Electronic Design

Snap-Action Switches Still Rule The Roost

Snap switches were created nearly 80 years ago to make chickens happy. The first snap-action precision switch was developed in 1932 for chicken brooder temperature control, the same year Charles F. Burgess, owner of Burgess Laboratories, secured a contract for the manufacture of 10,000 chicken brooders.

Historically Speaking

Burgess Laboratories developed a precise snap-acting switch mechanism that was designed so the switch plunger’s very short travel stored energy in a spring. The energy was used to transfer the moveable contact with a positive snap. The switch provided the stable operating characteristics required for the temperature control in the chicken brooder (Fig. 1).


1. Snap switches are used to help ensure the correct temperature to make chickens comfortable and to optimize their egg production.

Later, Burgess formed Micro Switch Corp., which Honeywell International bought in 1950. Through the years, snap-acting switch technology has been enhanced and perfected, but the same basic principles remain in today’s versions.

Basics Of Operation

According to the National Electrical Manufacturers Association, a precision snap-acting switch is “a mechanically operated electric switch having pre-determined and accurately-controlled characteristics, and having contacts where the maximum separation between any butting contact is 1/8 inch.”

A precision snap-acting switch consists of a basic switch used alone, a basic switch used with an actuator or actuators, or a basic switch used with an actuator or actuators and an enclosure. Snap-action technology utilizes a plunger and spring. Pent up energy increases in the spring as the plunger is depressed. When the plunger can’t hold it anymore, the energy is released through the movement of the spring.

More specifically, with the plunger in the completely released or free position (FP), the common contact is against the normally closed (NC) contact. In this condition, the NC circuit of the switch can carry current. This means there is electrical continuity between the common terminal and the NC terminal. The common terminal is electrically insulated from the normally open terminal.

As the plunger is depressed, it reaches the operating point (OP). The distance between the FP and the OP is called the pre-travel (PT). At the OP, without further movement of the plunger, the common contact accelerates away from the NC contact. Within a few milliseconds, the common contact strikes, bounces, and comes to rest against the normally open contact.

Because the mechanism is designed for “snap-action,” the common contact cannot stop partway between the NC and normally open contacts. The NC circuit is now open and the normally open circuit is closed. As the plunger is depressed past the OP, the normally open circuit remains closed and the NC circuit remains open.

Variations of snap-action switches are used in a multitude of applications, from chicken brooders, baseboard heating, boiler controls, furnaces, whirlpool tubs, and commercial soda machines to missiles, submarines, computers, and medical devices. The uses for this technology truly do run the gamut. Anyone looking for precision measurement would be remiss if they discounted this proven technology, which is nearly eight decades strong.

Perfect Measurement Apps

The measurement conditions for snap-action switches are temperature, pressure, and position detection. In turning a circuit on or off, the average precision snap-acting switch changes its resistance by about 13 orders of magnitude in a couple of milliseconds, when its plunger moves about 0.001 in. This same switch can control a 15-A, 240-V heater or the millivolt-milliamp circuit of a receiving antenna.

These devices can be used in very high-precision, high-repeatability of position, temperature, and pressure switch applications. The ability to switch high voltage and current means they can control high loads directly, like motors and actuators, without additional components such as relays.

Temperature is the most common sensing measurement. Temperature regulation is important for comfort in some applications and vital in others, such as chicken brooders. Switches are usable in thermostat controls in both residential and industrial sectors. They are deployable in baseboard heating, boiler controls, and furnaces, automatically turning the heat on or off depending on temperature settings.

Pressure sensing is the second most common measurement. Strain gauge snap-action switches are used to detect changes in hydraulic pressure. In addition, residential furnaces often use switches to prevent potential hazards. For example, switches can detect and measure back pressure, which could be caused by various obstructions in the tubing such as a bird nest. That pressure would shut the heat down before a fire results or carbon monoxide is vented back into the home.

Position measurement is the third most common use, particularly in microwave ovens. There are three or four switches on the door mechanism alone, and one cuts the power if the door opens, protecting the user from radiation. Riding lawn mowers use an interlock switch in the seat to sense the rider. Another switch engages and disengages the clutch. Both are critical for user safety (Fig. 2).


2. Micro Switch snap-action switches are designed for presence and absence detection where physical contact with an object is permissible. They also excel in applications requiring simple on-and-off actions.

Advantages

Snap-action switch technology’s performance has been field-proven. Its simple operating premise has survived decades of technological improvements and demanding environments. Why? Simply, because it works. Therefore, when you’re choosing a switch for an application, you should unquestionably consider snap-action.

Along with a proven track record, the technology offers a long mechanical life. Snap-action switches last longer than many alternative technologies due to a simple yet rugged design. The switches also can be designed with various materials and casings, which allows for flexibility and customizability as well. Some switches are waterproof or completely sealed within the chosen casing materials. Switches aren’t sensitive to most temperature fluctuations or electronic interference. And, they don’t need auxiliary power to run them.

It’s important to understand the capabilities of snap-action. These switches are open or closed. There’s no in between. If your application requires more comprehensive or continuous measurements, consider a sensing product.

Switches are the essential building blocks of any application. But what’s inside the switch can make a difference in the application’s success or failure. Consider the switch, its construction, and the vendor carefully when determining your application needs and requirements.

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