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Mechanical Design

An Introduction to Solenoids

Like cavemen did thousands of years ago with fire, you might have just struck upon an incredibly revolutionary idea. As you coin down your concept and think of ways to make it as simple as possible, you’ll probably come across mechanical functions that require linear action. Energy can be converted into linear motion by an immediate action, but how can an instant conversion of energy to motion be achieved?

Don’t panic! Instead, thank Mr. Ampere for inventing solenoids. If you’re not familiar with solenoids, this article is meant to illustrate their working principle, different types, and basic selection criteria.

What are solenoids?

Solenoids are simply coils of wire that, when electric current is applied, create a magnetic field that exerts a force over a metallic element. This force can be determined by the following equation:

F = (N*I)2 μ0 A / (2 g2)

μ0 = 4πx10-7

F is the force in Newtons

N is the number of turns

I is the current in Amps

A is the area in length units squared

g is the length of the gap between the solenoid and a piece of metal.


As the magnetic field is created instantly, solenoids make a great fit for applications that require immediate action. Their simplicity makes them inexpensive, and they can pack a lot of power into a very small form factor. However, since their force is proportional to the position of their moving elements, they work best for short distances, making them ideal for valves, interlocks, electromechanical relays, etc.

Impact of solenoids on industry

Many applications benefit from the simplicity of solenoids to automate complex tasks. Just inside a car you might find plenty of them: a couple in the transmission used to change between gears, a few used in the door locks, another at the ignition circuit, etc.

As a matter of fact, almost every industry has been powered by a solenoid at some point: Pick-and-place robots in manufacturing process lines use solenoids to open and close clamps to handle products; gates at storage silos are driven by pneumatic or hydraulic cylinders; and some health monitoring systems in emergency rooms use solenoids to displace critical fluids, such as blood or medicines, into patients’ bodies.

Main applications of solenoids

Electromechanical relays

When a solenoid is setup to actuate switch contacts when its coil is energized, it is called an electromechanical relay. Relays are useful to isolate control power from the load power. That click you might have heard coming from the thermostat of an air conditioning unit is a relay closing the circuit that turns on or off the unit. If you’ve been on the road during sunset or sunrise, you might have wonder how all those lights are turning on or off remotely—they’re operated by relays.

Mechanical interlocks or operators

If you’ve ever walked into a building where you had to wait until someone remotely unlocked the door for you, most likely, a solenoid was keeping the door locked. Like the door locks in a car, when you press the lock button, and electrical current flows through solenoids that lock the doors, keeping them from opening. Some schools still use a solenoid to hit the bell that frees children for recess, and some fire alarm systems are based on solenoids.

Pneumatic and hydraulic solenoid valves

Pneumatic and hydraulic actuators are found in many industries. They are preferred over actuators that only use electricity as their main power supply because they require less maintenance and are cheaper to install and operate. Sometimes, some control logic is even embedded into pneumatic/hydraulic circuits through solenoid valves with different configurations.

Solenoid pneumatic and hydraulic valves are classified by the amount of ports where air can flow through the valve, known as ways; the amount of positions the valve has; and whether the position remains after action, or if it returns to a certain position. They are sized by a flow factor, which refers to the amount of force required to exert and can be determined by the following equation:

Cv = Q x (SG/dP)^0.5

Cv is the flow factor of the valve

Q is the flow [GPM]

dP is the pressure drop across the valve (inlet minus outlet pressure) [psig]

SG is the specific gravity of the medium (fluid or gas)

Typically, a hydraulic/pneumatic actuator’s data sheet will specify the amount of pressure and flow required to properly operate it.

In pneumatic and hydraulic circuits, there are times when the forced needed to be exerted by the solenoid is too big (for example, to open or close the water supply pipe of a whole neighborhood).

In these cases, rather than having the magnetic field act on its own to perform the action, in what is known as a direct circuit, a membrane is used to take advantage of the fluid. The solenoid only acts over a small portion of the fluid, known as pilot, to control a pressure differential that uses the same medium to open or close the membrane. This is known as indirect control.

example of indirect control


Types of solenoids

Solenoids can be found indistinctively in any application, as their types are usually defined by the following pros and cons:

Linear Solenoid (also known as tubular solenoids when encased in a metal protection): can push or pull a plunger.

AC Laminated


  • Great amount of force

  • Longest stroke

  • More efficient


  • Produce a fair amount of heat

  • High inrush


DC C–Frame


  • Lower inrush

  • Stroke can be more controlled


  • Less efficient (without special control circuit)

  • Shorter stroke than AC solenoids

  • Less force than AC solenoids


DC D–Frame


  • Lower inrush

  • Stroke can be more controlled


  • Less efficient (without special control circuit)

  • Shorter stroke than AC solenoids

  • Less force than AC solenoids but more force than C-Frame DC solenoids


Rotary Solenoid: can create a non continuous rotary motion of the plunger. This type is used when a rotary motion of the plunger simplifies mechanical design.

Designing the electrical circuit for solenoids

Even though solenoids are fairly simple to power, some key concepts need to be kept in mind. Whenever you’re designing the electrical circuit for a solenoid, you need to identify the inrush and holding amperage in the manufacturer's datasheet. The current constantly consumed while the solenoid is being actuated is known as the holding amperage, while inrush current, or pull-in current, refers to the current required to actuate the solenoid from its initial to its final position.

Depending on the coil size, the inrush current can be much greater than the holding amperage. The bigger the difference between those amperages, the bigger the force exerted by its magnetic field. Inrush current typically doesn’t last long enough to burn a time-delay fuse or to trip a breaker; however, electricians usually pick fuses or breakers twice or three times the size of the holding amperage of a solenoid.

Main Takeaways

Solenoids are great solutions for controlling valves and electromechanical switches or mechanical interlocks. Their simple operating principle and instantaneous response makes them ideal for applications that need to pack a large amount of power into a small space, while being quick, consistent, and robust. Now, it’s time to put your creativity to work and take advantage of magnetism to power your next invention.

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