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IO Shield: Did I burn my open drain FET?

Created Mon, 18 Jun 2012 14:45:10 +0000 by Tareq


Mon, 18 Jun 2012 14:45:10 +0000

I just found out why was all my effort to run a stepper motor with the Basic [color=#0000FF]I/O[/color] shield in vain..

The reason I found is when I tested the output signals of the connectors on [color=#400000]J7[/color]&[color=#400000]J8[/color] the potential between the positive([color=#FF0000]+[/color]) and pin 5 is the same as the negative ground(12V), while it was very different with the other pins 3,9&6 with values varying around 2V.

The resistance is only 2ohm across Q2 (horizontally reads"C85") in the upper half of it which goes to pin5. the others read 750ohm. I did wire the supply's ([color=#FF0000]+[/color]) and ([color=#0040FF]-[/color]) opposite a couple of times <<---YOU DID WHAT!?- and I am not sure wither it is the reason i got that transistor burned. I don't even dare to ask "can i get it fixed?" I really dear this shield the most and I can see that [color=#008000]Digilent[/color] put their hearts and soul in it. but the stupid positioning of color=#BF00BF(+)[/color] on [color=#400000]J6[/color] made me reverse the polarity many times >_<'.. It should be changed!
By the way, are the Q1 a,b and Q2 a,b the open drain FETs or the other small D5-D8 ,that read (14) are?? Thank you very much.


Thu, 28 Jun 2012 21:42:33 +0000

To check if one of the FETs is shorted, I would measure the resistance between the pins on the screw terminal connectors (J7 or J8) and ground (Pin 2 of J6). If you read a low value of resistance, then that transistor is damaged.

Reversing the polarity of the power supply connection on J6 could definitely damage the transistors. This will forward bias the internal diode in the FET and excessive current can then flow through the transistor. Sorry that you don't like my choice about which pin to use for + and which for -.

Q1 and Q2 are the transistors. They are On Semiconductor NTHD4508NT1G dual n-channel FETs. In theory, a damaged transistor can be replaced, but they are fine pitched, surface-mount devices and rework on these kinds of device generally requires a pretty skilled technician. I know that I can't do it myself.

The diodes D5-D8 are there to clamp the high voltages that can result from rapdily switching off inductive loads (such as motor coils). Without these diodes, the resulting voltage spikes would easily destroy the transistors.

Gene Apperson Digilent