Join Barron Stone for an in-depth discussion in this video Protect against flyback voltage, part of Electronics Foundations: Semiconductor Devices.
- [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.
- 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