some questions about voltage regulator - Electronic Circuits Projects Diagrams Free

miaomiaooh · 5th November 2006, 09:18 AM

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 · 5th November 2006, 12:19 PM

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 · 5th November 2006, 02:03 PM

Quote:

Originally Posted by 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.

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 · 5th November 2006, 05:24 PM

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 · 5th November 2006, 06:00 PM

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 · 6th November 2006, 12:39 AM

Quote:

Originally Posted by 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.

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

audioguru · 6th November 2006, 12:49 AM

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 · 6th November 2006, 05:03 AM

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.

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

Similar Threads
Title	Starter	Forum	Replies	Latest
DC current or voltage regulator, for an analog multimeter	steelgod	Electronic Projects Design/Ideas/Reviews	4	10th June 2006 09:31 AM
voltage regulator and PIR sensors	manz	Datasheet/Parts Requests	2	25th December 2005 04:13 AM
How to use Voltage Regulator in MultiSim8?	Almazick	General Electronics Chat	0	1st August 2005 03:23 AM
ecg 957 voltage regulator spec sheet	inkedallusion	Datasheet/Parts Requests	1	5th January 2003 07:11 AM
40-120V DC to 5V@120mA Voltage Regulator.	gasson	Electronic Projects Design/Ideas/Reviews	2	30th October 2002 04:43 PM

5th November 2006, 09:18 AM	(permalink)
miaomiaooh	some questions about voltage regulator 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??

5th November 2006, 12:19 PM	(permalink)
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. __________________ "Everything that is done in the world is done by hope." -Martin Luther "There are two ways to live your life. One is as though nothing is a miracle. The other is as though everything is a miracle."-Albert Einstein

5th November 2006, 05:24 PM	(permalink)
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. __________________ Uncle $crooge

5th November 2006, 06:00 PM	(permalink)
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! __________________ I also post at the following sites: http://www.stop-microsoft.org http://www.heated-debates.com Screen name: Aloone_Jonez And http://www.silicontronics.com, same screen name as here.

6th November 2006, 12:49 AM	(permalink)
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. __________________ Uncle $crooge

6th November 2006, 05:03 AM	(permalink)
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.