Flawn:
Be very careful with stuff you find on YouTube!=C2=A0I don't believe that s=
olution
will work at all in an application where you need 500 mA @ 5v to power a ra=
ther
expensive I/O unit. The 9v battery will not be able to provide enough curre=
nt to
power the USBpre for very long;=C2=A0it will die quickly. Then the Zener tr=
ick is
only useful for peripherals that draw a small amount current since it opera=
tes
in shunt regulator mode (don't even try it if you value your USBpre!).
You really need a proper circuit that uses an IC built for this purpose,=C2=
=A0in
either=C2=A0a series regulator circuit (wastes power & generates lots of he=
at) or a
switching regulator circuit (gets the most out of your battery, can be very=
efficient). The reason why commercial power supplies seem expensive is beca=
use
of the engineering needed to produce=C2=A0efficient power conversion that h=
as
fail-safe features to protect the powered device.
I would look at Maxim or National Semiconductor or Texas instruments. They =
make
good power conversion ICs and have lots of application notes to help you ou=
t.
Take care,
Greg
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