5 steps to select a basic switch

A precision snap-action switch, commonly used to detect temperature, position, and liquid levels, is typically available in three models. Here are five steps to select the right basic switch for an application.

06/23/2013


Honeywell Micro Switch snap-action switches are available in many configurations to meet an engineer's specific requirements. Courtesy: Honeywell Sensing and ControlA precision snap-action switch is typically available in three models. 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 basic switches have existed since the 1930s, and this small-form-factor electromechanical switch has evolved over the years to meet the requirements of a variety of applications including aircraft, appliances, boiler controls, medical devices, sprinkler systems, test equipment, timers, and vending machines. Snap-action switches are commonly used to detect temperature, position, and liquid levels.

Whether engineers are looking for a temperature controller in residential baseboard heating or industrial boiler controls, or a level switch in a large oil tank, they need to consider five key switch specifications: physical size, electrical requirements (voltage/current), reliability (mechanical/electrical life), environmental concerns (hazardous environments, temperature range), and agency approvals.

Here are five steps to select the right basic switch for your application.

Step 1: Understand how a switch's physical size impacts other characteristics

Size matters when selecting snap-action switches. Switch dimensions directly relate to other device characteristics including current range, travel and operating force. For example, one of the smallest snap-action switches available in the market measures .50 in. x .236 in. x .197 in. (LxWxH). While this tiny switch may be a good choice for a compact circuit breaker to detect the status of the circuit, it typically handles between .1 to 3 amps (A) and features a short travel.

Applications requiring higher amps could require a larger switch. As an example, in oil tank applications where the snap-action switch is used to detect the level of liquid, the switch needs to offer a longer travel and higher current. Typically, in level switch applications where the switch is directly driving a pump, the switch needs to handle a lot of current. This requires a large basic switch with ratings of 20 A or 25 A at 125 V ac or 250 V ac.

Tip: The smaller the switch, the shorter the travel and the less current the switch can handle.

The physical size of the switch also impacts operating force. In an ideal world, engineers look for switches with a low operating force and high current capacity. But, there is a tradeoff between these two specifications. To provide a high current range and still maintain good contact, the snap-action switch needs more robust springs, which translates into a higher operating force and a larger switch. Operating force can range from 2 g for plenum air-movement type applications to 8 ounces for applications such as solenoids that require a high operating force. One of the largest switches in the market measures 1.94 in. x .69 in. x 1.3 in. (LxWxH).

Engineers also need to pay attention to differential travel – the distance between the switch's trip and reset position. Threshold levels vary depending on the application. For example, in temperature switch applications, the on/off operating points should be as close as possible, requiring sensitive differential travel as low as 0.0001 in. However, in liquid-level pump applications, for example, too tight of a differential may cause the fill pump to cycle more often, shortening the pump's lifetime.

Step 2: Know your electrical requirements

Snap-action switches can typically handle from 5 mA at 5 V dc up to 25 A at 250 V ac. A snap-action switch line that offers many options from low energy to power-duty electrical ratings allows these switches to be used in more applications.

Engineers must know the rated current and voltage (ac or dc) of the application to select the right switch for the job. Because there is a big push to lower energy consumption of a variety of equipment across all industries, snap-action switches should 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, such as industrial-grade pump applications.

In addition to load requirements, circuitry must be selected. Switch contacts are either normally open (NO) or normally closed (NC). With NO contacts there is no current flow between the contacts. When the switch is activated, the contacts are closed and the circuit is completed. With NC contacts, there is current flow between the contacts. When the switch is activated, the contacts are open and the circuit is broken.

Step 3: Consider environmental conditions

Environmental requirements can play a big role in the selection of snap-action switches, particularly in high-reliability and critical applications, such as industrial controls, medical devices, and military equipment. Understand the environmental conditions of the application, including contaminants in the air that could potentially get into the switch, fluids the switch will be subjected to, and operating temperature requirements.

For harsh environment applications, look for switches that offer a wide operating temperature range and are environmentally sealed. A highly robust snap-action switch will operate between -65 F (-54 C) to 350 F (177 C) to handle a variety of applications. Also, consider switches that meet at least IP67 to protect against the ingress of liquid. This could eliminate a lot of time spent on designing an enclosure to achieve the same protection level.

See steps 4 and 5, on the next page


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