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Author Topic: Standalone Arduino on 6 AA: voltage drop from diode, ideas to circumvent?  (Read 2983 times)
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You don't have to use a P channel MOSFET, see diagram C in attached image.


* reverse_protection.gif (12.06 KB, 521x337 - viewed 53 times.)
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Thanks for the extra ideas Cookies.  One question though : what does it mean (in the description for the N-channel option) by
Quote
If low-side switching will work
What would you need to check to ensure that was the case?

Geoff
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@Nick: I meant the AA battery in combination with the DC DC booster smiley-razz So it would be nice to find a N-channel mosfet which is compatible with 1, 2 or 3 AA batteries. Or do I miss out on something here ? Granted: one battery will not last endless, but suppose I would like to try smallest package possible it would be nice to know that it works

@Cookies: very elegant. What do they mean with soft start ? I understand the voltage at the gate will rise slowly, so... the fet opens slowly instead of a single switch turn on ? Why would I want this, is it more pleasurable for my IC ?
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Thanks for the extra ideas Cookies.  One question though : what does it mean (in the description for the N-channel option) by
Quote
If low-side switching will work
What would you need to check to ensure that was the case?

I can't really think of a situation where low-side switching wouldn't work.

@Cookies: very elegant. What do they mean with soft start ? I understand the voltage at the gate will rise slowly, so... the fet opens slowly instead of a single switch turn on ? Why would I want this, is it more pleasurable for my IC ?

Soft start might used to limit inrush current to capacitors and maybe to reduce popping with audio amplifiers, for microcontroller stuff you want the power to turn on and stabilize as fast as possible, so don't use soft start.
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Just wire a jumper across the diode, and double check polarity when connecting
an external power source. Diodes are for babies [just kidding ;-)], but once you
wire up a power source with the correct polarity, you can't plug it in backwards,
after all.
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...but once you wire up a power source with the correct polarity, you can't plug it in backwards, after all.
I truly wish that were the case in my experience too smiley
I can't really think of a situation where low-side switching wouldn't work.
Excellent.  I was concerned there might be occasions where you shouldn't use it but now you've said that, I've got far more N-channels in the kit and more options for small/cheap N's to by the looks of things too.

Thanks for the clarification, Geoff
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...but once you wire up a power source with the correct polarity, you can't plug it in backwards, after all.
I truly wish that were the case in my experience too smiley
I'm assuming you're using the Power jack, and not hacking into the Vin header pin,
which is 50% gotcha, and 50% blew my Mega board to kingdom come.
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I'm assuming you're using the Power jack, and not hacking into the Vin header pin,
which is 50% gotcha, and 50% blew my Mega board to kingdom come.
Actually I've only trapped a real Arduino in two projects.  I normally prototype on Arduino then flesh out a board around either ATMega or ATTiny for the final build.  Plenty of opportunities to smoke stuff, all part of the fun smiley
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Actually, all atmega's are soldered to veroboard and - for the moment - powered by 6 AA batteries. The work is done by students learning electronics. Mistakes will be made - the entire idea is learning to build and debug something - so the board goes through several debugging cycles, meaning connecting and disconnecting the power source many times. Reverse polarity protection has already shown a good choice on several occasions smiley-wink
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Actually, all atmega's are soldered to veroboard and - for the moment - powered by 6 AA batteries. The work is done by students learning electronics. Mistakes will be made - the entire idea is learning to build and debug something - so the board goes through several debugging cycles, meaning connecting and disconnecting the power source many times. Reverse polarity protection has already shown a good choice on several occasions smiley-wink
Yeah, with students, you get all of Murphy's gotchas, and a few he never thought of
too. We see that around here everyday with noobees.
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Hi again

Has anyone got a pet MOSFET that they use for these lower voltage reverse polarity protection challenges?  Reason for asking is when I started to look in the parts I have on hand, my logic level MOSFETs that might be useful for protecting reverse polarity on a 5V system are either more expensive than the rest of the Bill of Materials for the project combined or, as in the case of the 2N7000 N-channel which are under 10c each and I have heaps of...actually appear to have a unreasonably high resistance, therefore a power loss which can be as bad as a diode.

The 2N7000 has a RDS(on) at VGS = 5 V, ID = 100 mA of 6 Ohms max.  By my maths (which I'm hoping is flawed) that's a forward voltage of 0.6V.  That doesn't appear to be much of a gain over a cheaper diode.

What MOSFET have you found for the 6V situation, Jack?

Geoff
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Has anyone got a pet MOSFET that they use for these lower voltage reverse polarity protection challenges?  Reason for asking is when I started to look in the parts I have on hand, my logic level MOSFETs that might be useful for protecting reverse polarity on a 5V system are either more expensive than the rest of the Bill of Materials for the project combined or, as in the case of the 2N7000 N-channel which are under 10c each and I have heaps of...actually appear to have a unreasonably high resistance, therefore a power loss which can be as bad as a diode.

Power mosfets in TO220 packages are relatively expensive, but if you can accept an SMD package then there are plenty of low-cost devices available, e.g. http://uk.farnell.com/nxp/nx2301p/mosfet-p-ch-20v-2a-sot23/dp/1894738.
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You don't have to use a P channel MOSFET, see diagram C in attached image.

Perhaps I'm looking at this wrong, but shouldn't the Drain and Source be reversed in figure C?



No, in C, when the power is connected correctly the MOSFET will turn on and current will flow though the MOSFET backwards from S to D, bypassing the body diode, which is fine in this case, normally if the S and D are swapped and the MOSFET is turned off the body diode will conduct. If power is connected backwards the diode will block and since the MOSFET is now off that will also block.
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