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Hello,

I've been pondering this for a while.  In the last according to pete from sparkfun, he mentioned something called backwards protection in regards to the battery.  Supposedly it would protect the circuit if you put the battery in backwards (Vcc into ground, ground into Vcc).  Exactly how would that work?

Thanks!
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Diode would be my guess, Shottky type to minimize voltage drop.
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Series protection or parallel protection. Series one does a drop in voltage (on the diode, ~0.3V with schottky, 0.7V with normal Si diode), the parallel does not, but the diode must be rated to withstand the short current (it depends on the battery type).


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IIRC, I've also seen implementations using Mosfets where the voltage drop is less than with a Schottky. More components, more complication, but if a low voltage drop is desired, it's one way to do it. But for KISS purposes, a Schottky in series seems like a good place to start.
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Ontario
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There was a recent thread on this same topic.  Look here: http://arduino.cc/forum/index.php/topic,117998.0.html

Personally, I like the "crowbar" approach that pito calls "parallel protection", but combined with a fuse which blows immediately under short circuit current.
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Arduino's get by with a simple reverse polarity protection diode.
You can see it between the barrel jack connector and the 5V regulator at the upper right of the schematic:
http://arduino.cc/en/uploads/Main/Arduino_Uno_Rev3-schematic.pdf
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Personally, I like the "crowbar" approach that pito calls "parallel protection", but combined with a fuse which blows immediately under short circuit current.
Great suggestion - no voltage drop and protection to boot. In the interest of board space and user happiness, I like using PTCs.
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PTC won't protect against reverse voltage tho. By the time they blow, the damage is done.
With schottky diode, voltage drop is only 0.3V.
If you're powering from 4 AAs, the small drop gets you down to the high side of 5.5V operating voltage (6V Absolute Max). 0.7V drop probably preferable to get down to 5.3V.
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Absolutely, as with any circuit, the specifics determine what's best.

I still use PTCs even if they may not totally protect the circuit downstream from them, as they do isolate problem circuits eventually. That in turn allows a collection of boards to fail gracefully.
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PTC won't protect against reverse voltage tho. By the time they blow, the damage is done.

The idea is that the circuit is actually protected by a fat rectifier that creates a short across the power lines if the polarity is wrong.  Then the short circuit current blows the fuse or the PTC or what have you.  The circuit itself sees only the -0.7V voltage drop of the rectifier.

When power is hooked up the right way around, there's no voltage drop at all from the protection circuit -- if it uses a fuse.
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