Since their invention in the 1930s, microswitches, small electromechanical switches, have been continuously developed to meet a variety of application requirements, including aerospace, home appliances, boiler control, medical equipment, sprinkler systems, testing equipment, timers, and vending machines. Microswitches are commonly used to detect temperature, position, and liquid level.
Precision microswitches typically come in three types: basic, with operating handle, and with both operating handle and housing. Microswitches employ a push-button and spring design.
Whether engineers are looking for thermostats for residential heating or industrial boiler control, or level switches for large oil tanks, they need to consider the following five key characteristics: dimensions, electrical requirements (voltage/current), reliability (mechanical/electrical life), environmental factors (hazardous environments, temperature range), and institutional certifications. The following five key steps can help you select the right microswitch for your specific application.
Step 1: Understand how the switch's dimensions affect other characteristics.
Size is important when selecting a microswitch. Switch size is directly related to its characteristics, including current range, travel, and operating force. For example, the smallest microswitch currently on the market measures 0.50 inches x 0.236 inches x 0.197 inches (L x W x H). While such tiny switches are suitable for detecting circuit conditions in compact circuit breakers, they typically only interrupt currents from 0.1 to 3 amperes (A) and have a short travel.
Applications requiring higher current typically necessitate larger switches. For example, in oil tank applications, the microswitch used to detect liquid level needs to provide a long stroke and withstand high current. Typically, in level switch applications, the switch directly drives the pump and carries a significant current. This necessitates a large microswitch with a rated current of 20A or 25A at 125VAC or 250VAC voltage.
Tip: The smaller the switch size, the smaller its travel and the smaller the current it can interrupt.
Additionally, the switch's dimensions also affect the operating force. Ideally, engineers need a switch with low operating force and high current capacity. However, in reality, a trade-off is required between these two parameters. To provide a large current carrying capacity while maintaining good contact, microswitches require stronger springs, which increases the operating force and size of the switch. Operating forces range from 2 grams in pressurized pneumatic applications to 8 ounces in solenoid valve applications (which require greater operating force). The largest switch on the market is 1.94 inches x 0.69 inches x 1.3 inches (L x W x H).
Engineers should also pay attention to the differential travel of the switch—the distance between the tripped and reset positions. The threshold value varies depending on the application. For example, in temperature switch applications, the on/off operating points should be as close as possible, with a differential travel as low as 0.0001 inches. However, in level pump applications, an excessively short differential travel can cause the pump to start and stop too frequently, shortening its lifespan.
Step 2: Understand your electrical requirements
Microswitches are typically capable of interrupting currents from 5mA/5VDC to 25A/250VAC. The microswitch product line offers a range of products from low-power to high-power load types, suitable for a variety of applications.
Engineers must know the rated current and voltage (AC or DC) required for a specific application in order to select the appropriate switch. Because various devices across all industries strive for low energy consumption, microswitches must be able to operate at low current (logic-level loads) and DC voltage.
However, some applications require switches capable of interrupting large currents and high voltages, such as industrial pump applications.
In addition to load requirements, engineers must also select appropriate circuits. Switch contacts must be either normally open (NO) or normally closed (NC). For NO contacts, no current normally flows between the contacts. When the switch is activated, the contacts close, and the circuit is completed. For NC contacts, current normally flows between the contacts. When the switch is activated, the contacts open, and the circuit is broken.
Step 3: Consider environmental conditions
Environmental requirements have a significant impact on the selection of microswitches, especially in high-reliability and critical applications such as industrial control and medical equipment. Understanding the environmental conditions of the application includes contaminants that may enter the switch from the air, the liquid in which the switch is located, and the required operating temperature.
For applications in harsh environments, you need to choose a hermetically sealed switch with a wide operating temperature range. Highly reliable microswitches can operate from -65°F (-54°C) to 350°F (177°C), easily handling a variety of applications. At the same time, ensure the switch has at least an IP67 protection rating to prevent liquid ingress. This eliminates the need to spend considerable time designing a housing that achieves the same level of protection.
Step 4: Determine the required service life
Switch reliability is critical. You need to determine exactly how many electrical and mechanical lifespans an application requires. Different switches have different contact materials, housings, and terminals to meet the electrical and mechanical lifespan requirements of different applications.
A highly reliable microswitch can operate 10 to 20 million times before mechanical failure and 50,000 to 100,000 times under maximum load before electrical failure.
Regarding the slight price differences between switches of different quality, you need to consider the total cost of use. This is because you're not only paying for the switch itself, but also for ensuring its long-term trouble-free operation. In many cases, the additional cost of using a more reliable switch can be offset by its lower warranty costs during use.
Step 5: Inspection agency accreditation requirements
Choosing switches that meet various international electrical requirements can help simplify product design for different regions. Some major standard certifications include UL certification in the United States, cUL or CSA certification in Canada, ENEC certification in Europe, and CQC certification in China.
When selecting a micro switch, you need to consider several factors, such as form factor, load requirements, operating environment, and reliability. Therefore, it is essential to understand the actual application requirements: What size switch is needed? What is the required rated power? Is it a low-voltage application? Does the switch need to be sealed? Does the switch need to withstand extreme temperatures? Does it require certification from international organizations? Is it a critical application? Once these questions are clarified, selecting the appropriate switch for the application becomes much easier.
