Join Barron Stone for an in-depth discussion in this video Use a MOSFET as a switch, part of Electronics Foundations: Semiconductor Devices.
- [Instructor] MOSFETs are popular for building electronically controlled switches because of their incredibly high input impedance at the gate terminal and their ability to drive high-power electrical loads. The device or circuit that's controlling a MOSFET doesn't need to provide much current to operate the MOSFET as a switch. It just needs to be able to raise or lower the gate voltage enough to put the transistor into the saturation or cutoff modes. The circuit to create a low-side switch with an in-channel enhancement mode MOSFET has a lot in common with a circuit for a low-side switch using an NPN bipolar junction transistor.
The BJT has its emitter terminal tied to ground and the MOSFET has its source terminal tied to ground. The two transistors also have their respective collector, or drain terminals, connected to the electrical load the switch is going to be turning on or off, in this case a motor attached to a 12 volt power supply. The difference between these two circuits comes into play when connecting the base and gate terminals. The BJT switch needs to have a resistor in series between the base and the controlling device to limit the amount of current the BJT will draw into its base when the switch is turned on.
Since the MOSFET has such a high input impedance at its gate, there's no need to put a current limiting resistor in series with it. The gate can be directly connected to the control signal's source. However, I should include a pull-down resistor between the transistor's gate and the source terminal to help the gate return down to zero volts when the switch is turned off. The control signals for the low-side MOSFET switch are similar to the control signals for a low-side BJT switch.
To turn the motor on I need to provide a positive control voltage or a high signal. To turn the motor off I need to lower the control signal down to zero volts or ground. The pull-down resistor is included to provide a guaranteed path for any built up charge at the gate terminal to drain down to ground. The switch can turn off quickly and fully when the control signal drops to the low state. Since, I don't want this pull-down resistor to draw too much current when the control voltage is in the high state to turn the switch on, I usually use 10 kilo ohms for my pull-down resistors.
If I want to control the MOSFET switch with a micro-controller board, like in Arduino or a Raspberry Pi, I need to make sure to use a MOSFET that can be turned on and saturated at a low enough voltage to work with the output levels from these micro-controllers. The digital output pins on many Arduino boards output a five volt signal to represent digital high but Raspberry Pi boards can only produce 3.3 volts from the digital output pins. Unfortunately, a lot of MOSFETS require 10 to 15 volts to turn on, which mean you can't control it directly from a micro-controller.
When you're choosing a MOSFET for your next project always check the data sheet to make sure it will have a low enough gate to source threshold voltage for what you're doing. The RFP30N06LE N-Channel MOSFET is a popular choice amongst hobbyists for controlling high powered devices directly from low powered micro-controllers. It has a gate to source threshold voltage that's low enough to make it compatible with your typical three to five volt micro-controller output signals.
Additionally, when it's turned fully on, it has a tiny little resistance the source and drain that's less than 47 milliohms. This transistor can handle up to 60 volts between the drain and source terminals when it's turned off and when it's turned on, it can handle up to 30 amps of continuous current flowing through it. Those specs make it useful for a wide variety of applications, but if you're going to be using it to switch a load with more than about an amp of current, be sure to strap a heat sink onto it or you'll quickly end up destroying your transistor.
- Semiconductor materials
- Diode applications
- Rectifying a signal
- Detecting the signal peak
- Protecting against large signals, reverse current, and flyback voltage
- Special purpose zener diodes, Schottky diodes, and photodiodes
- NPN and PNP bipolar junction transistors
- Using a BJT as a switch
- Field effect transistors
- Differences between BJTs and MOSFETs
- Operational amplifiers
- Op-amp applications
- Comparing signals
- Buffering signals
- Amplifying signals
- Filtering signals
- Combining signals