Back to the Basics: Understanding Industrial Buttons and Switches
This article features an in-depth discussion of the various options for buttons and switches, including function, styles, shapes, and illumination options for all kinds of control panel inputs.
Industrial Switches and Pushbuttons
In our modern daily lives, we all interact with buttons and switches. The switches on keyboards, the light switch in the kitchen, and the controls on the car radio all have similar properties and features. But what makes them similar yet different, and just how many types of buttons and switches are out there?
What’s Inside an Industrial Switch?
A simple switch is designed to control an electrical load in a closed circuit. That load could be a light, a motor, or even a heating element. The switching device will typically consist of a small metal actuator that moves in a vertical or horizontal motion which actuates the opening or closing of contacts.
When the contacts are closed, electrical current can flow to the load, but when the contacts are open, the current flow stops. Because the switch needs to open and close its contacts under load, there are some design considerations to be considered when selecting a switch for your application.
Figure 1. Control panels almost always contain some combination of switch or button inputs for users. Image used courtesy of Adobe Stock
Types of Switches
Switches come in a variety of shapes, sizes, and current ratings. Selecting a switch that is not rated for your circuit load could cause premature failure of the switch or even a fire.
Although there are many kinds of devices that ‘switch’ a signal on and off, this category is often called the rotary or quarter-turn switch, distinguishing them from buttons and sensors. Sometimes trade lingo can apply the switch label to many devices, which is just fine, but this discussion will reserve it for the rotating applications.
Some switches will only have an off-and-on position and are designed to remain in that state until the switch is operated again. These are called maintained, retentive, or latching.
There are also spring-return switches, called momentary, which have a rest normal position, then when actuated, they require the operator to hold the switch in that position. When released, the switch will return to the rest position.
Some varieties of switches have three positions with various combinations of maintained and momentary functions on each side of the center position. In the case of switches mounted directly to printed circuit boards (PCBs), rotary switches can have many more than three positions, but this is uncommon for traditional industrial switches.
Industrial switches have interchangeable contact blocks, meaning you can customize your switch for normally open or normally closed contacts. Depending on the make of the switch, contact blocks might be stackable to build a custom switch operation that can control multiple circuits. Common switches used for industrial automation are the rotary and rocker switches, while toggle switches are commonly used in automotive applications.
Figure 2. A lineup of typical industrial selector switches. Image used courtesy of Omron
Types of Pushbuttons
A pushbutton is similar to a switch in that it’s designed to control electrical circuits by opening or closing contacts with the use of an actuator. The difference is the direction in which the actuator is moved.
With a pushbutton, the face of the device is operated with a push in an inward direction toward the contacts. The actuator rides on grooves in the body of the button, which reduces the number of moving parts within the button.
Just like switches, industrial pushbuttons can have stackable contacts, which add to the functionality of the pushbutton. Typically, a pushbutton function is momentary, meaning as long as the pushbutton is actuated only until the pressing force is removed.
Some pushbuttons have a maintained or latched function that keeps the contacts closed even after actuation and, when pressed again, will return to the open state. These often-called on-off buttons have internal components that keep the contact blocks in their actuated state until the pushbutton is pressed again.
Figure 3. Metal-cased industrial push button. Plastic cases are also very common. Image used courtesy of Schneider Electric
Shedding Light on Simple Control Devices
Knowing when a button or switch is activated is vital in automation. An operator should be able to know the state of a button by simply glancing at the button. For this reason, some switches and pushbuttons have illumination options.
Every button or switch manufacturer approaches illumination a little differently, but typically industrial buttons and switches will have the option to purchase with a small LED bulb or incandescent bulb mounted inside the switch body. This bulb aligns with the button cover, and when the current has been applied, the face of the button will light up. Different colored button faces can be purchased to illuminate different colors.
Some manufacturers allow for the light bulb to be easily replaced, while others (although more rare) only offer non-serviceable arrangements where an entirely new module must be purchased.
Illuminated switches work in the same manner. The indicator can be wired in series with one of the contacts to give an indication of the button press, or it can be wired directly to a control system if you want the light to flash or remain energized after the button is released.
Figure 4. Illumination in a switch or button is usually found in the base of the module (left) with the top operator cap containing the clear or colored faceplate assembly. Image used courtesy of Mouser
Figure 5. This button unit includes a contact block of the light. Image courtesy of Grainger
Make-Before-Break and Break-Before-Make
In some high-voltage applications, the switch needs to move the contacts in a quick manner to prevent arching. Panel disconnect switches work in this fashion. As the actuator is moved from the off to on position, a spring will assist the actuator and prevent partial movement of the contacts.
When your application requires moving an electrical supply from one circuit to another, a special switch called make-before-break (MBB) is used. This switch will make the new connection before breaking the original connection. These switches are typically used in power industry applications where power needs to be applied to a circuit and disconnected from another circuit.
Another special switch is the break-before-make (BBM). This switch works in the opposite function as the MBB switch. The first connection is broken before the second connection is made. Again this style of switch is typically found in power industry applications, but there might be low- or medium-voltage applications where this style of switch can be used.
Don’t Overlook the Mighty Switch and Button
Without switches or buttons, life would be very different—electrical circuits would not be possible because there would be no way to control the circuit. While buttons and switches work in a similar fashion, they have different roles in automation.
Some might argue that HMIs are replacing physical buttons and switches with digital equivalents. There might be some truth to this in some cases, since the price and overall size of HMIs have greatly reduced, but some customers still prefer the tactical feel and reliability of the industrial switch or pushbutton for simple user interface panels.