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Schematics

The above is part of my project design. 'BRAKE_CTL_3' is output pin from uController. It sends 0 or 1 to drive Q4 to turn it on or off. 'BRAKE_LSD_3' connect to one end of Relay. Other end of relay is connected to +12V. I would expect my design to turn relay on or off. However, relay is always turned on(closed). When I measured the pin output at 'BRAKE_LSD_3', the voltage switch from 0 to 3.3V instead of 0V to 12V(ideally). I dont know whats wrong with my design? Can anyone help?

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  • \$\begingroup\$ Ten bucks says the mosfet has too high a VGS for 3.3v. Either add a transistor to control the mosfet and invert logic, or replace the mosfet. \$\endgroup\$
    – Passerby
    Commented Apr 30, 2015 at 19:39
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    \$\begingroup\$ @Passerby - The Diodes Inc data sheet says that the ZXMS6004FF will be ON to handle a Drain current of 0.9A with a Vin of 3.0V at 25C as long as the device is mounted on a 15x15mm square of 1 oz copper. You can send be the ten bucks. \$\endgroup\$ Commented Apr 30, 2015 at 20:23
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    \$\begingroup\$ @Passerby Its an "IntelliFET" so it switches sharply at 3V. \$\endgroup\$
    – brhans
    Commented Apr 30, 2015 at 20:27

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Your MOSFET is connected upside down.
The Source pin shound be connected to "ground"/0V and the Drain pin should be connected to your relay/"BRAKE_LSD_3".

Remember that most MOSFETs have a "body diode" in parallel with the Drain and Source.
If you look at the symbol of your MOSFET you'll see a little arrow in the middle - that gives you a hint as to which way the diode will allow current to flow through the MOSFET no matter whether its turned on or off.

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  • \$\begingroup\$ Okay do you know why its still give me voltage change from 3.3V to 0V though \$\endgroup\$
    – ynan
    Commented Apr 30, 2015 at 20:14
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    \$\begingroup\$ Its probably related to the fact that this isn't a simple MOSFET, but one of them new-fangled "IntelliFET" thingies and there's a current path from the 'IN' terminal to the 'S' terminal - maybe through whatever they've got in there for ESD protection. \$\endgroup\$
    – brhans
    Commented Apr 30, 2015 at 20:24
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    \$\begingroup\$ @ynan - Turn the part around and be happy. \$\endgroup\$ Commented Apr 30, 2015 at 20:26

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