wallwarts and regulation..

[Tony Allgood]

Scott said of the 7805:
Essentially, the output circuit of these regulators
consists of two transistors that form a voltage divider.  Too high an
input voltage will cause the lower transistor to conduct excessive
current through the ground pin.


The 7805 and others are series pass regulators, there is no shunt
transistor. It relies on the output load to sink current. There should only
be a small current down the ground pin only. This is more to do with the
quiscent operating current of the regulator itself, not the load. This has
to be taken into account when designing 317 regulators as well. This does
change with input pd, but only slightly. However, it may be partially
dependant on temperature which means that the output pd will drift as the
input pd changes due to a change in dissipated heat within the regulator.
Mind you, this is small and should not cause too much problem in most cases.
I used the 78L05 as a 7V5 regulator and it produced a stable 7.5 volts as
predicted by the potential divider attached to its ground pin, ie. the
regulator thinks it sees 5v across OUT and GND. This should not be effected
by input pd. This is not a fluke, it does work. Likewise, the circuit can be
configured with a 7905 to produce negative rails. You will have to keep the
resistors of low values, 330R or so for the bottom one, to keep the affects
of the changing ground currents. The latter is true for the 317 also. The
317 is essentially a 1.2V regulator.

Tony Allgood, Cumbria, UK

e-mail: oakley at enterprise.net

My latest project; a rack mounted VCF module with warmth. See it at:

http://aupe.phys.andrews.edu/diy_archive/schematics/effects/filter.html