Varistors are widely used! How to choose a varistor
2023-08-01 09:37:32
Varistors
The ideal resistor in a resistor is a linear resistor. But linear resistors are rarely available so to speak. This is because the resistance value of the element is affected by temperature, and such resistance value is called the temperature coefficient of resistance. The change in current flow causes the resistance temperature to change, and the resistance value also changes with the temperature.
As a voltage-limiting protection component, varistors are usually used in circuits because of their non-linear volt-ampere characteristics. It plays a role in circuits to withstand over-voltage, and when over-voltage occurs at the two ends of the varistor, the varistor can clamp the voltage to a relatively fixed voltage value, absorbing the excess current to protect other components.
Varistor in the circuit plays the role of switching protection, with an inductive load circuit is suddenly disconnected, the overvoltage can exceed the power supply voltage of a number of times. The overvoltage will cause arc and spark discharge between the contacts, which will damage the contacts of contactors, relays, electromagnetic clutches, etc., and shorten the service time of the equipment. Varistors have a shunt effect at high voltages, so they can be used to protect contacts by preventing spark discharges at the moment of contact disconnection.
In addition, varistors are small in size, light in weight, stable in performance, will not be affected by external factors, large through-current capacity, fast response and other advantages in the electronic components market to occupy a place in the electronic circuits and home appliances in a wide range of applications.
how to choose the varistor
When selecting the varistor, the specific conditions of the circuit must be considered, and the following principles should generally be followed:
1、Selection of Voltage Sensitive V1mA
According to the selection of power supply voltage, the power supply voltage continuously applied to both ends of the varistor can not exceed the "maximum continuous operating voltage" value listed in the specification. That is, the maximum DC operating voltage of the varistor must be greater than the DC operating voltage of the power line (signal line) VIN, i.e., VDC ≥ VIN; for 220V AC power supply pressure sensitive selection, we should take into account the amplitude of the fluctuation of the grid voltage, the value of the selected varistor pressure-sensitive voltage, we should leave enough margin. The general fluctuation amplitude of the domestic power grid is 25%. Should be selected for the pressure-sensitive voltage of 470V ~ 620V piezoresistor is more appropriate. Selection of voltage-sensitive voltage a little higher varistor, you can reduce the failure rate, extend the service life, but the residual voltage slightly increased.
Especially pointed out for the varistor voltage selection criteria is higher than the supply voltage, in order to meet the need to protect the protection device at the same time, as far as possible, select the varistor with high voltage, which not only protects the device, but also improves the service life of the varistor. For example, to protect the device withstand voltage Vdc = 550Vdc, device operating voltage V = 300Vdc, then we choose the pressure-sensitive resistor should be pressure-sensitive voltage of 470V varistor, pressure-sensitive voltage range is (423-517), the pressure-sensitive voltage up to the negative error of 470-47 = 423Vdc is greater than the device's power supply voltage of 300Vac, to the large positive error of 470 + 47 = 517Vdc is less than the device voltage 550Vdc.
Selection must also be noted:
(1) must ensure that the voltage fluctuations in the most large, the continuous operating voltage will not exceed the maximum allowable value, otherwise it will shorten the service life of the varistor.
(2) In the power line and the earth between the use of varistors, sometimes due to poor grounding and the voltage rise between the line and the ground, so it is usually used than the line and the line between the use of occasions higher nominal voltage varistor.
2. Selection of flux
The nominal discharge current of the varistor should be greater than the required to withstand the surge current or the maximum surge current that may occur in the equipment work. Nominal discharge current should be calculated according to the piezoresistor surge life time rating curve in the impact of more than 10 times the value of the maximum impact of the flux of about 30% (i.e., 0.3IP) or so.
3、Selection of clamping voltage
The clamping voltage of varistor must be less than the maximum voltage (i.e. safety voltage) that can be withstood by the protected parts or equipment.
4、Capacitance Cp selection
For high-frequency transmission signals, capacitance Cp should be smaller, and vice versa 5, internal resistance match (ResistanceMatch)
Protected components (line) internal resistance R (R ≥ 2Ω) and varistor transient internal resistance Rv relationship: R ≥ 5Rv; for the smaller internal resistance of the protected components, in the case of not affecting the signal transmission rate, as far as possible, the use of large capacitance varistor.