Re: Voltage regulation

On Thu, 10 Jan 2008 14:10:17 -0600, Chris W <1qazse4@xxxxxxx> wrote:

Suppose you have a 12V supply and need to power a 5V device. If that
device is a simple resistive load like say an incandescent light bulb,
you can simply use the right size resistor to drop the voltage. If the
device is more complex and does not draw a constant current, the
resistor obviously won't work. Something like a 7805 would be the
easiest solution. In both cases you are wasting a lot of power. If the
first case over half the power is just generating heat in the resistor.
I assume the same, or close to the same, amount of power is lost in
the 7805 regulator.

If you needed to light two 5V light bulbs then you could hook them in
series and waste a lot less power by using a smaller resistor.

Now for my question, can you do something similar by using 2 7805
regulators hooked up in series to power 2 separate loads? Alternatively
and more what I want, can you also have the outputs hooked up in
parallel to power one 5V device?

Sounds like you already have a grasp of the principles. In theory
with two regulators in series (and the ground returns isolated from
ground) each regulator would do its best to keep the voltage constant
to its load (barring oscillations or other anomalies).

But if one regulator/load is drawing less current than the other -
that regulator would have lots of input voltage to play with and the
other might not. So it would only work with very close unvarying
loads and enough voltage headroom to keep two regulators happy -
nothing that suggest higher efficiency.

Best bet is to use switching regulators to supply the loads from a
single source. National Semiconductor has a line of "simple
switchers" (now in their 3d or 4th generation) that are very easy to
apply and very efficient - and still available in packages you can
hand solder to without a magnifying glass.
--
.

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