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How do you energize a relay?

Relays are electromagnetic switches that are controlled by an electromagnet. When current flows through the electromagnet coil, it generates a magnetic field which pulls the relay armature and changes the switch contacts. There are a few key steps to properly energizing a relay:

Requirements for Energizing a Relay

To energize a relay, you need:

  • A power source – This provides current to the relay coil. Often a low voltage DC power supply is used.
  • Relay coil – Creates a magnetic field when energized. Check the relay specifications for the proper coil voltage.
  • Switch contacts – Connects or disconnects the relay contacts when the coil is energized or de-energized.
  • Load circuit – The circuit being switched by the relay.

Relay Coil Voltage

The first step is to determine the proper coil voltage for the relay. This can usually be found on the relay datasheet or labeled on the relay itself. Common coil voltages include 5V, 12V, 24V, 120V, etc. It’s crucial to apply the rated coil voltage – applying a lower voltage may not fully engage the relay, while a higher voltage can damage the coil.

Power Supply

The relay coil needs to be powered from a DC power supply capable of providing enough current for the coil. A benchtop power supply works well for testing and prototyping. In a permanent installation, the coil power may come from an AC-DC converter or directly from a battery.

Coil Current

In addition to the coil voltage, the coil current rating must be checked. The power supply must be able to provide the rated coil current, which may range from a few milliamps up to a few hundred milliamps, depending on the relay size and type. This current rating is often listed on the datasheet as the “pickup current” or “pull-in current.”

Flyback Diode

When the relay coil is de-energized, the magnetic field collapses very quickly, inducing a high voltage spike across the coil. This can damage transistor switches and other components. To suppress this voltage spike, a flyback diode should be connected across the relay coil. The diode provides a path for the induced current and clamps the voltage.

Connecting the Circuit

Here are the basic steps for connecting a relay:

  1. Connect the positive terminal of the power supply to one end of the relay coil
  2. Connect the negative/ground terminal of the power supply to the cathode (striped end) of the flyback diode
  3. Connect the anode of the flyback diode to the other end of the relay coil
  4. Connect one of the relay contacts to the load being controlled
  5. Connect the other relay contact to the load’s power source

This completes the basic relay driving circuit. When power is applied to the coil, the magnetic field will pull the armature and change the state of the switch contacts, energizing the load circuit.

Controlling the Relay Coil

In many applications, the relay coil needs to be turned on and off under electronic control rather than being constantly energized. Some options for controlling the relay include:

  • Transistor switch – An NPN or N-channel MOSFET transistor can switch the relay coil on and off.
  • Microcontroller I/O pin – Setting a microcontroller I/O pin high or low will directly control power to the relay coil.
  • Relay drive IC – Dedicated ICs are available to interface logic signals with the relay coil.
  • Solid state relay – An SSR has a transistor output that can be used to control a traditional magnetic relay coil.

The transistor, I/O pin, or drive IC can be employed to control the positive supply line going to the relay coil. When activated, the coil is energized. When off, no power flows through the coil.


Here are some common problems and solutions when energizing relays:

  • Coil not energizing – Check that the proper coil voltage is applied. Verify the power supply is on and connections are good.
  • Humming noise – Usually caused by a faulty coil. Replace the relay.
  • Chattering – Indicates faulty or dirty contacts. Clean or replace the relay.
  • Overheating – Make sure the coil power supply can provide enough current. Reduce the load on the contacts.
  • No switching action – Check the coil voltage and power supply. Test the continuity of the contacts.

Sample Relay Wiring Diagram

Here is a diagram showing a typical wiring scheme to energize a 12V relay coil:

The 12V DC power supply feeds current through the relay coil. The flyback diode protects the transistor switch. When the transistor is activated, it completes the circuit through the coil, energizing the relay. The normally open contacts close, allowing current to flow to the load.


To recap, the key steps in energizing a relay are:

  • Select a relay with the proper coil voltage for your application
  • Use a power supply that can provide enough current for the coil
  • Connect a flyback diode across the coil
  • Wire the coil, diode, and power supply together
  • Connect the relay contacts to your load circuit
  • Control the relay by switching the positive coil supply line with a transistor, I/O pin, etc.

Following these guidelines will allow you to successfully drive a relay in your circuits. Just remember to always check the coil voltage and power requirements when using a new relay.

Relay Type Typical Coil Voltage Switch Current Switching Speed
Small signal 3-12 VDC 1-10 A 2-5 ms
Power 12-120 VDC 10-30 A 5-20 ms
High current 12-48 VDC 30-80 A 10-50 ms
Reed 3-24 VDC 0.5-1 A 0.2-2 ms
Mercury wetted 12-120 VAC/DC 0.5-10 A 1-5 ms

This table provides an overview of some common relay types and their typical coil voltages, contact ratings, and switching speeds.

Small Signal Relays

Small signal relays are general purpose relays for lower power applications. They have coils that operate from 3-12V DC and can switch 1-10 amps maximum. The contacts open and close in 2-5 milliseconds.

Power Relays

For switching higher voltages and currents, power relays are used. They have coil voltages from 12V up to 120V DC. Contact ratings range from 10-30 amps. Switching time is 5-20 milliseconds.

High Current Relays

When you need to control very high current loads, a heavy duty relay is required. They operate on 12-48V DC coils. The contacts can switch 30-80 amp loads. Speed is 10-50 milliseconds.

Reed Relays

Reed relays have very fast switching times down to 0.2 milliseconds. Coil voltages are 3-24V DC. They switch relatively small loads up to 1 amp.

Mercury Wetted Relays

For AC or DC loads, mercury wetted relays can handle 0.5-10 amps. Coil voltage ranges from 12-120V AC/DC. Switching speed is 1-5 milliseconds.

As you can see, there are quite a few relay types available to meet different voltage, current, and speed requirements. Make sure to select a relay that is properly rated for your specific application.

Example of Energizing a Relay in a Battery Charger

Here is an example of how a relay would be energized in a lead acid battery charger circuit:

Battery Charger Circuit

  • Input: 120V AC from the wall outlet
  • Step-down transformer to convert 120V AC to 12V AC
  • Bridge rectifier to convert the 12V AC to pulsing DC
  • Filter capacitor to smooth the pulsating DC to steady 12V DC
  • LM317 voltage regulator configured as constant current source
  • Adjustable resistor to set charging current (e.g. 5A)
  • Relay contacts connect the charging current to the battery when energized

Energizing the Relay

For this battery charger example, we need a relay that can switch a 5A load at 12V DC. A common automotive 12V relay would be suitable. Here are the steps to energize the relay coil:

  1. Select a 12V automotive SPDT relay, with a 12V coil and 10A contacts
  2. Use a 12V bench power supply to power the relay coil
  3. Connect a 1N4004 diode across the relay coil for flyback protection
  4. Connect the negative terminal of the 12V supply to coil pin 1
  5. Connect the diode cathode to coil pin 2
  6. Connect the diode anode to coil pin 1
  7. Connect the positive 12V supply to coil pin 2 through a switch
  8. Closing the switch energizes the coil and closes the relay contacts
  9. The closed contacts connect the battery charger output across the battery

This energizes the relay, allowing battery charging current to flow when the coil is activated. The diode prevents the coil voltage spike when power is removed.

Safety Tips When Working with Relays

Here are some important safety guidelines to follow when energizing and using relays in your projects:

  • De-energize and disconnect power before modifying or adjusting any relay wiring
  • Ensure wires going to the relay contacts are properly rated for the load current
  • Use caution when working with high voltage AC relay contacts
  • Allow relays to properly cool after prolonged activation before handling
  • Make sure the relay is securely mounted in its socket
  • Check for damaged or burnt relay coils and replace as needed
  • Verify the proper coil voltage before applying power
  • Be aware of high voltage transients that can occur when deactivating coils
  • Cover any exposed contacts or terminals for safety

Always exercise great care when working with relays. Make sure proper precautions are taken to avoid electrical hazards from high voltage, current, or temperatures.