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Old 12-24-2016, 12:42 AM #33
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Default Re: Did I kill my driver?

A diode to protect against reverse polarity comes at signficant performance cost. The part itself is only a few cents, but it drops half a volt if it is a skottky diode, probably a full volt if it's an ordinary diode.

Half a volt may not sound that bad, but if your batteries run at 4 volts nominal it still wastes 15% of energy just for that protection.

There are considerably better ways to do this, like using mosfets to protect against reverse polarity. These are a bit more expensive and take up some more board space (at least 1 mosfet and 1 resistor), and still waste a few percent of power.

Another method is to use a fuse and 'crowbar' diode, which shorts reverse polarity so the fuse blows. This is only practical if your circuit already has a fuse for some other reason though, otherwise it's way too large for use in pointers.

So yes, there are good reasons to design drivers such that they get destroyed with reverse polarity, which have nothing to do with component cost.


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Old 01-04-2017, 04:07 PM #34
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Default Re: Did I kill my driver?

Thank you for the information, and good ideas Benm! While I have a few power diodes laying around that can certainly do the trick, they get hot regardless. I took your advice and picked up a nice little p-channel mosfet. Its forward resistance is almost zero, it's voltage drop is close enough to 0 that it won't be noticeable, and the allowable gate voltage is much higher than the batteries that I am using, so I was able to solder the ground directly to the gate. It works great, and handles 20 amps without even getting warm. The cost was $.89, so more than a diode, but it's not contributing to the heat load, so well worth it. (Of course, I bought another $20 in parts from Digikey while I was at it...) Thanks again!

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Old 01-05-2017, 01:24 AM #35
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Default Re: Did I kill my driver?

Glad to hear you were able to make use if this approach!

It's not that common to find it on battery operated products, as there usually is some physical way that avoids batteries inserted in reverse making elecrical contact. This mostly applies to button-top batteries though, like your typical AA/AAA consumer alkaline.

Using the p-channel mosfet is ideal if you have the space for it - losses are very low compared to series diodes and protection is good against randomly mis-inserted batteries (they will not do that good a job at high reverse voltages, but that doesn't randomly happen often).

Anything that can use flat-top lithium cells and such should really have reverse protection as it's not that hard to accidentily insert the batteries the wrong way around.
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