>> How would I get 7.2V from a 7812
>>
> You raise the voltage on the reference pin (what is often called
> the ground pin) of a 7805 by 2.2V. In your case a 2.4V Zener would
> probably do, anode to 0V, cathode to centre pin of 7805. A resistor
> goes from 7805 o/p to
> k of the zener to supply a couple of mA to the zener. You can also
> do this with 3 Si diodes to raise the o/p 1.8V-ish
>
> Or like this
>
> http://www.talkingelectronics.com/Projects/PlugPackRegulator/PlugPackR
> egulat or.html
>
>> or a lm317 regulator?
>>
> http://www.geocities.com/tomzi.geo/lm317/lm317.htm

I don't think either of these are an appropriate solution for the
OP's situation. No offense Jinx (this isn't meant personally towards
you!) but I'm always amazed at how many people on the PIClist
advocate "jury rigging" the jellybean standard voltage regulator
circuits to do something there's a better solution for or they really
weren't designed to do. Yeah, I know -- common parts in the bin and
all that... but crummy engineering and bad design decisions are still
that in a hobby project too.

In the OP's case, he was looking for 5.5W @ 7.2V from a nominal 12V
source. That's 763 mA. Using any linear regulator is going to mean
3.7W of power dissipated by the regulator at a nominal 12V input and
full rated output. This circuit screams for a buck switching
regulator. Properly designed, it will run efficient and cool and
won't be excessively noisy. It needn't be expensive either. If really
low output noise is required and simple LC post filter will take care
of that.

National, TI, Linear Technology, Micrel, and (ugh) Maxim all make
highly integrated devices that can easily handle these requirements.
Follow the application notes carefully and the circuit will work with
little fuss. Most of the typical online distributors will stock the
parts (including standard magnetics, capacitors, etc.) that will be
required. You might even be able to get a small quantity of sample
parts. Often the manufacturers have an evaluation board (especially
for SMD chips) that has DC in and DC out terminals that you could
easily incorporate into a hobby project. If not, many sources of
cheap prototype PCBs have been discussed in the PIClist many times.
The best part of this solution is that it will be efficient, won't
generate much heat and will work properly.

Matt Pobursky
Maximum Performance Systems



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