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General Electronics

A diode conducts electrical current one direction and not the other.
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.
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

Flyback Diode

It's next to a DC Relay.
What is it, and why is it there?

By Douglas Krantz

Schematic of a relay coil and a flyback diode.
A relay consists of an electromagnet and a moving armature. As it moves, the armature opens or closes electrical contacts (make or break contact).

Someone Thinks the Flyback Diode is Important

Manufacturers all

over the world spend

good money

installing these

diodes, they must

think they're


Snubber - Protects the Circuit Components and Reduces RF Interference to Other Circuitry

A realy coil is an electromagnet
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 Coil with the Applied Voltage
Voltage, an electrical force, pushes and pulls on the electrons

Relay Turning On

When the relay is

first turned on,

voltage (electromotive

force or EMF) is applied

to the ends of the coil.
Electrons move along a wire like a Train moves along Tracks
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.

EMF Pushes and Pulls Electrons

Like a train being

pushed and pulled

by locomotives,

The magnetic field is being built up around a relay coil.
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.

Building Magnetic Field

As the electrons travel,

they put energy into

creating a magnetic

field, pulling in the

armature of the relay.
A turned off relay
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.

Relay Turning Off

Intuitively one would

think that ending the

applied voltage stops

the current like a water

faucet, releasing the

The current can't just be turned off in a relay coil.
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.

It's Not That Simple

According to the

First Law of


energy cannot be

created or destroyed,

it can only be

A magnetic field being produced around a coil
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.

The Magnetic Field is Still There

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 and the wires make 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.

The Collapsing Magnetic Field is a Generator

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.
Electrons moving as they are forced to by the collapsing magnetic field.
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 Magnetism Creates Its Own EMF

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

As the magnetic field collapses, it goes past the wires creating a generator.
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.

Danger -- High Voltage

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 magmetism generates greater voltage as the field collapses faster.
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 Turn Off, the Greater the Voltage

The quicker the

electrons are stopped,

the faster the magnetic

field collapses;

and the greater the

generated voltage

spike keeping

the electrons moving.
The magnetic field collapses as the current is turned off.
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.

Something Has to Give

This voltage is going

to be passed through

the circuit to whatever

is stopping the current.
Sparks across switch contacts as the magnetic field collapses
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.




get little


jumping the

A transistor switch being burned out by the voltage generated in the relay coil.
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

The flyback diode shunts the current back into the coil as the magnetic field collapses.
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.

What Can Be Done About This Voltage Spike?

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.
The electrical current normally bypasses the flyback diode.
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.

Doesn't the Diode Normally Short Out The Whole Circuit?

Normally, when external

voltage is applied to the

coil, the flyback diode is

reverse biased and will

not conduct any

A realy coil is an electromagnet
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.

The Flyback Diode is Forward Biased Only While the Relay is Being Turned Off

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

Relay and flyback diode.
The flyback diode is a snubber, reducing the impact of the voltage produced by the collapsing magnetic field of the relay coil.

The Flyback Diode Protects the Circuit

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


voltage spike.

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