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I have this circuit built (updated to remove short-circuit):

Circuit

I'm experiencing some weird behavior though that I don't understand.
1. When I close the switch, nothing happens. No current flows through the MOSFET, according to the Ammeter.
2. If I disconnect the ground of the Arduino from the negative terminal of the power supply, current flows through the MOSFET, even when the switch isn't closed!

Have I hooked this up incorrectly? Why is the MOSFET letting current through? Any ideas?

Note: Data Sheet for MOSFET
Note 2: When I use this exact setup but replace the 12V power supply with a battery, and the component with a resistor and an LED, it works fine.

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    \$\begingroup\$ It would help if you conformed to standard schematic drawing conventions. Signal flow from left to right and higher voltage on top and lower voltages low on the page. If you did that you'd quite easily see see that your NMOS is in backwards. \$\endgroup\$ – placeholder Oct 10 '12 at 0:17
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For starters, you seem to be shorting the "Arduino Digital Output" to ground whenever you close the switch (push the pushbutton).

Secondly, the n-mos is not connected properly, it should be placed with its source to ground and its drain to the load. The ammeter should also be on the high side, not the low side, because it interferes with the Vgs being applied.

Thirdly (some may put this as first in priority), when you put a voltage source in your schematic, don't place it upside down because it obfuscates the schematic (I wish the "rotate" tool in schematic entry programs was disabled in such cases).

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    \$\begingroup\$ So is this what it should look like? \$\endgroup\$ – Mason Oct 10 '12 at 1:06
  • \$\begingroup\$ @Mason That looks much more reasonable. You should also have a 10k resistor between the MOSFET gate and source to ensure it's off when the Arduino output is high-impedance (i.e. when it's not powered). \$\endgroup\$ – Adam Lawrence Oct 10 '12 at 1:24
  • \$\begingroup\$ @Mason that drawing is a lot clearer. Why don't you update the main drawing? I will note that you have a line drawn across the power supply. \$\endgroup\$ – placeholder Oct 10 '12 at 1:35
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The gate needs a 10k resister to ground, this is why it won't work...Your input to the gate is grounded out.

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The MOSFET conducts because there's an intrinsic diode between source and drain. Since you're using a N-channel device, your circuit is forward-biasing it, so it conducts. The only time it's not "there" is if the MOSFET is biased on and it's shunted by the conductive channel (i.e. current will flow through the channel instead of the diode).

Look closely at page 1 of your datasheet and you'll see the diode in the schematic symbol.

Look closely at page 2 of your datasheet and you'll see a section entitled "Drain-Source Diode Characteristics and Maximum Ratings".

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