Magnetic switch has the difference between contact and no contact

- Sep 30, 2019-

A magnetic switch is a line switching device that is controlled by a magnetic field signal, also called a magnetic switch. Commonly used magnetic switches are available in single and double contacts.


The magnetic switch means that it is induced by a magnet. This "magnetic" is a magnet, and there are several kinds of magnets. The working principle of the magnetic switch, the commonly used magnets on the market are rubber magnetic, permanent ferrite, sintered NdFeB. The switch is a reed switch.


When a magnetic substance is close to the glass tube, under the action of the magnetic field lines, the two reeds in the tube are magnetized to attract each other, and the reeds are attracted together to connect the circuit connected to the node. After the external magnetic force disappears, the two reeds are separated due to their own elasticity, and the circuit is broken.


Magnetic switch has contact and no contact difference

Unlike the reed contact that has never been used for cylinder piston position detection, the contactless magnetic switch uses a magneto-sensitive (sensor) and an electronic circuit, and is therefore also referred to as an electronic magnetic switch.


Features: Depending on the internal components, the two types of magnetic switches have various features as described below.


1) Life expectancy:


2) There are contacts


The life of a contact switch is nominally 5 million. However, depending on the load conditions, the life value will be quite different, generally distributed between several million to 15 million back.


b) no contact


The interior of the contactless switch is semi-permanent* due to the absence of mechanical contacts.


Semi-permanent: It does not mean that there is no limit to life.


2) Oscillation phenomenon:


a) there are contacts


The springing of the reed contact during opening and closing will cause the output signal to oscillate.


b) no contact


Due to the use of triodes


ON/OFF


Action, so the output signal will not oscillate.


3) Opening and closing capacity:


a) there are contacts


Since there is a limit to the current that can flow through the reed contacts, large-capacity opening and closing cannot be performed.


b) no contact


Although there is a certain limit to the heat loss based on the triode, it is larger than the upper limit capacity of the contact switch. In addition, there is a certain amount of margin for the instantaneous variation (such as inrush current).


4) Internal voltage drop: Is there a contact? The contactless switch must meet the following conditions:


Power (load) voltage - internal voltage drop > load operating voltage


When using PLC: ON voltage) * If the above conditions are not met, the load cannot be normally turned ON (or kept OFF).


a) there is a contact because the indicator light (LED)


In series with the loop, the internal voltage drop also includes its voltage drop in the forward direction.


Strength value: 2 ~ 3V


b) no contact


The two-wire type does not allow the voltage to be set too low for the electronic circuit to operate.


Strength value: 4.0 ~ 4.5V?


The three-line type can be ignored.


5) Leakage current: The contactless switch must meet the following conditions: load operating current (when using PLC: OFF current) > leakage current


a) there are contacts


There is no leakage current.


b) no contact


Even if the switch is off in the two-wire type (when OFF, the current (leakage current) of the drive circuit operates to the load. Strength value: 1 mA or less.


The three-wire type, like the two-wire type, requires a current that can drive the circuit to operate. However, since the power line (brown) and the signal output line (black) are independent, the current flowing from the signal output line is very small and can be ignored.


6) Hysteresis interval:


a) There are contacts due to the hysteresis characteristics of the material of the reed contact, and the values of the ON/OFF magnetic fields are different, resulting in a hysteresis interval (ON/OFF position difference. Strength value: 0.55 mm to 1.0 mm


b) Non-hysteresis interval ratio strength value: 0.3mm~0.6mm 7) Influence of external magnetic field When the external magnetic field (such as the magnetic field generated by the dissolved current) exceeds the detected magnetic field strength of the cylinder, is there a contact? No contact switch will cause a malfunction.


a) When the contact has a malfunction under the AC dissolved magnetic field, the number of opening and closing of the reed contact increases rapidly, which will reduce the service life of the switch. Countermeasure D-P7 (for AC/DC magnetic field) cannot be used for standard cylinders.


b) No contact will affect the magnetic switch even if a malfunction occurs. D-P5 (for AC magnetic field) can be used for standard cylinders. )


Internal structure:


a) The contacts are simply constructed based on the reed contacts and the glass epoxy board. For mechanical vibration? The tolerance of the impact is very weak.


b) The contactless part consists of a susceptor and other electronic components, and the parts are mounted on a ceramic or glass epoxy circuit board.


Although the composition of the parts is more complicated than that of the contact switch, the mechanical strength is relatively strong.


3. Have a contact? No contact switch selection:


By comparing there are contacts? Contactless features, in reliability? life?


The performance of the contactless switch on the signal output oscillation phenomenon is better. In addition, there are indeed limit to the mechanical and electrical life of the contact switch. However, the content of the advantages and disadvantages mentioned here, in practical applications, has almost no examples of problems.


It is difficult to distinguish between the use of contacts and contactless switches, and it is difficult to distinguish them according to their advantages and disadvantages in various fields. Ultimately, it is determined according to the user's design thinking on the device. Please refer to the attached information for ideas on this.


4. Operation block diagram of the contactless magnetic switch (three-wire type) Description of each part:


1) The susceptor is shown in the figure on the right. The input circuit consisting of two parts is a bridge circuit. Usually, the voltage between A and B is in an equilibrium state. When the piston is close to the magnet, a potential difference is generated between A and B. This potential difference is the detected input signal.


2) Amplifying section/comparing section Since the output signal of the magnetoresistive resistor is very small, it is necessary to amplify the signal to a level convenient for processing by the amplifying section. The comparison unit compares the amplified output signal set number with the fixed level of the corresponding cylinder, and outputs a detection signal.


3) The output unit flows current to the load based on the detected signal. The load drive output (NPN triode output) loop and the action display lamp output loop are independent.


4) D1? D2 Amplifying unit output signal detection level comparison unit checks the voltage of the sense resistor A-B. The piston movement D1 is for preventing the power supply from being reversed, and D2 is for preventing the surge voltage of the output unit. The use of contact and contactless switches is distinguished: the form of the semiconductor unit is generally evaluated as high reliability and long life. Most of these evaluations are expressed by the product failure rate, that is, the relationship between the failure rate and time as shown in the following figure.


Among them, the starting point of the contact type wear failure time is what happens at what time and what can be used as the point of judgment.