Small changes in gate potential produce large changes in the current flowing from source to drain.

Here is a simplified circuit diagram of a MOSFET used as a switch.

Because MOSFETS can be damaged by electrostatic discharge, the inputs can be protected as shown below. The zener diodes prevent input voltages greater than about +/- 15.7 V.

The diagram below shows a diode used to protect the MOSFET from back EMF if an inductive load is used. The diode allows the load current to die away gradually when the MOSFET is switched off.

• R1 and R2 form a potential divider. The input voltage is reduced by about 1% by these components.
• D1 and D2 are 15 volt zener diodes. This means the voltage at the FET gate can never be more than +/- 15.7V. This makes electrostatic damage to the FET very unlikely. If a very large voltage was applied to the input, R1 would fail. This component is not expensive and is easy to replace.
• The FET will turn on if a positive potential is applied to the input. (+ 4 to 6 volts needed).
• If an inductive load is being used, D3 provides a path for the inductor current to die away slowly when the FET is turned off suddenly. D3 can be omitted if the load is resistive.
• R3 is optional. It acts as a dummy load.