Extreme noob electronic question

[Paul Maddox]

Marc,

I'm going to need a voltage regulator to bring down 9v to 5v with a
> LM7805 using this rough schematics
>
> http://snesdev.antihero.org/wp-content/uploads/2008/03/lm7805.gif


Yup, that'll work.. It might be worth adding a diode on the input just in
case you connect the power up backwards :)


> I undestand the capacitors are used to stabilize the ac changes in the
> DC supply. Does that mean I can't use polarised caps ?
>

in this case, they're not there for that reason.
in the old school, you used to have to rectify AC, then use capacitors to
smooth it into DC, which is what I think you're referring to.

Now the 7805 will take a 9V DC signal and make it 5V DC, so there's no need
to smooth the signal. Check the data sheet for your 7805 as they can often
run from 6V to 18V for input, but not all. But note, it takes 9V *DC* and
regulates it to 5V DC. i.e. if you have 9V AC going in, this just won't
work for you :)

The purpose of the capacitors here is to get rid of any high frequency
noise.
Consider;
every time a transistor switches on, it pulls a bit of current, it takes
the power supply a split second to recover from the dip and provide a bit
more current. when it switches off, the power supply has to adjust again.

Now, stuff in a micro with 1000's of transistors all switching on and off
at different times, and your nice clean 5V rail is suddenly full of junk.
these capacitors are there to minimise the amount of Junk getting into the
regulator (c1) and any residual on the output or reflected noise from the
circuit (C2).

Bigger capacitors work at lower frequencies, so the bigger the capacitor
the more low frequency noise it will get rid of.
Bigger capacitors are bigger physically and to keep the size down, they're
often made "polarised" this helps keeps the physical size small. you can
get non polar 100uF capacitors, but they massive and expensive.

Paul
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