What Is A Solenoid?



Often times when working on high amperage applications a solenoid is incorporated into the wiring system. This is due to their ability to control a large output of power with very little power input. But, what is a solenoid? Some would call them “relay switches”, but really they are just giant relays – not switches – because they cannot be manually actuated like toggle switches, for example. With the ability for a solenoid to carry such a large load, they are an option to consider when a relay is unable to handle the power load required for the application (i.e. snow plows).





A solenoid will have a different number of contacts based on its actuation and duty. In this example, we took an insulated continuous solenoid with SPST actuation (the most common actuation type):

In this instance, the two smaller contacts in the center on top are the control terminals for the coil. Essentially, these two smaller contacts for power and ground function the same as pins 86 and 85 on a relay.




Then, the two larger contacts (on either side of the smaller contacts) are what control the circuit accessory. These larger contacts act as pins 30 and 87 on a relay. Because this is a SPST solenoid, only one accessory can be activated within this circuit.

On the bottom, you’ll see the two mounting brackets which assist in – you guessed it – mounting. But these brackets also support grounding the system when using a non-insulated solenoid.  


Magnetic Field & Functionality

The magnetic field of a solenoid is created by energizing the coil inside. Essentially, if you open it up, you’ll see the coil wrapped around the core of the solenoid and the t-shaped shaft or “plunger”.



When the circuit is activated, the coil is energized by the power sent from the power contact. This energy moves the plunger head to connect the two larger contacts (power and ground). 


img_1916 and 1917_1.jpg

If you look inside a solenoid, when it’s energized you’ll see the plunger head get drawn in to connect the two large connects, as shown above. 


Once this connection is made, power is sent to the accessory. In most cases, this results in an ON activation state.


Things To Consider When Choosing A Solenoid

With solenoids, there are all sorts of choices to make. Let’s take a look at what some of the key areas to consider are when choosing a solenoid for your next build.


Grounded vs Insulated

Whether you choose grounded or insulated will largely depending on what you’re working on. If you are building your own application, the choice is up to you. With a grounded solenoid, you will have one less ground wire to run, which can make things easier. However, if you are replacing a solenoid in an existing application, most commonly you’ll want to replace grounded with grounded and insulated with insulated. The exception to this rule is if there is a problem with grounding the circuit. Let’s say there was originally a grounded solenoid being used, but because the mounting brackets are not flush to the mounting base or there’s significant corrosion the circuit isn’t grounding. In this instance, moving to insulated so you can add in a new ground wire may be the best option.

At a glance, it’s relatively easy to tell if a solenoid is insulated or grounded. When it’s grounded, it will not have the grounding contact sticking outside the housing (rather, it is inside), so only three terminals are exposed (two large & one small). If it’s insulated, you will see the grounding contact sticking out from the outside of the housing of the solenoid, so you’ll see four terminals (two large & two small) because you have to wire in the grounding wire.


Continuous vs Intermittent

When choosing between continuous or intermittent, it simply depends on how long your application will run. For example, to start a car there’s just a brief period of time when a surge of power is needed: when the key is initially turning in the ignition to get the motor to turn over. Since there’s only a short period of time that a strong in rush of power is required, intermittent would be best. However, if you have an application that requires significant draw for an extended period of time, choosing continuous is best. This would be for an application like a cooling fan, for example.

Here’s a rule of thumb when choosing between intermittent and continuous:


Intermittent  0-15 seconds of power at in rush amperage rating
Continuous  0-15 seconds of power at in rush amperage, then 15+ seconds at the steady amperage rating


The in rush amperage is basically the very maximum amperage to push through a circuit, and is typically just for the initial start up of the circuit. The other standard amperage rating you’ll often see is the steady amperage rating.

With continuous duties, a solenoid can stay on for long periods of time but can handle lower currents due to the length of time they remain on. Conversely, intermittent duties allow for higher current loads for much shorter periods of time.


So, that’s what a solenoid is, how they work, and how to choose one for your next build. Next time you start a job, consider a solenoid for those large power loads when a relay just can’t do the job. And if you have questions, give our Technical Support a call at 1.800.654.4757. 
img_1858 flipped.jpg

Posted in Relays & Power Distribution, Switches.