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.
That is your mistake.
The positive terminal is connected to the board supply voltage (12V)
Although the negative terminal won't work either,
Ok i understand, So how can i fix it, what need to be done ?
Looking into it but you will probably need another transistor to drive the IRLZ44
This will make it work but your RAMPS board must only be powered by 12V, NOT 24V
Also, I not sure if what you did damaged the IRLZ44N and/or the RAMPS board.
What is the resistance of the 120cm Nichrome wire?
I would estimate about 5ohm.
About 0.5kW at 48V 
You probably need beefier power supply and mosfet.
You can follow @jim-p's schematic and remove the transistor and 12V supply. Connect the port pin directly to the junction of R1 and R2. Eliminating the transistor will prevent around 1.5mA from flowing into the input protection diode. I don’t see any issue with this, though some people might disagree.
If the circuit switches too slowly, you can insert an HC gate between the port pin and the MOSFET gate(s). In that case, move R1 to the port pin to ensure it stays off during reset and setup.
Make sure to attach a reasonably sized heatsink. If you want to add another MOSFET in parallel, place them on the same heatsink, and ensure each has its own gate resistor.
Thank you @jim-p the Ramps is definitely power by 12v power supply, and the output voltage of D8 is 12v, ill replace the Mosfet and give this a try, ill keep you updated
Hi @gilshultz thank you for your reply, but i can't remove the 12v supply its the D8 terminals of the ramps that will control the heating of the Nichrome
To me it means you need beefier power supply as well. Or higher resistance. Or lower voltage...
48V / 5Ω = 9.6A. And you have a 5A power supply?
Yes i've made a mistake sorry about that 
, i've deleted the comment
No problem!
An IRLZ44 has an Rds(on) of 0.028 ohms (quoted at 5v). This means it would be dissipating about 2.6 watts and would really need a heat sink.
No hurry
That's incorrect. You will need to redesign your system. Operating power supplies at their limits is not a good practice, and overloading them, as you’re planning, can be dangerous.
I agree,
but I'm not the OP 
Sorry wrong button.