> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Jai Dhar
> Sent: Monday, January 27, 2003 9:54 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: [OT]: Linear regulator question
>
>
> Hello,
>
>  I am using a +12 and +5V regulator in my psu, and both are being
> fed from a
> full wave bridge rectifier ( I guess I don't need the negative
> part ).

Skip the nonsense about "the negative part".
A full wave bridge makes the "negative part" positive.

> Anyway,
> My transformer ends up putting out 30V at the positive end of the
> rectifier.
> The specs for the +5 regulator say that 25V should be the max for
> input, but
> the +12 is ok.

If you have a center tap transformer, you could get 15 volts for your
5 volt regulator from a wire connected to the center tap.
(Add another filter cap on the line.)

> My question is that would it be fesable to feed the +5V
> regulator from the +12V one? I suppose this would mean that both
> my 5 and 12
> outputs could only have a max. current of 1.5A out since they are
> being fed
> off the same regulator... this correct?

Basicly correct...

> If this isn't fesable, how can I use
> the +5 regulator safely. Would a voltage divider do? It seems too
> simple, I
> don't know, there are a lot I do'nt know about regulators, and maybe this
> wouldn't work. Anyway, all your help is appreciated.

Use a series resistor to drop some of the voltage before the regulator.
You could even hang a zener after the resistor as a shunt pre-regulator.
>
> Jai
>
>
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