At 11:10 AM 3/7/2003 -0500, you wrote:
>On Fri, Mar 07, 2003 at 08:32:34PM -0800, James wrote:
> > Is there a way to get regulated 15 V from a 7812 12 V regulator ?
>
>All 78XX regulators are designed to keep their regulated voltage between the
>Vout and GND (ADJ) pins. So putting a voltage divider from Vout to GND so
>that the midpoint is 3V and attaching that node to the ADJ pin will do what
>you want.

That's rather a bad idea because the current from the GND pin is large
(several mA) and quite variable.

It is, however, constant *enough* that the drop across a zener or LED or
diode will be fairly constant. But not a resistor.

>Note that the LM317 is little more than a standard 1.2V regulator.

The current out of the ADJ pin is only uA. In place of that, you must
draw at least a few mA from the output pin, which is often done by
the divider. So, it's quite different from a 78xx regulator. Try looking
at the schematics sometime if you want to see the differences- the LM317
is quite a bit more complex.

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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