Limit switches: features, classification and principle of operation of electrical limit switches

Limit switch (limit switch) - a device that gives control signals when moving the actuators to the extreme or locking position. It is also called a limit switch. For example, to limit the lifting of loads above the permissible height, such devices are mounted on cranes. They provide a trip signal to the system, cutting off voltage to the starting motor contactor. That is, it is used as a limiter for the movement of mechanisms.

Device types

Experts divided these devices into three large categories, in particular, they are:

1. Mechanical. They are triggered by contact with a lever or a wheel, while the contacts are closed or open, warning, possibly control signals are given.
2. Contactless. They are triggered when any metal objects or other objects are approached, to which the commutation has been configured.
3. Magnetic. It is activated by approaching a magnet to a certain distance. They are also called reed switches.

Areas of use

Each limit switch is used in different areas of life. Relatively spheres of application, they can be divided as follows:

1. Protective. Installed in order to protect the mechanism or personnel from accidental actions. For example, a cage that lowers workers into a mine will not move until all doors are fully closed. This is important from a security point of view.
2. Functional. Responsible for turning on or off lighting and other electrical mechanisms. The simplest example: turning on the light in the refrigerator when opening the door.

It is worth noting that many people do not even think about how often they have to encounter similar mechanisms:

• using household appliances;
• while driving in the car and in the auto industry;
• buying furniture;
• v production process for solving various tasks of a technological nature.

Design features

Electrical limit switches include the following components:

1. Frame. Available in dielectric or conductive materials.
2. Movable part launched by acting on the contact group. The contact group is absent in reed switches and non-contact units.
3. Contact part. It can be composed of both make (normally open) and break (normally closed) contacts. When choosing, you should pay attention to the current and voltage indicators that will flow through the contacts, because they completely depend on how long the operating life of the switch will be.

Group of contactless devices

They were developed to counterbalance mechanical, they are more perfect. They work from transistor switches, the open position of which sets a small resistance. Contactless start-up and shutdown prevent burning of contacts when electrical circuits are broken. Proximity switches can be:

1. Capacitive. Interact with the human body. When you approach it, an electrical capacity will arise, triggering the multivibrator circuit located in the middle. With further approximation, the capacitance increases, and the pulse frequency decreases. Such elements have significant sensitivity. The functionality depends on the plate attached to the capacitors.
2. Inductive. Such electronic devices respond after the movement of magnetic materials, are equipped with metal or non-magnetic cores. When the triggering object moves, electrical impulses are applied to the threshold element, a close / open signal is obtained.
3. Optical. A special subspecies, such switches have an infrared LED and a transistor that can pick up an incoming signal. The optics work regardless of the level of natural light. If the LED beam is interrupted, the photocell will be closed and the execution engine will turn off.
4. Ultrasonic. Quartz sound emitting elements are not complete here. They can also act as motion or volume sensors. When they get into their zone of action, the amplitude of the sound signals of a certain frequency will change, which is not distinguishable by the human ear.

Non-contact switches are used in the manufacture of numerous sensors for monitoring bulk materials and liquids, dispensers.

Mechanical limit switches

They have different designs depending on the industry of application. They are usually divided into three large groups:

• push-button;
• roller;
• lever.

In most cases, they are used by metallurgists, builders and mechanical engineering specialists. They are additionally equipped with rubber seals, and include a group of NO and NC contacts.

One subcategory is called microswitches, the principle of their operation is the same as conventional ones. Only the dimensions are much smaller. The working stroke can be only a few millimeters, so a small lever and roller are included in the structure. Careful response tuning is required.

Magnetic and automotive varieties

Electromagnetic limit switches (reed switches) interact with a permanent magnetic field. Made up of one or more contacts with special ferromagnets. If a magnet approaches them, the signal to the control circuit is closed and starts. The main advantage of the devices is that there is no mechanical effect at all. This significantly increases the service life. Magnetic switches are manufactured in glass or plastic cases, their dimensions are very small.

Automotive limit switches - parts of lighting and signaling systems. In their design, there is only one terminal with the ability to connect a positive potential. The negative terminal is a body that is pressed against the metal components of the car body. In this case, it is important to ensure that the part of the body where the device will be located is completely free of paint.

Spindle and pneumatic models

Spindle devices limit the movement of mechanisms or act as limit switches in cyclic control. The rotating mechanism switches the internal contact group. They are used on winches and other shafts that rotate at low speed. Equipped with several contact groups.

Pneumatic limit switches are devices that stop the flow of compressed air or other gases after pressing a button or lever. Another type of them is triggered when a certain level of pressure is reached in the system.

Experts remind: all work on connecting and disconnecting switches should be carried out in a complete absence of voltage. The check is carried out by means of a voltage indicator, and metal housings are subject to mandatory grounding.