A (Metal-Oxide-Semiconductor Field-Effect Transistor) MOSFET as a switch can be used to turn things on and off, just like how you flip a light switch at home. But instead of your hand, the MOSFET uses an electrical signal to control its switching.
In this article, we will learn what a MOSFET is, how it works as a switch, and where it’s used.
Table of Contents
What is a MOSFET?
A MOSFET is a small electronic part that controls the flow of electricity in a circuit. Think of it as a gate that either lets the electricity flow through or stops it.
There are two main types of MOSFETs:
- N-channel MOSFET: It allows electricity to flow when you apply a voltage to it.
- P-channel MOSFET: It stops electricity from flowing when a voltage is applied.
We will mostly talk about the N-channel MOSFET because it is commonly used in circuits as a switch.
The Parts of a MOSFET
Before we learn how it works as a switch, let’s look at the basic parts of a MOSFET:
- Gate: This is like the switch handle. When you apply voltage here, the MOSFET turns on or off.
- Drain: This is where electricity enters the MOSFET.
- Source: This is where electricity leaves the MOSFET.
When the MOSFET is “on,” electricity can flow from the Drain to the Source. When it’s “off,” the electricity cannot flow.
How a MOSFET Works as a Switch
A MOSFET can be used in two states: on or off. These states depend on the voltage applied to the Gate.
- When the MOSFET is OFF:
- If there is no voltage applied to the Gate of an N-channel MOSFET, the path between the Drain and Source is blocked.
- No electricity flows through, and the MOSFET is off, just like when a light switch is in the off position.
- When the MOSFET is ON:
- When you apply a voltage (usually a small one) to the Gate, it opens the path between the Drain and Source.
- This allows electricity to flow from the Drain to the Source, turning the MOSFET on, just like when a light switch is flipped to the on position.
The voltage you apply to the Gate is like the “control signal” that decides whether the MOSFET allows electricity to pass or not.
Why Do We Use MOSFETs as Switches?
There are many reasons why MOSFETs are great switches in electronic circuits:
- Fast Switching: MOSFETs can turn on and off very quickly. This is important in devices that need to switch rapidly, like computers or smartphones.
- Low Power Consumption: MOSFETs need very little power to control them. This makes them efficient and good for battery-powered devices.
- Compact Size: MOSFETs are very small, so they can be used in tiny devices like microchips.
Circuit Problems (MOSFETs as Switches)
Problem 1: Turning On a Light Bulb with a MOSFET
Circuit Description: You have a 12V battery, a light bulb, and an N-channel MOSFET. The light bulb is connected to the battery through the MOSFET. The Gate of the MOSFET is connected to a 5V signal.
Question: Will the light bulb turn on when 5V is applied to the MOSFET’s Gate?
12V Battery (+) --- Light Bulb --- Drain (MOSFET) --- Source (MOSFET) --- 12V Battery (-)
|
Gate (MOSFET) --- 5V signal
Solution:
- Identify the components:
- The Drain is connected to the light bulb.
- The Source is connected to the ground (negative side of the battery).
- The Gate receives a 5V signal.
- How an N-channel MOSFET works:
- The MOSFET will only turn on (allow current to flow) if the voltage applied to the Gate is higher than the threshold voltage (typically around 2-4V for most MOSFETs).
- Step-by-Step Analysis:
- A 5V signal is applied to the Gate.
- This voltage is greater than the threshold voltage, meaning the MOSFET will turn on.
- When the MOSFET is on, current can flow from the Drain to the Source (from the light bulb to ground).
- This completes the circuit and allows the light bulb to turn on.
Conclusion: Yes, the light bulb will turn on when 5V is applied to the Gate of the MOSFET.
Problem 2: Controlling a Motor with a MOSFET
Circuit Description: You have a 9V battery, a small motor, and an N-channel MOSFET. The motor is connected between the battery and the MOSFET. The Gate of the MOSFET is connected to a 3.3V control signal from a microcontroller.
Question: Will the motor run when the 3.3V control signal is applied to the Gate of the MOSFET?
Circuit Diagram:
9V Battery (+) --- Motor --- Drain (MOSFET) --- Source (MOSFET) --- 9V Battery (-)
|
Gate (MOSFET) --- 3.3V from Microcontroller
Solution:
- Identify the components:
- The Drain is connected to the LED and resistor.
- The Source is connected to ground.
- The Gate is connected to a 5V signal.
- Step-by-Step Analysis:
- A 5V signal is applied to the Gate of the MOSFET.
- Since 5V is greater than the threshold voltage (typically around 2-4V), the MOSFET will turn on.
- When the MOSFET is on, current flows from the 5V power supply through the LED and resistor to the Source of the MOSFET.
- The LED will light up because current is flowing through it.
- Calculating Current:
- The resistor limits the current to protect the LED.
- Using Ohm’s Law: I = V / R
- Voltage across the resistor = 5V – (Voltage drop across the LED, typically 2V).
- Voltage across the resistor = 3V.
- Resistor value = 100Ω.
- Current through the circuit: I = 3V / 100Ω = 0.03A = 30mA
Conclusion:
- If the MOSFET's threshold voltage is less than 3.3V (like 2V), the motor will run.
- If the threshold voltage is higher than 3.3V, the motor will not run properly.
Problem 3: MOSFET as a Switch for an LED
Circuit Description: You have a 5V power supply, an LED, a 100Ω resistor, and an N-channel MOSFET. The LED is connected to the 5V supply through the MOSFET. The Gate of the MOSFET is connected to a 5V control signal.
Question: What will happen to the LED when the 5V control signal is applied to the MOSFET’s Gate?
Circuit Diagram:
5V (+) --- LED --- 100Ω Resistor --- Drain (MOSFET) --- Source (MOSFET) --- Ground
|
Gate (MOSFET) --- 5V signal
Solution:
- Identify the components:
- The Drain is connected to the motor.
- The Source is connected to ground.
- The Gate is connected to a 3.3V signal.
- Check if the MOSFET turns on:
- For the MOSFET to turn on, the Gate voltage must be higher than the MOSFET’s threshold voltage (which is around 2-4V).
- Step-by-Step Analysis:
- The Gate voltage is 3.3V.
- This voltage may or may not be enough to fully turn on the MOSFET, depending on the threshold voltage.
- If the threshold voltage is 2V, then the MOSFET will turn on and the motor will run.
- If the threshold voltage is 4V, then the 3.3V signal is not enough to turn the MOSFET fully on, and the motor will not run or will run very weakly.
Conclusion: The LED will turn on, and about 30mA of current will flow through the circuit.
