wallwarts and regulation..

Scott Gravenhorst chordman at flash.net
Sat Mar 7 19:17:25 CET 1998

DOH! Thanks Tony, I sit corrected.  I see in the National data book
that in this configuration, load regulation is described as:

[(r1+r2)/r1] * Lr 

where Lr is the load regulation factor of the 7805 with the ground pin
grounded.  r1 is fixed and connected from the output of the reg to
it's ground pin and r2 is variable and connected from the ground pin
to "real" ground.  Does this expression say that load regulation
worsens as r2 is increased?  

On Sat, 7 Mar 1998 13:16:09 -0000, "Tony Allgood"
<oakley at enterprise.net> wrote:

>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:

-- Scott Gravenhorst

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