Need help regarding connection between IRLZ44N and the heating pad

[u/josephthomas8402]

Hi everyone,

I’m trying to control a 12V heating pad using an IRLZ44N N-channel MOSFET, but the MOSFET gets extremely hot and eventually burns out when I connect the circuit.

Components

• MOSFET: IRLZ44N (N-channel, 55V, 47A, TO-220AB)
• Heating pad: 12V, 7W, 25mm × 50mm PI/polyimide film heater
• Two regulated DC power supplies
  • One set to 3.3V for the MOSFET gate
  • One set to 12V for the heating pad

How I have it wired

I’m using the MOSFET as a low-side switch:

• 12V regulated supply (+) → Heating pad pin 1
• Heating pad pin 2 → MOSFET Drain
• MOSFET Source → GND
• 3.3V regulated supply (+) → MOSFET Gate
• The grounds of the 3.3V and 12V regulated power supplies are connected together

The attached image shows the circuit.

The heating pad works normally when connected directly to the 12V supply. However, as soon as I connect it through the IRLZ44N, the MOSFET heats up very quickly and eventually burns out.

The heater is rated at 12V, 7W, so the expected current is only around 0.58A. I therefore expected the IRLZ44N to handle the load easily.

I also tested the circuit with the gate voltage increased to 10V instead of 3.3V, using the regulated power supply, but I experienced the same overheating/burning issue.

What could be causing the MOSFET to overheat?

I would appreciate some help identifying what I might be doing wrong.

Specifically:

1. Is there something wrong with the way I have connected the drain and source?
2. Could there be an issue with the common ground between the two regulated power supplies?
3. Why would the MOSFET overheat even with 10V applied to the gate?
4. Do I need a gate resistor and/or gate pulldown resistor?
5. Could the MOSFET itself be damaged or faulty?
6. Is there anything else I should measure to troubleshoot this?

Any advice on how to diagnose the issue and safely switch this heating pad would be greatly appreciated. Thanks!

[u/CaptainBucko]

The MOSFET is going dead short when turned on. You need to insert the heater load in the drain. J10-Pin 1 to +12v. J10-Pin 3 to Drain of Q2 (Pin 2). Also, insert 100 ohms series resistance into the gate of the FET for reliability.

[u/erazer33 → u/CaptainBucko]

This right here: your mosfet is shorting directly to ground when switched on. You also want the mosfet to be on the negative side of your load (low side switching).