What’s a Solenoid?



I’ve worked in a factory for almost fifteen years, and hear the term “solenoid” tossed around a lot by maintenance. I have no idea what one is, so I smile and nod.

In: Engineering

It’s a coil of insulated wire that creates a magnetic field when a current is passed through it.


A solenoid is a piece of metal that’s wrapped in a coil (of a metal wire) through which electricity is passed. That whole contraption is called a solenoid.

A solenoid is a coil of wire that generates a magnetic field when an electric current runs through it.

They are probably using that word interchangeably with “electromagnet”

It is an electromagnet with a rod in it. When you apply power to the electromagnet it will move the rod which usually does something useful. For example actuating an electrical switch or a valve. It is a quite useful thing on machines as it allows you to control fairly big controls using electrical control signals which means you can easily automate the machines.

In your setting a solenoid is going to function similar to a relay. A solenoid valve for example is going to take a signal from the PLC and use it to open a pressurized air valve and send it to a pneumatic cylinder. The electrical portion and pneumatic valve portion come as a single unit.

It is an switch controlled by electric current that most often turns a valve on and off. This can be for liquid or gas. You have a coil of wire that when an electric current is applied to it becomes a magnet (electromagnet) which then interacts with the steel plunger in the valve sucking it up into the ‘on’ or ‘open’ state allowing the liquid or gas through the valve.

Depending on the situation, I prefer an electrically actuated ball valves for water applications because they are much more reliable than those 3/4″ sprinkler solenoids that seem to burn up in a few weeks to a few months of use.

Bonus explanation of the applications between a relay, solenoid, and a contact switch with an automotive focus: [https://info.waytekwire.com/blog/relays-vs-solenoids-vs-contactors-a-comparison](https://info.waytekwire.com/blog/relays-vs-solenoids-vs-contactors-a-comparison)

[This video](https://youtu.be/AvwBcu3e9O4) by Jeremy Fielding is a pretty good intro to relays, contactors & solenoids. It’s essentially an electromagnet that moves a core designed to push or pull things.

You know what a hydraulic piston is, right? You apply hydraulic pressure and the piston moves either in or out depending on the configuration? A solenoid is that, but electric instead of hydraulic. Also, they are almost always super tiny and super fast, compared with the hydraulic version.

They’re used all over the place in mechanical systems, like for pulling levers, changing gearing, actuating a clutch or a brake, etc.

A solenoid is a type of ‘linear actuator’. So is a hydraulic or pneumatic piston. It actuates a mechanical element or linkage in a linear fashion. (Compare ‘linear’ to ‘rotational’ as in a motor… which also has electric, hydraulic, and pneumatic counterparts.)

Edit: Anticipating further questions:
A stepper motor is a rotational actuator which can move an accurate amount given an accurate amount of energy provided the resistance to movement doesn’t exceed a threshold. (If it does, the controller will have no way to know the position of the stepper motor.) I’m only aware of these existing in electric form.

A servo is a rotational actuator that can move an accurate amount given an accurate amount of energy and can provide force feedback to a controller so it’s position can be determined if it’s moved by or fails to move because of an outside force. I understand that these have electric and hydraulic counterparts.

It’s a railgun with a stopper / break / or limitation somewhere.

An ELI5 that was explained to me, I realize this would not actually work it’s imaginary.

Imagine some not magnetic pipe (plastic), with a rod in the middle. Wrap a jumper cable all the way around with positive and negative hanging off either end.
The only thing magic about this is that the jumper cables you’re familiar with are not precise enough nor is the battery in your car strong enough at the scale I think I’m painting.
Pop either end of your jumper cables onto a battery and an electric field shoots instantly through the cable from the negative to the positive. At the same time, the magnetic fields around the wire overlap in the middle and push the rod out the top just launching it far away.

Putting the rod back, and swapping the sides of the cable (in a way just turning the pipe upside down) launches the rod into the ground.

That’s a cool gun but its no good for a solenoid. Give our rod a long + shape, and our pipe a shape like two brackets [ ]. The middle of our + gets caught on the edges of the brackets and doesn’t launch. It just stays stuck until the power is turned off or reversed.

There are a lot of ways to stop a solenoid from launching but I thought just running into the pipe was the most ELI5. This rod can be used to push and pull objects, gases, or whatever depending on the parts. I personally have used small ones to make an Arduino push buttons on remotes but remain modular. These are so small and have so little force that they do impact the pipe with a little bit of padding.