Electronic – Logic-level transistor not switching

fetlogic-level

I am trying to use a transistor to drive a lamp from a Raspberry Pi (3B+) and I went after one that can be switched by the 3.3V logic.
I found the FQP50N06L to have a max Vgs threshold of 2.5V, so it should work (right?).
Here's the circuit I am using:

schematic

simulate this circuit – Schematic created using CircuitLab

The lamp is rated 0.75A @ 5V, so this setup should be working, but it's not. The lamp turns on if I use 5V at the gate and goes dim for 4.5V or so.

I would suspect I chose the FET poorly, but I'd appreciate someone else's input on the matter before going hunting yet again for another transistor and found out the circuit is wrong.

So what could be the problem here?

EDIT: Yes, I forgot the word "threshold" on "max Vgs threshold."

Best Answer

If the lamp is really 0.75A @ 5V, then it's resistance is closer to 7 ohms (when hot).

The Vgs of 2.5V is the threshold, where it will just start to conduct, so you have not chosen a good MOSFET. However, it probably would work, if the load resistance was a constant 7 ohms. But, it is not a constant 7 ohms, a cold filament can have 10 times less resistance than a hot filament. So, you never get enough current to heat up the filament.

enter image description here

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