MOSFET Overheating and Nichrome Wire Always On in 48V Setup with RAMPS 1.4

[omar587]

I'm using a RAMPS 1.4 board and thermistor to control the heating of 120 cm of 23-gauge N80 nichrome wire with a 48V, 5A power supply. The nichrome wire is meant to heat up when commanded via the D8 terminal on the RAMPS board, which normally drives a 12V heated bed.
I am using an IRLZ44N MOSFET to switch the 48V supply to the nichrome wire. The D8 positive terminal is connected to the Gate of the MOSFET through a 100Ω 1 wat resistor, and there's a 10kΩ pull-down resistor between the Gate and Source. The Drain of the MOSFET is connected to one end of the nichrome wire, and the other end of the wire is connected to the positive (V+) terminal of the 48V power supply. The Source of the MOSFET is tied to the negative (V-) terminal of the 48V power supply and the negative terminal of D8.
The problem is that when I power on the system, the nichrome wire heats up immediately—even when the RAMPS board is off—and the MOSFET gets very hot and smokes. I’m trying to figure out why the nichrome wire heats up without a signal from D8 and why the MOSFET is overheating. Any advice on resolving this issue would be appreciated.

 

[danadak]

Post a schematic to verify your system interconnect to mosfet.

If D8 is floating, the 10K in a electrically noisy environment may
permit mosfet turn on.

MOSFETS as they get hot Idrain drops, typically. Unless drive
circuit also sensitive to T and starts driving harder, eg. more Vgs
drive.

Regards, Dana.

 

[Bluejets]

1 watt level resistor unnecessary (1/4w would be normal selection) as mosfets are voltage operated, however........

As requested complete circuit diagram including power supplies and all connections. ( Not fritzing)