6 Factors to Consider when Selecting an Electro ...

Limit switches are used to convert a mechanical motion into an electrical control signal. The electrical signal activates when a cam engages the limit switch lever or plunger which makes or breaks an electrical contact inside the switch. This electrical control signal is then used to limit position or reverse the machine travel, or to initiate another operating sequence. It can also be used for counting, sorting or as a safety device. 

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Typical limit switch applications are in the control circuits of solenoids, control relays and motor starters which control the motion of machine tools, conveyors, hoists, elevators and practically every type of motor driven machine. 

Experience has shown that most limit switch failures are the fault of the installation. In some cases, an installation that is not perfect cannot be avoided, but most cases prove that proper application of the limit switch would have prevented failure.

Selection of the proper electro-mechanical limit switch for an application generally breaks down into two major decisions, choosing the proper actuator (lever) and choosing the proper enclosure. There are other considerations, such as what operating sequences are available, temperature rating and electrical rating. 

Before selecting a limit switch, these 6 factors should be considered:
1. What is the voltage and current that will be switched?
2. How many contacts are necessary? Are the contacts: Normally Open? Normally closed?

3. What is the environment like? Relatively clean? Dusty, dirty, smoky, wet, hot, cold, etc.?
4. How close can you get to the target?
5. Where should the limit switch be mounted?
6. What is best way to activate the switch? Lever type? Plunger type? Cat whisker? Other?

There are several general categories or classifications of limit switches on the market, the most common are electro-mechanical switches, non-contact proximity switches and photoelectric switches. The electro-mechanical limit switch should be the first choice because it is suitable for most applications and is generally the most economic to use.

When people choose a switch, they don't always have enough time to think about it, and they don't pay enough attention. Most switches are relatively low-cost and simple in construction. People may do not fully consider the features and functions they can provide when choosing. Depending on the application, some specific parameters will help the designer choose a proper switch. But because of the wide range of options available, people are often easily misled.

In this episode, we focused on 10 key factors that designers should consider when choosing a switch, and emphasized why it is important to choose a switch. A right switch can add value to your product and the entire brand.

1. Data Driven

The emergence of the Internet of Things (IoT) has generated a lot of data in various fields. These data need to be stored or transmitted more often. In addition, the Internet of Things devices also need to accommodate antennas, transmitters and additional circuits, which may have a great impact on the design space of the circuit board. Nevertheless, the switch still has to interact with the user, and at the same time withstand the harsh environment and interference. For example, medical device manufacturers are increasingly focusing their investments on the development of systems and platforms for storing, correlating, and analyzing these data. Therefore, designers must carefully select switches that can meet demanding performance specifications (including miniaturization and low power consumption).

2. Meeting Physical Demands

With the growth of wearable devices, the demand for switches is also increasing. Although wearable devices have been around for a while, there are different concepts when designing devices in different industries. For example, the components in medical wearable devices need to be resilient to cope with rough treatment and harsh environmental challenges, and they also need to be able to respond the first time and work normally every time. The challenge for designers is to ensure that each individual component is suitable for the environment required for the operation of these machines. The switch is an important part of the interface between the user and the device. Therefore, to ensure the consistency, service life and quality of the switch, it is very important to choose the correct switch.

To meet the requirements of many wearable devices, the switch must be waterproof, sweat-proof, and have low energy consumption to maintain battery power. The switch may need to reach IP67 or IP68 sealing level, and can work reliably within the signal current range of milliamps.

3. Dimensions

For example, in the industrial field, switches are used in a variety of applications, including CNC machine tool control, safety and position sensors, battery chargers, power supplies, and gas detection devices. They all have one thing in common, whether in height or board area, the space on the PCB is limited. As equipment functions increase, this form factor becomes more and more important and can also make the space on the PCB more valuable.

Ultra-small size switches can relieve component congestion and help reduce the size of end-user devices, making these devices more popular in the market. The ultra-small switch is very flexible, allowing electronic designers to add other components to the PCB space originally reserved for larger switches. Many types of automated sensors require touch-screen switches with a footprint of 3.0mm x 2.6mm. The latest touch-screen switches on the market now take up less than 2.0mm of space.

4. Rated Electric Power

When choosing a switch, another factor to be considered is the required signal current. Different product designs have very different power requirements. Some switches need to handle low signal current input from PLC and microprocessor, while some switches handle higher currents to drive relays.

Tact switches usually handle currents as low as 1 milliampere (mA) at a DC voltage of 32V, and some currents are even as low as 1 microampere (μA), while snap acting switches may need to handle up to 25 Ampere of current to control the motor circuit. The circuit breaker auxiliary switch presents a challenge to the designer because the same switch may be required to handle currents ranging from 1 mA to 10 amperes.

5. Use Scenario Considerations

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It is important to consider where and how to use the switch. Switches are usually used in harsh environments and need to be able to cope with any situations that may be encountered. Users interact with switches every day, from pressing buttons on gas pumps, opening doors on trains, pressing keyboards on ATMs, calling and selecting floors in elevators, and so on. One thing they have in common is that these public facilities are more likely to be abused or encounter harsh environmental conditions.

For the above-mentioned applications, highly rugged switches are required that can withstand rough handling in order to create human-machine interfaces (HMIs) that can stand the test of time. These switches differ from standard switches in construction materials and methods. For example, most anti-vandal switches have an impact resistance rating of IK08 or IK10 (the highest possible impact rating), and are sealed on IP67 intrusion protection (IP). A series of functional options are also required, such as LED lighting, function symbols, and termination styles such as solder lugs or quick-connect terminals to suit various applications.

6. Limited Options

For designers, the worst thing is that the switch options are very limited. Some basic parameters, such as the number of shots and throws, latch/transient action and other electrical parameters, must be considered. Environmental characteristics and reliability of the switch are also important. The correct analysis of the above factors means that the correct materials can be defined, especially the contacts, and the life cycle is consistent with the expected life of the equipment. The ergonomics and aesthetics of the switch often affect the user's experience and perception of the device.

Common options also include different actuator styles for buttons and push switches, termination options (such as solder lugs, quick connect terminals or wire connections), straight or right-angle mounting, surface mounting or through-hole, and illuminated or non-illuminated, etc. 

7. Sound and Feel

The sound and feel of the switch, also known as the sense of touch, is an important element of switch operation. Tactile sense provide important feedback for the user to know whether the switch is operating correctly. Choosing a switch with the correct tactile sense can bring a high-quality feel to any product. For example, as cars become more complex and feature-rich, tactile feeling becomes more important in the automotive industry. In fact, the sense of touch plays a vital role in the design of car interiors and control systems. Look, feel, and sound are important components of a car brand identity, and switches are the key to achieving this goal-the successfully selected switches are often redesigned on multiple models of the same manufacturer.

Tactile sense and acoustics can not only differentiate a manufacturer from other competitors, but can also be used to define the positioning of different product lines of the same manufacturer. It is important to choose a supplier who is willing to modify the switch components and materials (such as the shape and size of the metal dome) to obtain the correct switch.

8. Calculate the Cost

There is no doubt that when it comes to switch selection, cost is an important consideration. For example, the industrial market pays particular attention to cost and therefore chooses switches that are "good enough" for the application. To achieve this cost-saving choice, you can choose the most commonly used versions on the market-these versions do not require high performance, but will provide an economical price while maintaining strict quality control. If no additional features are needed, choosing a standard, off-the-shelf model will prove to be the best strategy.

9. Cost of Ownership

Although any solution must reflect the required features and requirements, it must also meet cost constraints in order to be commercially viable and meet customer needs. Here, "cost" not only refers to the cost of components, but must also include all costs, connection costs, and assembly costs required for an integrated solution-the so-called "total cost of ownership."

In dirty, humid or corrosive environments, many industrial applications can be used for up to ten years. The switches used in these applications need to conduct low signal currents (less than 50 mA) in the temperature range of -40°C to 85°C without oxidizing or corroding the contacts. Cycle life requirements range from 50,000 to 5,000,000. Failure to meet these specifications may result in the replacement of expensive terminal equipment.

10. Less is More

Although the next generation of innovative devices and products may require fewer switches, they will still play a vital role. For example, the future of connected, electric and self-driving cars will have more advanced human-machine interfaces, and they will need to meet consumers' expectations for high-quality consumer electronic products such as smartphones and tablets. In the future self-driving cars, more control functions will need to be integrated. As the car takes the driver to the destination, the driver will have more time for entertainment and even work. When choosing the appropriate switch, you need to take into account these increased expectations.

Any product is only as good as the sum of its parts. Whether it is a sophisticated surgical instrument, a sophisticated panel input system or a car key fob, the switch must be adapted to the specific requirements of the product design it uses. Considering these ten design factors, design engineers can ensure that they are choosing the best switch for their application.

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