Since your're new to electronics it might be fun to answer this question
experimentally. You can build a simple adjustable power supply and test the
behaviour of the 7800 series regulators directly.

The power supply can be built using the LM317 adjustable voltage regulator
and 4 external components. The actual circuit is available in the datasheet
(and you should start becoming familiar with datasheets) or through an
online search (try Google, for example).

Connect the power supply to the 7800 to be tested and the 7800 to the load
to be driven. If you don't have the actual load/device built yet, mock it
with a resistor or similiar device drawing the expected amount of current.
This is important because the voltage regulator may 'drop out' or loose
regulation at different points depending on current draw.

Then you're ready to experiment. Adjust the input voltage while measuring
the output voltage and observe the behaviour of the regulator when it
approaches the critical point where Vin < Vout.

Just as an aside, you don't want Vin to be too high either (e.g. 12V into a
7805, supplying 5V) because voltage regulators are very inefficient and
you'll simply waste power, which will be apparent by the heating of the
voltage regulator.

Hope this was helpful.

Shawn


>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? Does the regulator stop giving an output or does
>it just output the supply voltage? Any help would be greatly appreciated.


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