- [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.