Designing a MOSFET Circuit as a Voltage-Operated Switch

Basics of MOSFET as a Voltage-Operated Switch

The MOSFET can function as a voltage-operated switch in many types of electronic circuits.
It operates as a switch when it is in cutoff (off state) or in saturation (on state).
Remember as a voltage-controlled device, a MOSFET’s state is controlled by the voltage applied to its gate terminal.
The source-to-drain current pathway is turned on or off by changing this gate voltage.
When the gate voltage exceeds a specific threshold, the channel is ‘opened’, and current is allowed to flow - this is the MOSFET acting as a closed switch.

Step 1: Choose your MOSFET. You will need to decide between using an N-Channel or P-Channel MOSFET. This will depend on whether the switch needs to be ‘on’ when the signal is high or low, respectively.
Step 2: Design the input section, i.e. the component that will provide the input voltage to the gate. This could be a microcontroller, another transistor, or even a simple switch.
Step 3: Add a pull-down resistor to the gate of your MOSFET if you’re using an N-Channel or a pull-up resistor for a P-Channel. This resistor will ensure the gate voltage goes back down to 0V when the input signal is removed, keeping the MOSFET ‘off’.
Step 4: Plan power supply connection. If using an N-Channel MOSFET, connect the source terminal to ground and the drain to the negative side of your load. For a P-Channel MOSFET, it’s vice versa.
Step 5: Choose an appropriate load. This depends on what you want to switch on, it could be a motor, LED, etc. The voltage requirements of the load should be suitable for the MOSFET to adequately switch it on and off.
Step 6: Apply the appropriate gate voltage. Naturally, you’ll want to apply a voltage to the gate of the MOSFET high enough to overcome the threshold voltage.
Optional: Consider adding a snubber circuit to protect against voltage spikes if you’re controlling inductive loads like motors or relays.

If the MOSFET is not turning on, check the gate voltage to ensure it is substantial enough to exceed the threshold.
Always verify connections to and from all terminals, especially the gate and source/drain.
Confirm that the input signal is within the MOSFET’s operating parameters.
If the problem persists, consider exploring whether the MOSFET is damaged or needs replacement.

Practising designing and examining a MOSFET switch can aid in understanding the mechanism of the MOSFET and its characteristic behaviour in circuits.