> >> I am using ST 7812 regulator the problem I face is the voltage out I
> >> get normally is constant 11.9 or 12.03 V.  Some of the parts after
> >> use for fews day or 1 month, the voltage out drop to 8.9 V and it is
> >> unstable waving between 8.9 to 11.9 V. What is the reason of this and
> >> how to avoid this ?
> >>
> >> James
> >
> >
> >James, just to awake a possible problem;
> >
> >In 2001 we bought hundreds of 7805 produced by TI in Mexico, all of them,
> >without exception presented several different voltages, from 4.01 to 5.12V,
> >not a single one did show a voltage between 4.95 and 5.05V, not one.
> >
> >Unfortunately we only found it out, after 5 months of purchase, so the
> >supplier "could do nothing" about it.
> >
> >By other technical point of view, I can suggest you to check the current
> >and temperature over such 7812.
> >What is the Voltage at its input? what is the load to it?
> >
> >Are you using any electrolytic capacitor at the 7812's output? if yes,
> >remove it, keep only a small 100nF to filter high freq noises.  Any
> >electrolytic capacitor should be used at the input of the 78XX family, not
> >output.  The reason is that it can cause regulation delays and problems and
> >the 78XX "can" do some messy regulation.  By the same reason, the 100nF
> >capacitor is REQUIRED to avoid high freq to mess with the regulation.
>
>Actually, I've had 7805's go unstable with any LESS than about 10uf on the
>output...!

I usually have at least 47uF on the output of the regulator (and always a
100n) with
no problems. Longish power traces on boards make this a necessity to keep
things
like op-amps stable when driving loads. Just don't go overboard and stick a
2200uF
capacitor on the output! Also, remember to add a 100n close to the input
when the
reservoir cap is more the 40 or 50mm from the regulator.
David...

___________________________________________
David Duffy        Audio Visual Devices P/L
U8, 9-11 Trade St, Cleveland 4163 Australia
Ph: +61 7 38210362   Fax: +61 7 38210281
New Web: www.audiovisualdevices.com.au
___________________________________________

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