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Motorshield V2 protection fet Q1 AOD417
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Motorshield V2 protection fet Q1 AOD417

by dave_t on Fri Jun 17, 2022 2:24 am

I have been developing an sketch on an Arduino Mega 2560 rev3 with an Adafruit Motor Shield v2.3 to drive a single Adafruit XY42STH34-0354A stepper motor (with 35 Ohm coils) via the shield’s M1 and M2 connections.
So far this has been powered from a Rigol DP832 bench power supply set to 12 volts, and a current limit of 1 Amp. The Mega is being powered from the 12v supply, with the jumper inserted, connecting net +12V to net Vin.
A while ago, the PSU was inadvertently connected with reverse polarity, during which time the PSU was delivering (from memory) about 100mV at 1 Amp for a few minutes.
After correcting the polarity, the motor shield was drawing 285mA with the Mega attached, or 182mA when the shield is unplugged from the Mega. The reverse polarity protection MOSFET Q1 (a AOD417) was running very hot, up to 80 degC after being powered up for 22 minutes. The observed temperature rise far exceeds that expected due to the calculated max of about 5mW dissipation due to Drain-source current flow (Id) multiplied by the drain-source voltage (Vds(on)), measured at about 9mV.
Later I changed the reverse polarity protection scheme to a series SR540 schottky diode in the positive supply line, and bridged out the Drain to Source of the MOSFET. The PSU current draw was unchanged from before, with the shield alone drawing 182mA at 12V.
I then unsoldered the Gate of Q1, removing it from the Ground. This reduced to shield’s current draw to 10mA, or 112mA when attached to the Mega, and the MOSFET was, as expected, cool. I temporarily reconnected the gate to ground, measuring the drain current with a multimeter as 167mA (not what MOSFETs are supposed to do), this caused the PSU to rise from 112mA to 274mA (with the Mega connected).
This conclusively proves that the gate of Q1 was damaged (presumably by the previous reverse polarity incident). At no time has a voltage above 12V DC been applied, so the Q1 should have survived, as its absolute maximum gate-source voltage is quoted in the datasheet as +- 20V, and the drain to source volts as -30V, neither of which were exceeded.
I conclude that this example of the motor shield was made with an out-of-spec AOD417 as Q1. You may be advised to check your production stock of these devices.

dave_t
 
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Re: Motorshield V2 protection fet Q1 AOD417

by adafruit_support_mike on Fri Jun 17, 2022 7:11 pm

Hmm.. was the 100mV @ 1A limited by the power supply’s settings, or a measurement of the actual output independent of the settings?

If the meter was still set to 12V @ 1A, that could easily kill the mosfet’s body protection diode. From there, it would be reasonable to see maybe 100mV across the shorted connection.

adafruit_support_mike
 
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Re: Motorshield V2 protection fet Q1 AOD417

by dave_t on Sat Jun 18, 2022 2:45 am

The PSU was limiting at 1A, causing the volts to drop to about 100mV.
I am not seeking a replacement or refund as I have built circuitry on the prototyping area, I am happy with my alternative reverse protection, and you have no stock.

dave_t
 
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Re: Motorshield V2 protection fet Q1 AOD417

by dave_t on Sat Jun 18, 2022 9:44 pm

As a clarification, during the reverse polarity incident, no external meter was used. The quoted volts and amps were read from the display on the bench PSU. The only use of a multimeter was the later measurement of Vds across Q1.

dave_t
 
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