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I have a 40 W, 12 V resistance heater in the hotend. It also has a 100 kΩ thermistor built in. I am using an Arduino to read the temperature on the LCD. This is working fine.

I want to use this temperature using a MOSFET and the PWM signal that is generated by the ARDUINO. So I connected the PWM signal to the Gate of the MOSFET and connected the 12 V, 3 A supply to the drain of the MOSFET. And connected the Resistance heater of the 3D printer to the source and ground.

But the problem that I am facing is that the Current is flowing from drain and the source even without the gate signal to the MOSFET.

I am not able to control the current to the resistance heater.

Mosfet: IRFZ44N and the AC to DC adapter is 12 V, 3 A. PWM signal is max 2 V.

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  • $\begingroup$ please add a wiring diagram $\endgroup$
    – Trish
    Dec 20, 2018 at 11:24
  • $\begingroup$ As a side note, 12V at 3A is only 36W. Using that with a 40W heater is asking for trouble. $\endgroup$
    – Tom van der Zanden
    Dec 20, 2018 at 16:01

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connected the 12 V, 3 A supply to the drain of the MOSFET. And connected the Resistance heater of the 3D printer to the source and ground.

The MOSFET is damaged. If the gate is at $0V$, no current should be able to flow.

You're trying to use an N-channel MOSFET as a high-side switch. This is a bad idea, because it would require more than $12V$ at the gate to turn the MOSFET on properly. It would be better to set this up as a low-side switch so that the microcontroller can drive the gate directly. The MOSFET should go between the load and ground, not between the load and $+12V$.

The fact that you connected it like this is the very reason the MOSFET has been damaged. When the MOSFET is OFF, there is $0V$ across the heater. When the microcontroller starts to turn the MOSFET ON (by applying $5V$ to the gate, thus increasing $V_{GS}$ to above $V_{th}$), current starts to flow through the load and the voltage across the load increases. This in turn decreases $V_{GS}$, causing the MOSFET to turn off. You will end up with approximately $3V$ across the load and $V_{GS}=2V$. You've got $9V$ being dropped across your MOSFET and with a current of approximately $800mA$, the $7W$ of heat produced will quickly kill the MOSFET.

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