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Thread: Driving ATtiny11 with 9v battery
www.piclist.com/techref/power/batterys.htm?key=battery
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face BY : Russell McMahon email (remove spam text)



> > Using Digikey 100 pricing,
> >
> > BC337    $0.14
> > Zener       $0.04
> > Resistor   $0.02 say

> Found Fairchild LM78L05 for 10 cents at Arrow. I think, I'll use that.

Certainly the easiest solution.
You can also probably find suitable transistors and zeners at eg Arrow for
equivalently less as well for a total cost of maybe 5 cents in parts - at
which point it becomes not worthwhile ;-)

The 78L05 has far better regulation than a zener based arrangement but is
still inferior in a few ways. These probably don't bother you in this
application but are worth noting in passing.

Quiescent current is 3 mA typical and 5 mA worst case. The simple transistor
with zener arrangement someone suggested can have a quiescent current
designed around your maximum current drain and actual transistor beta and
will usually be lower than the 78L05 for many applications. But this is
probably not too important here.

Worse though, the dropout voltage is over 1.5v at 25C at 1 mA and more like
1.8v at 25C at 70 mA (it gets worse at lower tempeatures). This means the
battery voltage can not be less than about 6.6 volts. The 1 transistor
regulator does better in this respect. Again though, it's not the end of the
world in such a simple application if you can't use the tail end of the
battery power without regulator dropout.

I'd say the 78L05 at $US0.10 is a good compromise. To put the quiescent
current in perspective - using an Alkaline 'PP3' 9V transistor battery with
no laod at all you'd get about 100 to 150 hours of battery life with the
regulator alone as load.



       Russell McMahon






{Quote hidden}

regulator.
> >
> > Note that Digikey prices for 78L05 in 100 quantity are as low as $US0.30
> > OR you can get the superior 7805 for the same money!
> > But, then you'd have to use them :-)
>
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