[sdiy] Variable Voltage Regulator as replacement?

[patchell]

Atom 'Smasher' wrote:

> > > an old trick of mine, for getting weird (uncommon) voltages, is to take a
> > > 78xx regulator, and add a resistor in series with the "ground" pin. this
> > > can be used to get a few extra volts from the regulator.
> >
> > errr, what about using diodes to get that right voltage? Wouldn't the use
> > of resistors depend on the current draw to determine just how much voltage
> > is added?
> ==============================
>
> you can use diodes, in series with the "ground" pin, and take advantage of
> the forward voltage drop.
>
> it's been a while, and i forget, but for some reason i remember that the
> voltage between the pin, and the "real" ground is not the same as the
> difference between the advertsed and the actual voltage at the output.
> (ie: a silicon doide, with a 0.7V drop, and a 7805, won't give an output
> of 5.7V.... however, that's if i remember correctly... i sometimes don't.
> anyway, for most purposes, it's easier to figure out what value resistor
> to use, and use only one, than to string together several diodes.)
>

    No, adding a diode in the ground leg of a three terminal regulator will raise
it by one diode drop.  I used to do this all the time when I needed a 5.6 volt
supply.  The exact voltage you get may vary depending on the diode.  Remember,
the lower the diode current, the lower the diode voltage drop, but, it is an
exponential function.

    I have also used zener diodes in the ground leg.  Like if I need a 10 volt
regulator, you put a 5 volt zener in the ground leg of a 5 volt regulator.

>
> however, the current through the "ground" pin on the 78xx regulators
> changes only trivially, while the output current goes from zero to max.
> this means that you can use a resistor, and the voltage across it will not
> change significantly.
> that's the key that makes the trick work.
>
>         ...atom
>
> ----------------Void-If-Detached----------------
> http://smasher.suspicious.org/fs1r  Yamaha FS1R
>
>         "Pick battles important enough to fight
>          and small enough to win."
>                 -- Johnathan Kozol

--
 -Jim
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