MOSFETs for Makers: Getting Started with High-Power Control

Let’s quickly go over 3 key factors to consider when choosing a transistor to act as a switch:

• Drain-Source Voltage: This is the maximum voltage that we can safely switch without damaging the MOSFET. Generally, datasheets will specify an "absolute maximum" and of course, it's always best to stay below this value with some safety margin.
• Continuous Drain Current: This is the maximum amount of current that can safely flow through the MOSFET continuously.
• Gate Threshold Voltage: When the voltage on the gate pin reaches this threshold voltage, the MOSFET starts allowing current to flow. If the voltage is lower than this threshold, the MOSFET won't let current pass through. For the transistor to be "fully-on" the gate voltage needs to be higher than this value - by how much is a case-by-case... and a conversation for another time.

For this guide, we will be using the P30N06LE MOSFET, a very common MOSFET that is suitable for a wide range of applications. With a drain-source voltage of 60V and a continuous drain current of 30A, it is beefy enough to power most components you will encounter with plenty of headroom to spare. Its gate threshold voltage lies somewhere between 1 and 2 Volts (not every MOSFET comes out of the factory the same), but the digital logic from most microcontrollers alternates between 0 Volts and 3.3 or 5 Volts, which aligns perfectly with this MOSFET's gate threshold voltage range. We often refer to MOSFETs with such characteristics as "logic-level MOSFETs" since they're happy to be switched by 3.3-5V signals.

Note: These big beefy MOSFETs can get very hot, very quickly. Although this MOSFET is rated up to 30A of continuous current, it may only be able to handle this with adequate cooling. Without using something like a heatsink, we may only be able to achieve 5-ish Amps of continuous current.

Mosfets also come in smaller and cheaper varieties such as the 2N7000. Whilst this one is only rated to about 100 milliamps, it is still rated for up to 60 Volts. This may be useful if for example you are switching a relay, or driving some 12V LEDS at only a few milliamps, and don't need to use bulky and more expensive MOSFETs.

There are also inexpensive and ready-to-use MOSFET modules that remove a lot of the setup hassle that we will see in later examples.

If we call the component we wish to control a load, we can use a set of nice universal instructions for wiring up a MOSFET as a switch:

• Connect the MOSFET's source pin to ground (or the negative terminal of our voltage supply).
• Connect the MOSFET's gate pin to a microcontroller output pin.
• Connect the load from the positive terminal of the power supply to the MOSFET's drain pin.

That might be a little hard to decipher so let's wire up our LED and resistor load. Any microcontroller running at 3.3 or 5 Volts will work but for these examples will be using a Raspberry Pi Pico.

Now we have a circuit wired up, but nothing interesting will happen until we set some voltages on the MOSFET gate pin with the Pico. We will be using Thonny IDE to program our PIco. If you need a refresher on how to do so, check out our guide.

So, plug the Pico into your computer, open up Thonny and copy-paste the following code into a new script:

from machine import Pin
import utime

MOSFET = Pin(15, Pin.OUT) # Initialise the pin that is connected to the gate pin of the MOSFET 

while True:  # Loop forever
    MOSFET.value(0)  # Turn the  MOSFET ON
    utime.sleep_ms(1000) 
    MOSFET.value(1)  # Turn the MOSFET OFF
    utime.sleep_ms(1000)

from machine import Pin
import utime

MOSFET = Pin(15, Pin.OUT) # Initialise the pin that is connected to the gate pin of the MOSFET

try:
    while True:  # Loop forever
        MOSFET.value(0)  # Turn the  MOSFET OFF
        utime.sleep_ms(1000) 
        MOSFET.value(1)  # Turn the MOSFET ON
        utime.sleep_ms(1000)

except KeyboardInterrupt:
        MOSFET.value(0)  # Turn the  MOSFET OFF

This script is pretty straightforward and is just going to continuously set the GP15 pin to 3.3 Volts for 1 second then to 0 Volts for a second.

The try: and except: part of the code:

try:

(something something)

except KeyboardInterrupt:
        MOSFET.value(0)  # Turn the  MOSFET OFF

Ensures that when we stop the script from running, it returns the pin to 0 volts so we don't accidentally leave our load running.

Now let's spice things up. A MOSFET can switch fast... really fast. This means we can pass a PWM signal from the pin of our microcontroller to the transistor and it will be able to switch our power supply with the same PWM signal.

Using the same setup, we are going to plug in the Pico, open up Thonny, then paste the following code into it:

from machine import Pin, PWM
import utime

MOSFET = PWM(Pin(15))  # Initialize a PWM object on the pin that is connected to the MOSFET gate

MOSFET.freq(1000)  # Set the PWM frequency to 1 kHz

try:
    while True:
        # Ramp up from 0 to full duty cycle
        for duty in range(0, 65536, 655):  # Iterate through duty cycles from 0 to 65535 in increments of 655
            MOSFET.duty_u16(duty)  # Set the duty cycle of the PWM output
            utime.sleep_ms(10)  # Wait for 10 milliseconds before incrementing the duty cycle

        utime.sleep_ms(1000);
        
        # Ramp down from full duty cycle to 0
        for duty in range(65536, 0, -655):  # Iterate through duty cycles from 65535 to 0 in decrements of 655
            MOSFET.duty_u16(duty)  # Set the duty cycle of the PWM output
            utime.sleep_ms(10)  # Wait for 10 milliseconds before decrementing the duty cycle

        utime.sleep_ms(1000);
        
except KeyboardInterrupt:
    MOSTFET.duty_u16(0)
    

This code is very straightforward. It cycles the PWM duty up from 0 to full over 1 second, then holds it there:

    # Ramp up from 0 to full duty cycle
    for duty in range(0, 65536, 655):  # Iterate through duty cycles from 0 to 65535 in increments of 655
        MOSFET.duty_u16(duty)  # Set the duty cycle of the PWM output
        utime.sleep_ms(10)  # Wait for 10 milliseconds before incrementing the duty cycle

        utime.sleep_ms(1000);

And then it ramps down from full duty cycle to 0 over 1 second, then holds it there:

    # Ramp down from full duty cycle to 0
    for duty in range(65536, 0, -655):  # Iterate through duty cycles from 65535 to 0 in decrements of 655
        MOSFET.duty_u16(duty)  # Set the duty cycle of the PWM output
        utime.sleep_ms(10)  # Wait for 10 milliseconds before decrementing the duty cycle

        utime.sleep_ms(1000);

Save the script to the Pico and Run it – If everything is working you should now see the motor ramping up and down as a result of our PWM cycle. Like before, we can use this same PWM code to power anything that can be controlled with a PWM. Feel free to try it on our initial LED example and you should see the same PWM cycle as the motor!

So far we have only talked about N-type MOSFETs, but there are many other types of transistors available. Transistors can be complex, so without diving into too much theory let's touch on some other types you may encounter and what applications they may be more appropriate in.

P-type MOSFETs

P-channel MOSFETs are very similar to N-channel MOSFETs but behave a little differently. In the opposite fashion to an N-channel, the gate voltage that we are supplying with our microcontroller must be higher than the voltage we are switching. This means a P-channel MOSFET cannot control a 12 Volt circuit with a 3.3 Volt signal. But what this does mean is that we are able to easily switch a negative power source, so we could switch a -12 Volt circuit with a 3.3 Volt signal, something an N-channel cannot do (as the N-channel needs a gate voltage to be lower than the circuit voltage). Additionally, whilst we place an N-channel between ground and the load, we place a P-channel between the positive Voltage and the load. This is called high-side switching and is sometimes sought after in circuit design.

BJT Transistors

We have been talking about MOSFETs but another type of transistor is the BJT (bipolar junction transistor). BJTs are often used in other than applications than switching (such as in amplifier circuits) but can still very easily be used as a switch. BJTs also utilise another naming convention for its pins: the base, emitter, and collector, and like MOSFETs also come in 2 types, NPN and PNP (similar to N-channel and P-channel). As we've seen, to "flick" the MOSFET as a switch we need to supply a voltage to one of its pins, but when using a BJT we will need to supply a current instead. BJTs are also often a little cheaper than MOSFETs.

Relays

While not a transistor, a relay is another form of switch that we can use in a very similar fashion and you can check out our guide on how to do so. Relays typically contain an internal mechanical switch that is flicked when a voltage is supplied. However, this means that they are a little less responsive as a physical mass needs to be moved around inside the relay, and that we cannot use PWM signals nor switch our circuits very fast as this would wear out the relay. But what relays do offer is better electrical isolation (as there is a physical separation in the circuit when switched open) and the ability to switch AC power much easily as transistors are more suited for controlling DC power.

The sky is the limit to what you can control with your microcontroller. If you've got a MOSFET and power supply big enough to drive your load, you can control it, and this is such an incredibly powerful tool to have as a maker. If you make something cool with this MOSFET circuit, please show us on our forums, and if you have any questions or need some help, start the conversation below. We're full-time makers and happy to help.

Happy Making!