What is a Flyback Diode?
Like a check valve with water, a diode freely conducts electrical current in one direction, and stops the electrical current from flowing the other direction.
It's next to a DC Relay. What is it, and why is it there?
By Douglas Krantz
Someone Thinks the Flyback Diode is Important
consists of an electromagnet and a moving armature. As it moves, the armature opens or closes electrical contacts (make or break contact).
Manufacturers all over the world spend good money installing these diodes, they must think they're important.
Snubber - Protects the Circuit Components and Reduces RF Interference to Other Circuitry
Moving electrons in a coil of wire produce magnetism and it's the magnetism that pulls the armature.
A flyback diode is actually a type of snubber circuit. A snubber circuit protects the rest of the circuitry from a magnetic coil. A snubber circuit also reduces the RF interference being transmitted from the circuit.
To understand why the diode protects, let's look at the inner workings of the relay, which we can think of as electromagnet.
Relay Turning On
Voltage, an electrical force, pushes and pulls on the electrons
When the relay is first turned on, voltage (electromotive force or EMF) is applied to the ends of the coil.
EMF Pushes and Pulls Electrons
Think of a train consisting of boxcars (electrons), and small locomotive engines (electromotive force) between each of the cars. The atoms making up the wire can be considered as the tracks guiding the train.
Like a train being pushed and pulled by locomotives, electrons are being pushed and pulled by the electromotive force
, and move along the wire.
Building Magnetic Field
Starting the movement of electrons isn't instantaneous, although it all happens so fast it's hard to even measure. Like a train starting out, the electromotive force (locomotive) starts moving the electrons (boxcars), and it's the movement of the electrons as they speed up that builds the magnetic field.
As the electrons travel, they put energy into creating a magnetic field, pulling in the armature of the relay.
Relay Turning Off
The relay has been turned on, the magnetic field has been stable, and now the electromotive force that had been keeping the electrons moving is removed. Expecting the current to immediately stop is like turning off the train locomotives and expecting the train to immediately stop.
Intuitively one would think that ending the applied voltage stops the current like a water faucet, releasing the armature.
It's Not That Simple
It's a two way street. Moving electrons create a magnetic field; a changing or collapsing magnetic field moves electrons. In other words, the electrons, like the momentum built up in a moving train, can't just be stopped.
According to the First Law of Thermodynamics, energy cannot be created or destroyed, it can only be converted.
The Magnetic Field is Still There
The magnetic field was built with electromotive force pushing and pulling electrons; the magnetic field returns the energy to the train of electrons by producing its own electromotive force.
We converted electrical energy into building up the magnetic field; the energy is still there, even after the electromotive force (voltage) is removed.
The Collapsing Magnetic Field is a Generator
A generator in a power plant produces electromotive force by moving magnets past coils of wire; the relay coil produces electromotive force as the collapsing magnetic field moves past the wires in the coil.
When the voltage is turned off, because the electrons are starting to slow down like railroad cars coasting to a stop, the magnetic field starts to collapse.
The Magnetism Creates Its Own EMF
Power is fed back into the train. The generated electromotive force produced by the collapsing magnetic field is like turning on the train locomotives again. Enough energy is given back to electrons that their movement will keep up the magnetic field.
The electrons don't just stop: the collapsing magnetic field puts its energy back into an effort to keep the electrons moving. It generates electromotive force in the coil, giving the electrons some extra push
Danger -- High Voltage
The voltage produced by this generator can be hundreds of volts; it will be whatever it takes to keep the electrons moving. This can be quite an electrical shock, even when the original voltage was only 12 volts.
This voltage can be seen on the terminals of the relay coil as a short term, reverse voltage spike.
Even when the turn-on voltage was only 12 volts, the generated spike can be hundreds of volts.
The Quicker the Turn Off, the Greater the Voltage
This is how the coil in a car works. Inside the coil, the magnetic field collapses fast, generating the 50,000 volts needed to jump the gap in the spark plugs.
The quicker the electrons are stopped, the faster the magnetic field collapses; and the greater the generated voltage spike keeping the electrons moving.
Something Has to Give
Inside an electronic circuit, this voltage will appear on the terminals of the relay, and from there be applied to the rest of the circuit.
This voltage is going to be passed through the circuit to whatever is stopping the current.
The voltage in the electromotive force causes electrical current to jump gap in the switch contacts that originally turned off the current. This sudden short-term surge of current will also produce electromagnetic interference (EMI), interfering with other parts of the circuit and possibly this EMI will be transmitted to nearby electronics.
Mechanical switches get little sparks jumping the contacts.
This jumping-the-gap is hard on semiconductors; their fragile junctions are no match for the high voltage of the relay's electromotive force.
Semiconductors get little sparks too, which punch holes through the junctions.
What Can Be Done About This Voltage Spike?
The flyback diode keeps the electrons moving by shunting them back into the relay coil. Because the electrons keep moving, the collapse of the magnetic field is slowed down, and the generated voltage will be much lower. Switch contacts and semiconductor junctions can easily handle these lower voltages.
The flyback diode, as a snubber, keeps the current flowing through the coil...
By shunting the current back into the coil, the diode shorts out the voltage spike.
Doesn't the Diode Normally Short Out The Whole Circuit?
The diode, as installed, is reverse biased. It will not conduct when the relay is turned on. There is no short circuit and no energy is wasted.
Normally, when external voltage is applied to the coil, the flyback diode is reverse biased and will not conduct any current.
The Flyback Diode is Forward Biased Only While the Relay is Being Turned Off
When turned on, the relay is a load to the power supply and does conduct; the diode is reverse biased and does not conduct. When first turned off, the relay is a generator of voltage; for a short time the voltage on the relay contacts is reversed and the flyback diode does conduct.
During the brief turn-off time of the relay, when external voltage is removed, the diode is forward biased to keep the transient voltage spike to a minimum.
The Flyback Diode Protects the Circuit
The flyback diode is a snubber, reducing the impact of the voltage produced by the collapsing magnetic field of the relay coil.
The reason manufacturers install these diodes next to DC relays is because at turn-off time, as the magnetic field is flying back, the flyback diode protects the circuit, and its components from the relay's damaging voltage spike.
Having serviced fire alarm systems for nearly 20, Douglas Krantz has compiled his knowledge of the causes of Ground Faults and how to reliably detect them into the book Make It Work - Hunting Ground Faults
. The book shows the three types of ground fault, what equipment should be used with each type of ground fault, and how to locate those hard-to-find ground faults.
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