Selecting a basic snap-action switch

BY DAN HERZOG, Sr. Product Marketing Manager, Honeywell, www.honeywell.com

Basic snap-action switches have been around in one form or another since the 1930s, when the first precision snap-action switch was developed for temperature control in a chicken brooder. The small-form-factor electromechanical switch has come a long way over the past 80 years. They are designed into a variety of applications including aircraft, appliances, boiler controls, medical devices, sprinkler systems, test equipment, timers, and vending machines.

Honeywell's MICRO SWITCH snap-action switches, designed for presence/absence detection and on/off actions, are available in millions of configurations to meet a designer's specific requirements. 

In general, a precision snap-action switch comes in three flavors. It can consist of a basic switch alone, a basic switch with an actuator(s) or a basic switch with an actuator and an enclosure. The snap-action comes from the plunger and spring design. Snap-action switches, double throw (SPDT), normally open (SPNO) or normally closed (SPNC), are commonly used to detect temperature, position and liquid levels.

Snap-action switches are used for many applications including temperature regulation.   One of the earliest uses (back in the 1930s) was in chicken brooders to control the temperatures to keep chickens happy and healthy. Some of those early switches are still in use decades later!   A slightly more mainstream use is for thermostat control where the switch is used to automatically turn heat on or off in residential baseboard heating or industrial boiler controls and furnaces, depending on the temperature settings. (See “SIDEBAR: From chicken brooders to industrial systems“)

Snap-action switches can also be used for level float detection in large oil tanks, where it's used to turn a pump on and off when reaching a set position level. But whether a designer is looking for a temperature controller in a chicken brooder or a level switch in a large oil tank, he or she needs to consider five key switch specifications: physical size, electrical requirements (voltage/current), environmental concerns (hazardous environments, temperature range), reliability (mechanical/electrical life), and agency approvals. These are all key elements to ensure a reliable switch that continues to operate when it's required. 

Physical size

The size of the snap-action switch needs to fit the application. The switch dimensions directly relate to other characteristics including travel, operating force and current range.  

For example, one of the smallest snap-actions available in the market measures 0.50 in. x 0.236 in x 0.197 in. (L x W x H). This tiny switch may be used in a compact circuit breaker to detect the status of the circuit. These switches typically feature a short travel and don't require much current. They can typically handle between 0.1 to 3 A. 

However, if the design calls for a snap-action switch to detect the level of liquid in a large oil tank, the design needs a larger switch with longer travel and higher current. Typically, in level switch applications where, say, the switch is directly driving a pump, the switch needs to handle a lot of current. A designer should look for a large, basic switch with ratings of 20 or 25 A at 125 or 250 Vac. One of the largest switches in the market measures 1.94 in x .69 in. x 1.3 in (L x W x H). 

The rule of thumb is, the smaller the switch, the shorter the travel and the less current the switch can handle. For example, switches smaller than the Honeywell V-Switch series typically have shorter plunger travel and lower current ratings. The V-Switch series size, and larger, can have any plunger travel length required and can handle current ratings up to 26 A. 

In addition, operating force and differential travel are impacted by the physical size of the switch. Designers are ideally looking for a low operating force with a high current capacity. But, in reality, there is a tradeoff between these two specifications. In order to provide a high current range and still maintain good contact, the snap-action switch needs more robust springs, which translate into a higher operating force. Operating force can range from 2 g for plenum-air-movement types of applications to 8 oz for applications involving solenoids  requiring a high operating force. 

Differential travel is the distance between the switch's trip and reset position. Ideally, a temperature-switch designer wants the on/off operating points of the switch to be as close as possible. Sensitive differential travel can be as low as 0.0001 in.  However, s ome designers need more differential; in the case of a liquid-level pump application, too tight of a differential would cause the fill pump to cycle more often shortening the pump life. Applications have different thresholds that they can handle.  

Electrical requirements

Switches must be selected based on rated current and voltage (AC or DC) to ensure the correct power rating for the application. Today, there is a big push to lower the energy consumption of a variety of equipment across all industries. This means snap-action switches need to be capable of operating at low currents (logic level loads) and DC voltages.  

However, there will always be a need for switches that can handle high current and high voltage. A snap-action switch line that offers a choice of low energy or power-duty electrical ratings allows the switch to be selected for use in a wider range of applications. Honeywell's MICRO SWITCH snap-action switch family, as an example, can handle from 5 mA at 5 Vdc up to 25 A at 250 Vac. 

Circuitry goes hand in hand with load requirements. Designers need to determine if the circuit needs a double throw (SPDT), normally open (NO) or normally closed (NC) switch. NO means the switch is not activated in its rest position, and there is no current flow. When the switch is activated, the contacts are closed and the circuit is completed. NC switches are activated in the switch position, and when activated, the contacts are open and the circuit is broken.

Environmental concerns

Meeting environmental requirements can be one of the biggest design challenges particularly in high-reliability applications including industrial controls, medical devices and military equipment. Designers need to understand the types of processes being used in the application including what contaminants are in the air that could potentially get into the switch, what fluids the switch will be subjected to, and whether the switch needs to meet extreme temperature ranges. These are all critical factors particularly where safety concerns are involved. 

Because snap-action switches can be used in a variety of harsh applications, such as golf carts or placed near an industrial oven, these switches should provide a wide operating temperature range. A highly robust snap-action switch will be able operate between -65 ºF (-54 ºC) to 350 ºF (177 ºC).

For harsh environment applications, look for switches that are watertight and meet the IP67 standard (Fig. 1 ). The IP code, part of the IEC60529 standard, specifies the degree of protection for enclosures to protect against the ingress of solid foreign bodies and liquid. This eliminates a lot of time spent by an engineer to design an enclosure to achieve the same protection objective.  

Fig. 1; Watertight basic switches work well because they are space-efficient miniature switches with excellent protection against water and dust. The size and construction of these switches makes them ideal for many applications because they can be adapted to various shapes, geometries and configurations.

The more capable the switching element, the more efficient the design and the less the engineer has to design around limitations. Make sure the switch is robust and highly flexible to enable as many applications as possible. 

Reliability

Snap-action switches generally offer a long mechanical and electrical life due to their rugged design. Since these switches can be designed with various contact materials, casings, and terminals, they are available in a wide range of electrical and mechanical ratings.

Generally, the choice of electrical and mechanical life cycles should be based on the application. For example, in applications where the cost of failure is low, electrical and mechanical life cycles don't need to be as robust. In these applications, the cost of replacing the switch will be less than the cost of the switch's failure. And vice versa for higher cost-of-failure applications.  

A highly reliable snap-action switch typically provides 10-20 million mechanical life cycles and 50K-100K electrical life cycles at full rated load.

Agency approvals

Check for agency approval requirements for your products. Switches should meet a variety of global electrical requirements. The basic ones include UL in the United States, cUL or CSA in Canada, ENEC in Europe and CQC in China.

In addition to the five technical considerations, designers need to consider basic snap-action switch lines that are based on a development platform approach. By providing a variety of options and versions, designers often find that they can save time and cut costs, particularly when they need to make changes to their original design. 

Picking a supplier   

When looking for a supplier, one thing to consider is whether their snap-action switch lines are very scalable. Does the supplier offer a variety of terminal options — such as quick-connect, PCB, solder, and screw as well as wired versions — and actuator interfaces including pin plunger, straight levers (short or long), roller levers, and custom levers? In addition to a full range of terminal and actuator options, their switches should provide a choice of operating forces, electrical ratings, and configurations to allow a designer to mix and match as many different characteristics as possible to meet their product requirements. 

In summary, there are several factors, including load requirements, operating environment, and physical size, to consider when choosing a snap-action basic switch. First, understand the application. Ask the following questions: What size switch is needed; what is the power rating for the application; does the switch need to be environmentally sealed; does the switch need to withstand high temperatures; are agency approvals required, and is it a critical application. 

Switch reliability is, of course, is a critical consideration. You must determine the maximum number of cycles of electrical and mechanical operations an application will require. A high-reliability snap-action switch can operate up to 10 to 20 million cycles before mechanical failure, and up to 50 to 100 thousand electrical cycles under maximum load before electrical failure. Take into consideration that the price differential between a high- and low-quality switch can be pennies. Designers should keep in mind total cost of ownership — it's not just the cost of the switch, but also the cost of potentially expensive equipment failure later.

Related Products: Electromechanical Switches