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I've connected the + of a 12v powersupply to a 12v LED-spot. The - of the LED-spot is connected to the source of a IRL540N mosfet. The drain is connect to the - of the 12v power supply and the ground of a arduino uno. The gate of the mosfet is connected to the 5v regulated output of the arduino.

If I connect the LED-spot directly to the 12v power supply, I measure 260mA.

The mosfet is getting very hot in notime. I've bought this mosfet as it should handle a gate voltage of 5v better. As I read the documentation of the mosfet the resistance of the IRL540N should be 0.05.

In this setup, if I disconnect the LED spot and measure the resistance between source and drain I measure 0.5 Ohm.

Do I have a problem in my setup, or should I use a different mosfet? I've read many posts on this subject on this website already but my background knowledge is too limited to pinpoint the problem.

My question is: Am I using the correct mosfet? I don't understand why the mosfet is getting very hot

enter image description here

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    \$\begingroup\$ Show us your schematic. The textual description says you connected it wrong way round. What did you really do? \$\endgroup\$ – Brian Drummond Jul 7 '17 at 14:03
  • \$\begingroup\$ I suspect you're using your transistor backwards, so all the current is flowing through the body diode instead of the channel. Are you able to control your LED by switching an Arduino pin from High to Low? If your LED is always on regardless of the gate level then this is the case. \$\endgroup\$ – Volodymyr Smotesko Jul 7 '17 at 14:04
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    \$\begingroup\$ Instead of posting "pictures of stuff with wires between" please try to post a simple schematic. There's an on-site schematic tool here even. \$\endgroup\$ – Lundin Jul 7 '17 at 14:11
  • \$\begingroup\$ actually, I think this photo in place of an actual schematic qualifiest this question for the "unclear what you're asking" close reason :( \$\endgroup\$ – Marcus Müller Jul 7 '17 at 14:29
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    \$\begingroup\$ Most meaningful measurement is missing. \$V_{DS}\$ when it is supposed to be on with the load connected. \$\endgroup\$ – Trevor_G Jul 7 '17 at 14:44
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You seem to have the source and drain terminals swapped over from your description (But not from the picture of the breadboard), the source should go to power supply negative, the drain the negative lead from the lamp.

Measure the actual gate voltage (Between gate and source), could it be 3.3V and not 5V? That mosfet will be unpleasantly resistive with only 3.3V on the gate.

Are you by any chance trying PWM dimming? That mosfet has a huge miller capacitance (All big vertical power fets do), and your gate stopper might be slowing the rise time so much as to cause the thing to spend significant time in the linear region if run above a few kHz.

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Do I have a problem in my setup, or should I use a different mosfet?

The IRL540N is an N channel MOSFET - take note

I've connected the + of a 12v powersupply to a 12v LED-spot. The - of the LED-spot is connected to the source of a IRL540N mosfet. The drain is connect to the - of the 12v power supply and the ground of a arduino uno.

You've connected the MOSFET the wrong way round - drain goes to the LED- and source goes to ground. If it were a P channel MOSFET then it would be correct but then your gate connection would be probably wrong.

I've bought this mosfet as it should handle a gate voltage of 5v better. As I read the documentation of the mosfet the resistance of the IRL540N should be 0.05.

Yes it should be OK if connected correctly.

I don't understand why the mosfet is getting very hot

Because it's connected the wrong way round and only partially conducting through the body diode (possibly ~250 mW being produced).

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I agree with the previous two answers provided. Ntype MOSFETs should have the following polarity configuration. Vg: (+), Vdrain (+), Vsource(-). The Electronics Companion by Fischer-Cripps is a particularly good resource for gaining insight into active electronic components. Here is a helpful photo of a BJT (similar principle for MOSFET). enter image description here

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