As seen in this LTSpice model, I am trying to control a white LED (3.6 Vf @ 20 mA) connected to a 7.5 voltage source with a 2N7002 N-channel MOSFET and a 5V control signal from an Arduino.
Given that my gate voltage is 5V from a micro-controller, I was expecting the MOSFET to basically act as a switch. By looking at the graph for the 2N7002, given that the desired current is 20mA and the Vgs is 5V, I was expecting a voltage drop across the transistor of close to zero such the source voltage was ~7.5V.
However, as seen in the simulation graph, the voltage across the transistor is in fact quite large, such that the source voltage is only ~3V (as opposed to the expected ~7.5V).
When I breadboarded this circuit, I got the same result, a source voltage of ~3V.
Can anyone explain why the source voltage of the MOSFET is much lower than expected? And can anyone please recommend a transistor that would effectively allow me in this situation to create a switch to drive the white LED with the use a 5V signal and a 7.5V supply power?