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Selecting Flyback Diodes for 5V Relay Coil Suppression

Published Date
Author Cadence PCB Solutions

Key Takeaways

  • The voltage generated during relay coil de-energization may be large enough to cause arcing in the control switch contacts and can reduce the switch life. 

  • The most common coil suppression technique in relays is to connect a reverse-biased diode in parallel with the relay coil. 

  • For a 5V relay, the reverse voltage rating of a flyback diode should be at least the coil voltage or working voltage.

Relay

Relays are an essential component in automatic control circuits

Relays are electrically operated switches that function with the help of an electromagnet, which is usually formed by a coil. The coil is energized with DC voltage and is specified as coil voltage in the datasheet. If the coil voltage is 5 V, then the relay is called a 5V relay.

Coil energization is the key to building an electromagnet, so de-energization of a coil causes harmful effects on associated relay components. Luckily, coil de-energization can be prevented by placing a reverse-biased diode, called a flyback diode, across the coil.

In this article, we will discuss relay and pin configurations, the operation of 5V relays, and the importance of flyback diodes in 5V relays.   

Relays

Relays allow easy control of devices that are remotely placed. A relay is triggered for low currents; however, they enable the control of high power systems.

Relays are essential components in automatic control circuits, load switching systems, star-delta converter-based motor speed controls, under-voltage protection, and over-voltage protection. They are available as modules in the market. 

Pin Configuration

Most relay modules come with common five pins. The ends of an inductive coil that acts as an electromagnet when energized corresponds to pin 1 and pin 2. The other three pins are common (COM), normally closed (NC), and normally open (NO). Usually, the common terminal is connected to the load end where the current enters the load. Depending on the application, the other end of the load is connected either to normally closed or normally open pins.

Let’s take a look at how 5V relays generally operate.  

The Operation of 5V Relays

In the case of 5V relays, the coil voltage must be 5 V. In 5V relays, pin 1 is connected to the positive terminal of the 5V DC supply, and pin 2 is connected to the negative terminal or ground through a control switch. This control switch can be a transistor, microcontroller, or anything that performs switching operations. The coil is energized only when the control switch is closed, so by using the control switch, the relay operation can be controlled.

Consider a 5V relay connected to a 5V supply with a control switch that is kept open. Let the output side of the 5V relay be in the normally open configuration. The load remains disconnected in the normally open configuration. When the coil is energized by triggering the control, the relay turns on and the current starts flowing in the load from the COM to NO. When the relay is configured to a normally closed configuration, the load remains connected all the time. Upon the application of the control trigger, the relay turns off, stops the current flow, and disconnects the load.

Now that we know what happens when a coil is energized, let's discuss coil de-energization and why a flyback diode is needed.   

Relay Coil De-Energization 

When a relay coil is energized, the current starts flowing through the coil. The steady-state value of the current through the coil depends on the DC resistance. The current in the coil produces a magnetic field. When the coil is de-energized, the magnetic field suddenly collapses. A change in the magnetic field generates counter-electromotive force (counter EMF) in the coil.

Relay coil de-energization and counter EMF affects relay circuits in the following ways:

  1. The counter EMF generated can be detrimental to the electronic driver used to control relay operation. The voltage generated may be large enough to cause arcing in the control switch contacts and can reduce the switch life. 

  2. Voltage spikes may initiate radiation, and the electromagnetic interference generated can disturb the operation of electronics in the vicinity.

To protect switching transistors or microcontrollers and adjacent electronics from the effects of interference, it is necessary to suppress the counter EMF generated in the relay coil and dissipate the stored energy in a controlled manner. A flyback diode in the relay circuit can be used to do just that.  

Flyback Diodes and Coil Suppression  

The most common coil suppression technique in relays is to connect a reverse-biased diode in parallel with the relay coil. The cathode of the diode is connected to the + 5 V DC and the anode to the relay coil end that goes to the ground. The diode connected across the relay coil is called a flyback diode or freewheeling diode.

When the coil is energized, the flyback diode is reverse biased and has no role in the relay operation. The flyback diode is forward biased by the counter EMF generated during coil de-energization. During coil de-energization, the flyback diode conducts and suppresses the counter EMF generated in the relay coil. 

Selecting Flyback Diodes for 5V Relays

The selection of a flyback diode is critical, as it influences the life of other components in the relay circuit. The selection of a flyback diode is based on the working voltage and working current in the energized relay coil.

For a 5V relay, the reverse voltage rating of the flyback diode should be at least the coil voltage or working voltage. Usually, the flyback diode is selected so that there is plenty of reserve in the reverse voltage rating. The flyback diode current rating should be at least the coil current or working current. Putting reserve in the current rating is followed in flyback diode selection.

Placing the flyback diode for a 5V relay physically closer to the relay coil is ideal. The CAD tools available from Cadence can help you with the proper placement of flyback diodes in relay circuits. Leading electronics providers rely on Cadence products to optimize power, space, and energy needs for a wide variety of market applications. If you’re looking to learn more about our innovative solutions, talk to our team of experts or subscribe to our YouTube channel.