Go Down

Topic: Reverse polarity protection on the MEGA ADK (where's the fuse?) (Read 481 times) previous topic - next topic

boggydew

Hi,

Can anyone explain the logic behind the reverse polarity protection circuit for the MEGA ADK.  It appears to be missing a fuse.  A reversed supply of greater than 1 amp will destroy the diode and then the voltage regulator in quick succession.

Moreover, even if the diode survives, the voltage across it will be between 0.6 and 1.1v (depending on the current),  exceeding the absolute maximum negative Vin for the LM2736 which is -0.5V.


Wawa

The Mega 2560 has a protection diode in series with + of the DC jack. No fuse needed.
The Mega ADK seems to have that diode between + to ground.
Seen many things with a protection like that completely blown and charred.
Best to never rely on that sort of reverse protection aye.
Leo..

DrAzzy

The correct wiring is clearly to have the diode in series with the positive side of the supply....

The crow-bar solution to reverse polarity doesn't seem to make much sense, as the supply will generally be able to put out enough current to blow the poor diode to kingdom come, after which it can freely destroy the rest of the board at it's liesure. Is it really wired with the diode like that? x_x
ATtiny core for 841+1634+828 and x313/x4/x5/x61/x7/x8 series Board Manager:
http://drazzy.com/package_drazzy.com_index.json
ATtiny breakouts (some assembled), mosfets and awesome prototyping board in my store http://tindie.com/stores/DrAzzy

boggydew

Yes it is really wired like that.

I downloaded the eagle files just to confirm the schematic and the copper doesn't lie.

https://www.arduino.cc/en/Main/ArduinoBoardMegaADK?from=Main.ArduinoBoardADK

https://www.arduino.cc/en/uploads/Main/ADK_MEGA_2560-Rev2-sch.pdf

It's quite bizarre. What are the chances of a comment from the board designer?

Cheers.

Grumpy_Mike

Quote
What are the chances of a comment from the board designer?
Zero point zero zero.

My guess is the thinking is that the diode where they put it does not limit the current into the Arduino normally and does not drop any voltage either. The "protection" comes from the power supply you plug into it going into current limit. Also it is not that easy to blow a 1A diode, with a 1A load it will take some time over heating. Anyway that is my guess, I am not saying I agree with it.

Go Up