Nmos transistor problem

[Jad]

Good morning; I want to do a simple relay controlled by my nucleo board; so I did a simple circuit with the 

STP55NF06L; however, it does not work as expeted, in real ife, even when I am not sending voltage to the gate, the transistor conducts electricity, however in the simulation, it works with a similar Transistor, i want to know if this transistor is correct or if I should use another one

 

[Peter BENSCH]

In your case, better than figure 3 of the data sheet is figure 4: Transfer characteristics. There you can see that significant current can flow from a gate voltage of approx. 2V. Table 3 also states the lower limit of the threshold voltage VGS(th) of 1V, above which drain current must be expected.

Although you have not described exactly what you mean by ‘conducts electricity’, as such a MOSFET is not infinitely high-impedance when switched off. The same table 3 mentions, for example, an IDSS of up to 10µA, which would already cause a drop of 1V at the 100kohms in your simulation.

In general, the STP55NF06L should work with a NUCLEO if you set the GPIO to push-pull and connect a real pull-down like the R13 in your simulation. An internal pull-down does not make sense to use in this case, as it can only be active when the MCU is powered. A physically present external pull-down, on the other hand, can pull the gate to GND even when the MCU is idle or switched off. Depending on the application, it may even make sense to provide a TVS between the gate and source to limit possible ESD voltages.

However, pay attention to what you want to drive with the MOSFET and, if necessary, provide protective components (keyword: freewheeling diode for inductive loads).

Regards
/Peter