I have a simple circuit working where I drive a laser module with a mosfet(2SK4017Q), a 10K resistor (gate-source) applying 3.3V on the mosfet gate. Mosfet drain is connected to laser module GND, and 5V applied to laser module Vcc. Laser specifications are: output: 5-10mw, working voltage: 4.5-4.8vdc, working current: 40mA. Microcontroller is a Teensy 3.5.
I wanted to miniaturize this but now I believe I have made a mistake choosing some wrong smd mosfet (2N7002). I say "now", because I have made this circuit to work once, but could not reproduce it again (...) In this thread I am trying to understand what's happening next - see picture.
I have removed the 10K resistor from its place (as shown) so gate is floating (when microcontroller is not sending any signal). Any value of resistor I tried until here produced just very pale light, and forward voltage on laser module terminal was about less than 2V.
Inductance (I hope this is the proper term) happens when I hold a (conductive metallic) screwdriver on the laser module trim pot (picture), and simultaneously another screwdriver on the mosfet gate, laser module light gets vivid again and circuit seems to work as expect. I am not sure how to start to fix it.