piclist 2004\07\01\094355a >
Thread: Driving ATtiny11 with 9v battery
www.piclist.com/techref/power/batterys.htm?key=battery
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face BY : David VanHorn email (remove spam text)



At 09:26 AM 7/1/2004 -0400, Mr MCU wrote:

>OK, one last time on this one ... is this a perfect world? ... no, but
>for the application it will work very nicely.   Everything is a
>compromise.  I don't pretend to be the best engineer, but I am one of
>the best designers.
>
>I was incorrect in stating NO power loss, what I should have said is
>that the current draw is ABOUT the same on both sides of the zener.

??? Kirchoff's law, the current everywhere in a series circuit is exactly the same.
(DC circuits, AC is different.)

>e.g. if the micro is using 1ma (when the LED is off) it will draw APPROX
>1ma from the 9V battery.  When the LED is on, (let say you want to use a
>cheap one and be somewhat bright so you set it up to draws 6Ma) then 7Ma
>(1ma + 6ma) is drawn from the battery.  If you put the micro into sleep,
>maybe it draws 100uA or so about from the battery.  Try that with your
>shunt regulator!  How much current would the shunt use from the 9V in
>all three of these conditions?

Of course the shunt zener actually regulates the voltage, where the series does not.
Vout = Vin-Vz, and the series zener wastes nearly as much power as a 7805-ish solution would, the difference being only the quiescent current of the regulator.

If you're going to have a significant load, then a 7805-ish linear would be a much better choice.


>As for voltage regulation, as an LED blinker, there will not be any
>significant difference if the micro sees 3.5 volts or 5, other than the
>brightness of the LED and possibly the speed of the blinking.  If they
>were controlled via the touch switch - no one would notice.

Clock speed varies with voltage, when using the internal RC osc.

>Sure a low Q linear regulator or a switcher would work, but at a much
>greater expense and NOT needed for THIS project.

Where was "THIS" project clearly defined?



> How is the series zener any more 'complicated' that the shunt?

It isn't, it just doesn't offer any significant advantage, and it has significant disadvantages. If you're so concerned over cost, then do an emitter follower with a 2N3904 on the shunt, and get moderate current output, and reverse polarity protection for another penny.

>As far as getting a newbie into trouble, I assume that the kit comes
>with instructions AND a description of operation.  If he gets into
>trouble, its because the author did a bad job of writing or the newbie
>didn't read it!

Students frequently make mistakes.
What <I> would do, is not worry about minimum power consumption, and instead design for a robust supply, and protect the micro from typical student mistakes, like inverted supply polarity, or disconnecting the ground before Vin.

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