Thanks everyone for providing me ideas and solution on the previous
issue.

OK, the output capacitor issue is solved. The solution is not to use EL
Cap at the output cap, it will cause instability.

The issue now is the input capacitor to the 7812.  The input capacitor I
am not sure I need to add into my circuit because the input voltage is
from an Unregulated Adaptor of 12 V. The distance from the Adaptor to
the 7812 circuit board is 3 meter wire (10 feets wire length).   The
worry of adding of the input cap is if the cap short, it will burn the
circuit.  What is the best value to add the input cap, I get a lots of
response to add around 1000 uF but I look at the 7812 datasheet, the
value suppose to be 0.33 uF.

If I don't add, what are the consequence and problem I might face.

Thanks again.



-----Original Message-----
From: Spehro Pefhany [mailto:speff@INTERLOG.COM]
Sent: 15 April 2003 3:21
Subject: Re: [EE]: 7812 Unstable

At 02:39 AM 4/16/2003 -0700, you wrote:
>15V      .------.
>-----o---| 7812 |---o----> 12V
>          '--.---'   |
>               |      _|_
>               |      --- 470 uF
>             |       |
>            _|_     _|_
>
>
>This is the diagram I am using, the current draw out is 450 m-600 mA
>only.
>Any circuit improvement required to prevent output voltage oscillation
?


The output capacitor is optional for stability (it improves transient
response so you should use it), but you NEED to have an input capacitor
close to the regulator (from 15V to ground).

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the
reward"
speff@interlog.com             Info for manufacturers:
http://www.trexon.com
Embedded software/hardware/analog  Info for designers:
http://www.speff.com

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