Motorshield V2 protection fet Q1 AOD417 reversed?

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rhormigo
 
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Motorshield V2 protection fet Q1 AOD417 reversed?

Post by rhormigo »

Is anywhere a good github link that shows up to date files and history on the Motorshield V2 to V2.3? I checked the link in the product webpage but the squematic files don't open correctly in the current Eagle. Looking to the PCBs and v2 that I can open, it suggests Q1 drain and source are reversed, so looks like this polarity protection doesn't work? I have one of the boards that has been around some time in my school lab where Q1 is in fact reversed. The PCB file V2.3 I can open appear to have this corrected, but the boards you are offering and squematics V2.2 in the web still show this reversed. In fact I carefully experimented to reverse polarity in power terminals from 0 to -1V (with a 100ma limit) observing as expected how the negative still goes across the diode in the fet in the board to about -0.7V where current starts ramping out of control as usually happen in these inversions.

At this point I don't know what I will get if I order now, and confuse how some like this has been undetected/corrected for long time. We are looking to buy a bunch for a project, but want to make sure we get the right thing.

Thank you,
Rick

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adafruit_support_mike
 
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Re: Motorshield V2 protection fet Q1 AOD417 reversed?

Post by adafruit_support_mike »

That's deliberate.

A mosfet's internal protection diode is reverse-biased when power is applied normally (drain positive to the source for an N-mosfet, source positive to the drain for a P-mosfet). They become forward biased when power is applied backwards.

FETs are inherently symmetric, so the drain and source can always be swapped. If you connect a P-posfet's drain to VCC and its gate to GND, the diode will conduct when power is connected correctly. That provides enough electrical connection to bias the gate at 0V, turning the mosfet on completely. At that point the mosfet shorts the internal diode so there's practically no voltage drop or power loss, as you'd get with a conventional polarity protection diode.

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rhormigo
 
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Re: Motorshield V2 protection fet Q1 AOD417 reversed?

Post by rhormigo »

I wasn't discussing how a fet works or if the shield doesn't work. It works! But the fet in that circuit is in a configuration of common gate to protect against polarity inversion (that happens often working with students), and I think the circuit in V2 doesn't work as intended, because with the source (and so cathode of the intrinsic body diode) connected to the power source will be forward to negative voltage. Sure the P fet it will conduct nicely with positive as the gate is under, but with negative voltage the diode will conduct under its forward voltage, as I experimented. More on this: https://components101.com/articles/desi ... protection

But otherwise, why then you have fixed this in the PCB V2.3 linked in your web here: https://github.com/adafruit/Adafruit-Mo ... eld-V2-PCB

My question remains, what is shipped these days PCB V2 or fixed PCB V2.3? or even more confusing, some other V2.2 that still looks wrong according to this schematic: https://cdn-learn.adafruit.com/assets/a ... 1396892649

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