I've looked all over and remember something about adding a resistor somewhere
to drain the mosfet so it shuts off.

Right now i have 3.5v going to the gate. It turns on when 3.5v is applied, but not turning off when the 3.5v drops. The Red LED just remains lit.

Any help would be highly appreciated

 

Add a 10K to 100K resistor from Gate to Source.

Add a resistor in series with the LED, else you will blow it up (unless it is a 12V indicator lamp assembly with an internal resistor).

According to the IRFZ40 data sheet, 3.5V is marginal to turn on this NFET. It will work for a light load, but not for a load that draws more than a 100mA.

 

Thank You,

I have a 4pk new package of NTE 1/4W 560 OHM 2% resistors. Would those work?

 

Thank You,

I have a 4pk new package of NTE 1/4W 560 OHM 2% resistors. Would those work?

Yes. The only downside is how much current you will have source from whatever drives the Gate?

 

I've looked all over and remember something about adding a resistor somewhere
to drain the mosfet so it shuts off.

Right now i have 3.5v going to the gate. It turns on when 3.5v is applied, but not turning off when the 3.5v drops. The Red LED just remains lit.

Any help would be highlyView attachment 78009 appreciated

Short circuit gate to source - if the MOSFET doesn't turn off, its toast. (You have got it right way round for the channel polarity haven't you?!).

If the MOSFET with G/S shorted turns off, replace the short with a resistor, anout 2k2 - 10k should do.

The MOSFET gate has capacitance - if it doesn't have a bleed resistor, you can put a static charge on the gate that will keep the MOSFET conducting for hours.

 

I suggest you watch these tutorials, they explain quite clearly how mosfets work in real applications