Introduction
A metal oxide semiconductor field-effect transistor, or MOSFET is a type of transistor that is used for opening and closing a circuit. MOSFETs are the most manufactured human artefact on the planet. In Arduino projects, MOSFETs are typically used for switching power consuming actuators between On and Off states. Typical actuators include motors and stronger light sources. For building arduino spectrometers, MOSFETs are needed for supplying power to stronger light sources, including xenon and broad spectrum infrared LEDs.
MOSFET
A MOSFET has three pins: gate (g), drain (d) and source (s). The drain connects to the actuator (the machine or device that is put to work) and the source to the external power supply; the gate determines if the connection between the source and the drain is ON or OFF. For an N-channel transistor, the connection between the drain and the source is by default OFF. Only when a positive voltage over a given threshold is supplied to the gate does the circuit between source and drain close and the MOSFET put ON. The power supply to the gate is usually of low voltage and negligible current. For Arduino compatible N-channel MOSFETS, the threshold for the gate to switch between OFF and ON needs to be less than 5 (or 3.3) Volt.
N-channel MOSFETs can only be used as switches for DC system, and applied for regulating the negative (-), or ground side of the actuator. That means that the MOSFET source pin should be connected to the negative (ground) side of the external power source. Battery in this case. The MOSEFT drain pin is then connected to the negative (ground) side of the actuator (xenon light in this example). When positive voltage is supplied to the gate, an electric current can flow between the source and the drain, and the actuator is turned ON.
5 volt alternatives
The N-channel MOSFET supplied with the Arduino Projects Book is an IRF520. As this MOSFET has a low gate threshold voltage of 4V it has been popular to use with Arduino 5 volt systems.
For more modest power needs, the BS170 MOSFET able to conduct 500 mA at a maximum of 60 V is popular. The BS170 is packaged in a 3-legged TO-92 capsule. Also this mosfet requires a 5 volt system to reach the fully open state and transmit 500mA, it is thus not suitable for 3.3 volt systems or lower.
Note how the positions of the G, D, S pins differ between the IRF520 and BS170 MOSFETs. The wiring with the BS170 thus differs compared to the MOSFET project (Mothorized pinwheel) in the Arduino Projects Book.
3 volt alternatives
Having switched to the arduino Nano33 system, I discovered that the BS170 MOSFET could not feed forward the 500 mA the my system solution with a xenon light required. Looking for an N-channel MOSFET with a lower gate threshold, I came across the following alternatives:
SI2300 (several versions with different ratings) available from e.g. digi-key, available as surface mount format SOT (small-outline transistor) 23.
SQ2310 (several versions with different ratings) available from e.g. digi-key available as surface mount format SOT (small-outline transistor) 23.
irlml2502pbf available from Elfa and also packaged as SOT-23.
IRLML2502TRPBF is yet another SOT-23 packaged N-channel MOSFET, available from digi-key but also fyndiq.se.
Looking for an alternative in a “TO” (three legged) package, I could only find T0-220 alternatives (and no TO-92 like the BS170). The price for a TO compared to SOT packaged MOSFET is about an order of magnitude. FQP30N06L is available from [fyndiq.se]https://fyndiq.se/produkt/10st-fqp30n06l-fqp-30n06l-60v-logik-n-kanal-mosfet-to-220-0-one-size-2bbde9ae34e94730/). The gate opens fully at 2.5 volt.