Jim Robertson wrote:
> My question  is if I change it over for a Zetec ZSR500C that is rated
> for 200mA does it follow that the output drive is lower impedance and
> therefore if I draw the same amount of current from it, it would not
> generate as much heat as the 78L05? Right/Wrong?

The impedence (current per droop voltage) is a function of the feedback and
other parameters inside the regulator.  This is independent of maximum
allowable current load.  In either case I expect the impedence to be
sufficiently low to not be an issue.  After all, that's what a regulator
does.

Impedence is also not related to power dissipation.  Dissipation is simply
(Vin - Vout) * Iin.  Note that Iin can be considered the same as Iout for
this purpose.  The difference is the regulator's ground pin current, which
will be negligeable in the realm where you have to worry about power
dissipation.

So the short answer is that replacing the regulator with a higher current
version will have no effect at all on the power dissipated in that
regulator.

> I am also looking at adding a heatsink if I can find a suitable size
> and shape. Any other ideas or solutions anyone. Thanks for any help.

Assuming the regulated output voltage is a fixed requirement and that the
circuit will draw what it will draw, the only way to decrease power
dissipation in the regulator is to reduce its input voltage.  Of course you
must still maintain the regulator's minimum required drop voltage at your
worst case current.  This can be only a few 100mV for a good LDO, or over 2V
for a brute force 7805.


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