- [Instructor] An inductor is an electrical component
that stores energy in a magnetic field
when current flows through it.
And the inductor will oppose any changes
to the amount of current passing through it
by very rapidly absorbing or releasing that energy.
An inductor wants the current through it to remain constant.
Now it's not just inductors that do that.
There are lots of electrical components
that exhibit inductive behavior such as motors.
An electric motor contains a bunch
of tightly wound coils of wire, much like an inductor,
and it stores and releases energy in a similar way.
From an electrical standpoint, a motor looks similar
to a resistor and an inductor in series with each other.
Now that's not saying that if you stick a resistor
and an inductor together you suddenly have a motor.
They're very different components.
But when a motor is used in a circuit,
electrically to the rest of the circuit,
it behaves similar to a resistor and an inductor.
As long as the current from the power supplier
remains constant, the inductive aspect
of the motor will be storing energy
but it basically acts like a wire allowing electrons
to pass through it without interference.
If I suddenly open a switch that cuts
off current to the motor, the motor will
very rapidly release its stored energy by inducing
a current to counteract that change.
That induced current will continue to push electrons
from one side of the motor to the other
in the direction that they were previously going.
But with that switch opened, those electrons can't continue
on the loop around the circuit.
So for a very brief period of time,
we end up with a higher density of electrons
on one side of the motor than on the other.
And that uneven distribution of electrical charge
means there will be a voltage across the motor
which is known as flyback.
The flyback is a voltage spike that occurs
when the source of current to an inductive component is
suddenly reduced or removed.
As the inductor rapidly unloads its stored energy
to counteract that change in current,
it creates a brief but significant voltage spike across it.
Flyback can be dangerous and potentially damage
other components in the circuit.
Fortunately that voltage spike is usually
very short lived and will dissipate over time
as the electrons find their way back
through the inductor to even out their distribution.
To demonstrate what flyback from a motor looks like,
I'll be measuring the voltage
across the motor for a computer fan.
I've chosen to use this fan because it only operates
at five volts, it has a relatively small inductance,
so it won't create a dangerously large voltage spike.
The fan is currently connected to a five-volt power source
and on the oscilloscope I can see that the voltage
between its two terminals is around positive five volts.
There's a lot of noise and ripples in the signal
due to the way the fan operates.
Now I'll set my oscilloscope trigger to capture
a single acquisition when it detects
the falling edge of a negative voltage spike
and I'll adjust the horizontal and vertical scale a bit
to capture a good view of it.
When I disconnect power to the fan,
the voltage suddenly drops.
And due to the inductive flyback,
the motor generates a negative voltage spike
of around minus 13 volts which is much greater
in magnitude than the original five volts across the motor.
Since the flyback voltage spike can potentially
damage the motor and other components
that are connected to the circuit,
to protect the circuit I need to provide a path
for the surge of current created by the motor's inductance
to take when that circuit is open.
I can create that path by inserting a diode in parallel
with the inductive load as shown here.
This is often called a flyback diode
but you may also hear it referred to as a snubber diode,
a suppressor diode or a clamp diode.
The flyback diode is oriented so that
when a circuit is closed, it will be reversed bias
acting like an open circuit so that all
of the current will flow through the motor.
When the switch to the motor is opened,
the motor will create a surge of current
but rather than creating a huge voltage spike,
this current will travel around the loop
created by the diode.
The path through the diode allows
the inductive motor to draw current from itself.
And the discharging current will loop around
and around over and over until the power is
eventually dissipated due to the resistance
of the wires and the diode.
The amount of time that it takes
for the inductive load to fully discharge
through the diode will vary depending
on the inductance and other factors,
but it's typically only a few milliseconds.
This is the same setup as before with the computer fan
except now I've added a flyback diode to the circuit.
When I open the switch to disconnect power,
I see that the voltage across the fan stops.
However, now I don't get that large
potentially damaging negative voltage spike
thanks to my flyback diode.