<blockquote>
  <p>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.</p>
</blockquote>

<p>The MOSFET is damaged. If the gate is at <span class="math-container">$0V$</span>, no current should be able to flow.</p>

<p>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 <span class="math-container">$12V$</span> 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 <span class="math-container">$+12V$</span>.</p>

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

CC BY-SA 4.0
2018-12-20T16:01:03.330Z
2018-12-22T15:08:29.497Z
2018-12-22T15:08:29.497Z
Tom van der Zanden
<p>I have a 40 W, 12 V resistance heater in the hotend. It also has a 100 k&Omega; thermistor built in. I am using an Arduino to read the temperature on the LCD. This is working fine.</p>

<p>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.</p>

<p>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.</p>

<p>I am not able to control the current to the resistance heater.</p>

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

CC BY-SA 4.0
2018-12-20T11:24:52.727Z
please add a wiring diagram
Trish
CC BY-SA 4.0
2018-12-20T16:01:43.547Z
As a side note, 12V at 3A is only 36W. Using that with a 40W heater is asking for trouble.
Tom van der Zanden
<pla><diy-3d-printer><hotend><fdm>
Trying to control temperature of hotend using PWM signal and MOSFET
user13895
# Trying to control temperature of hotend using PWM signal and MOSFET

(Asked by: username_0 on 2018-12-20)

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.

**Tags:** pla, diy-3d-printer, hotend, fdm
> **Comments:**
> **username_1**: please add a wiring diagram  
> **username_2**: As a side note, 12V at 3A is only 36W. Using that with a 40W heater is asking for trouble.  

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## 1 answers

### Answer by username_2 on 2018-12-20. Score: 2

> 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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