> I am new to electronics and have some questions about the 7800 series
> voltage regulators. What happens when the supply voltage goes below the
> regulated output voltage?

It starts a runaway nuclear reaction that obliterates the regulator, the
circuit board, and everything else within 20Km in a large fireball.  This is
generally considered "bad".

That failing, the output voltage drops below spec.

> Does the regulator stop giving an output or does
> it just output the supply voltage?

The regulator will continue trying to regulate.  In normal operation it
drives its pass element as hard as it has to in order to maintain the
desired output voltage.  When that output voltage goes low, the pass element
is turned on harder.  If the input voltage is below the output voltage set
point, it obviously can't maintain the desired input voltage, so that pass
element will be driven as hard as it can manage.  However, there will always
be some voltage drop accross the regulator, so the output voltage will be a
bit less than the input voltage.

Note also that the input voltage must be kept at or above the output
setpoint plus some margin for the regulator to work properly.  This margin
is often called the "dropout voltage", which is a rather high 2V for the
LM7805.  Therefore, the input voltage for a LM7805 must be at least 7V for
it to maintain 5V output over the full current range.


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