But when using power MOSFETs to switch either inductive or capacitive loads some form of protection is required to prevent the MOSFET device from becoming damaged. If the resistive load of the lamp was to be replaced by an inductive load such as a coil, solenoid or relay a “flywheel diode” would be required in parallel with the load to protect the MOSFET from any self generated back-emf.Ībove shows a very simple circuit for switching a resistive load such as a lamp or LED. The gate input voltage V GS is taken to an appropriate positive voltage level to turn the device and therefore the lamp load either “ON”, ( V GS = +ve ) or at a zero voltage level that turns the device “OFF”, ( V GS = 0V ). In this tutorial we will look at using the Enhancement-mode MOSFET as a Switch as these transistors require a positive gate voltage to turn “ON” and a zero voltage to turn “OFF” making them easily understood as switches and also easy to interface with logic gates.

We now know that there are two main differences between field effect transistors, depletion-mode only for JFET’s and both enhancement-mode and depletion-mode for MOSFETs. To overcome this problem Power Field Effect Transistors or Power FET’s where developed.V While connecting together various MOSFETS in parallel may enable us to switch high currents or high voltage loads, doing so becomes expensive and impractical in both components and circuit board space. We also saw that due to this very high input (Gate) resistance we can safely parallel together many different MOSFETS until we achieve the current handling capacity that we required.

We saw previously, that the N-channel, Enhancement-mode MOSFET (e-MOSFET) operates using a positive input voltage and has an extremely high input resistance (almost infinite) making it possible to use the MOSFET as a switch when interfaced with nearly any logic gate or driver capable of producing a positive output.

Transistor as a switch download