitors, then why change them to be 22,000 times smaller in value?? 15pF is almost nothing.

wat the function of capacitor for voltage regulator??if i dun place it,wat will happen??

 

[audioguru]

Look up the meaning of stability. The regulator's datasheet says they are for stability.
A regulator might oscillate without the recommended capacitors and the datasheet shows the output step response with overshoot and ringing when the output capacitor is too small.

 

[ShockSL]

The caps are for stability and they help regulate the output. The response of the regulator is some what slow. For example if you a using the regulator to drive a device that outputs a fast pulsating voltage output the regulator want be able to respond to the fast changing o/p current demand and the output of the regulator also begins pulsating.(Disastorous if you are driving a MCU). The caps works by providing the necessary current till the regulator can respond. So small caps will be ineffective depending on you application. A too large one will increase your start up times.