[sdiy] Variable Voltage Regulator as replacement?

[Atom 'Smasher']

> > 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.)

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

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