Relays and Solenoids: Electromechanical Devices

As automation expands, mechanical functions are relying more and more on electronic control. Solenoids provide an electromechanical interface for many of these applications. In addition, many electronic loads are often switched in and out. Relays are a form of solenoid that switches electronic loads. This blog is an introduction to relays and solenoids for the beginner with a few stories about the experiences I’ve had with each. The details of relays and solenoids are well explained in the references. There are also a number of instructional videos that can be found by searching “relays” and “solenoids” on YouTube.

A solenoid is basically an electromagnet that is created by applying current to a coil of wire. In physics we learn the “right hand rule” that determines the direction of the magnetic field when current is flowing in a coil of wire. By bending the fingers in a wrapped coil fashion, the magnetic field would be flowing out of the tip of an extended thumb when the current was flowing out of the wrapped finger tips.

The 'right hand rule'

The 'right hand rule'

When inserting an armature inside of the coil, the magnetic force will push the armature out towards the thumb when the coil is energized. The amount of force depends on the number of turns of the coil windings as well as the amount of current running through the coil. This force is used for all kinds of loads including engaging engine starting motors, power door locks, and moving valves such as those encountered at a soda machine. That “click” that you hear is actually the armature slamming against the valve as the magnetic force is applied.

There are actually two forces in a solenoid action. The first force is the force required to move the armature. The second force is a force that holds the armature in place. Because most solenoids have a spring holding the armature back, the electric force must be powerful enough to overcome the spring force and move or hold the armature in place. Of these forces, the holding force is less than the initial force that is applied to start the armature in motion.

Recently, I’ve experienced engine stalling in a diesel engine due to poor armature holding current. A solenoid that is used to control the fuel valve is failing by not holding the fuel valve open. The solenoid also prohibits starting by not pushing the valve open far enough. The latter failure is actually due to both a solenoid and a relay failing. A look at the wiring harness explains how the failure came about. When starting the vehicle, the solenoid is activated to open the fuel valve. This requires more power than the current used to hold the valve open to keep the engine running. The additional starting power is switched in by a relay. The combination of the relay contacts arcing and becoming more resistive along with the connectors also dropping voltage and a ‘hokey’ ground system has lowered the voltage at the solenoid to a point where it isn’t pulling the valve out far enough to start the engine. The result is fuel starvation when starting. The top photo (below), of the connector, with the blue markings shows three wires on the solenoid side of the connector:

  1. A red holding wire
  2. A white armature initiation wire
  3. A black ground wire

The top photo (above), of the connector, with the blue markings, shows three wires on the solenoid side of the connector

The top photo (above), of the connector, with the blue markings, shows three wires on the solenoid side of the connector

Note, however, that the green holding wire on the wiring harness side is much smaller in the bottom photo. The holding current is designed to be less at about 10.7 volts compared to the starting current at 12 volts. However, there is too much voltage drop in each current path, thus causing the problem while starting and running. Replacing the solenoid and relay might solve the issue. If not, reducing the voltage losses in the harness might be the solution. Finally, the ground wire runs through the harness which also adds additional drops. A more direct path might be the answer.

Relays are solenoids that move a set of contacts in order to switch a circuit in. Relays are used in abundance in automobiles. As loads increase, the current paths are kept local instead of running all the way to switches and control signals under the dashboard. Thus, relays take advantage of low level signals to power higher current loads.

Relays are often a lower cost choice than a semiconductor switch and associated support components. Relays are also isolated which means the switch circuit can reference a separate ground or the switched contacts can perform high side switching of the voltage. For automotive applications, relays offer blade terminals with an enclosed case that often has a mounting tab. This makes for easier plug-and-play versus a semiconductor circuit that requires isolation, a way to protect and mount the circuit board, and terminations to the semiconductor packages or circuit board traces.

A View of the Internals of a Relay Including the Contact and the Coil (From a tutorial on

A View of the Internals of a Relay Including the Contact and the Coil (From a tutorial on

In addition to voltage isolation, relays are a way to switch in large amounts of current including that for motors and other loads. As a result, there can be arcing of the switches especially for capacitive loads that have large initial current spikes. Contact failure is a common problem in relays. In extreme instances, the contacts actually weld themselves together into place. This occurred one time in a set of off-road lights I had installed on my truck. I had to pull off the freeway and unplug the lights in order to stop blinding oncoming traffic and burning holes in the heads of the drivers in front of me.

Another form of relay is the reed relay. Reed relays are oriented as an axial design where the contact also provides the spring tension that creates an open circuit. A coil is energized creating a magnetic field that closes the contacts. Reed relays carry less current than a standard electromechanical relay yet they switch faster.

 Reed Relay Diagram (Image courtesy of National Instruments white paper3)

Reed Relay Diagram (Image courtesy of National Instruments white paper3 )

Relays have lifespans that are related to the amount of cycles that the relay experiences. The more the contacts are opened and closed, the more arcing occurs. This lowers the lifetime of the relay. The following table shows the comparison of relay switching speeds, life expectancy, and current carrying capacity.

Comparison of Relay Types and Characteristics, from 'How to Choose the Right Relay3', National Instruments

Comparison of Relay Types and Characteristics, from “How to Choose the Right Relay3 ”, National Instruments

Switching relays were once prominent in voltage regulator applications for automobiles. The relay contacts were pulsed on and off to average the voltage coming from the alternator. For this reason, many classic vehicle owners transition their alternator to the more reliable solid state versions. Not only were the relay-based voltage regulators unreliable, they required a laborious adjustment procedure. In addition, many of the voltage regulator cases were conductive metal that had to be isolated from their metal mounting locations using rubber grommets. When the grommets dried out, cracked, and aged, shorting would occur in the regulator causing many a headache.

Relays, an option for load switching, have survived into the era of integrated semiconductors. They remain a viable alternative for many applications. Solenoids are appearing in more and more applications as the world becomes controlled electronically. I hope you enjoyed this introduction to relays and solenoids, electromechanical technologies.


  1. Relays, US Navy Crane Center document
  2. Magnetic force and field – Questions” web page
  3. How to Choose the Right Relay”, National Instruments white paper, publish Date: Aug 18, 2017
  4. Relays”, by Chris Woodford. Last updated: April 23, 2017

3 comments on “Relays and Solenoids: Electromechanical Devices

  1. AubreyKagan
    February 27, 2018


    Great introduction to the subject.  However I feel compelled to say that the National Instruments' table shown is so simplified as to be close to misleading. For a start there is no definition of a “Semi-conduct relay”, but there are many optically isolated SSRs (solid state relays). They say that reed re;ays will switch 10KV, but that is for a small minority of reed relays available. There are probably as many electromechanical relays with the same rating. And there is no mention of switching capabilty for AC.

    It does not mention that it is possible to generate zero-crossing switching with solid state relays (but then as I said, switching AC is not covered in the table)

    Another thing not mentioned is that switching electromagnetic devices results in back EMF which can in fact damage the driver, to say nothing of creating an arc that can interfere with the operation of sensitive electronics. This latter point is also true on the contact side of the relay.

    I discuss back EMF and its suppression in my blog “Back EMF and snubber”

    The table also refers to “power consumption” of the relays. It takes a certain amount of current to initiate an electromagnetic device as it overcomes inertia. Once activated it is possible to back off the holding current sigbnifcantly. I write about that in my blog “Saving power with relays and solenoids”



  2. AubreyKagan
    February 27, 2018

    Newcomers to relays would benefit from Jon Titus’ article for Sealevel Sytems “Control System Basics – Relays Explained”

  3. michaelmaloney
    September 25, 2018

    Thanks for sharing such a comprehensive piece of article which would definitely be helpful to those who are trying to lay out a setup on their own. However, I feel that there is a certain level of danger for interested parties to simply refer to texts and graphics alone before getting their hands on a real project. It would be a huge opportunity should an expert in the field like yourself to show a more in-depth explanation with hands-on experience to be gained by anyone keen. Workshops make a great platform for such a lesson to be conducted in a safe environment especially for newly interested individuals.

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