What should I pay attention to when selecting SMD patch varistor?

SMD chip varistorutilizes the nonlinear characteristics of the varistor. When overvoltage occurs between the two poles of the rheostat, the rheostat can clamp the voltage to a relatively fixed voltage value to protect the subsequent circuit. It has the advantages of small size, light weight, high installation density, strong seismic resistance, strong anti-interference ability and good high-frequency characteristics. It is widely used in computers, mobile phones, electronic dictionaries, medical electronic products, video recorders, electronic watt-hour meters and VCDs

Before selecting a rheostat, you should first understand the following relevant technical parameters: nominal voltage refers to the voltage value at both ends of the rheostat at the specified temperature and DC current. Leakage current refers to the current value flowing through the SMD patch varistor when a large continuous DC voltage is applied at 25 ℃. Grade voltage refers to the voltage peak at both ends of the rheostat when it passes through 8/20 grade current pulse. Flow rate refers to the specified pulse current applied on the waveform Flow (8/20 μ s) Peak current. The surge environmental parameters include large surge current IPM (or large surge voltage VPM and surge source impedance Zo), surge pulse width TT, short time interval between two adjacent surges TM, and the total number of surge pulses n within the predetermined service life of SMD patch varistor

In general,SMD patch varistorsare usually used in parallel with protected equipment or devices. Under normal conditions, the DC or AC voltage at both ends of SMD patch varistor should be lower than the nominal voltage. Even if the power fluctuation is serious, it shall not be higher than the large continuous working voltage selected in the rated value. The nominal voltage value corresponding to the large continuous working voltage value is the selected value. For the application of overvoltage protection, the voltage-sensitive voltage value should be greater than the voltage value of the actual circuit. The following formula is generally selected: VMA=AV/BC, where A is the circuit voltage fluctuation coefficient, generally 1.2; V is the DC working voltage of the circuit (effective value at AC); B is the voltage sensitive error, generally 0.85; C is the aging coefficient of the component, generally 0.9; The actual value of VMA calculated by this method is 1.5 times of the DC working voltage. The peak value should also be considered under AC condition, so the calculation result should be expanded by 1.414 times. In addition, it must be noted that:

(1) When the voltage fluctuation is large, the continuous working voltage must not exceed the large allowable value, otherwise the service life of the rheostat will be shortened

(2) When using a rheostat between the power line and the ground, sometimes the voltage between the line and the ground increases due to poor grounding, so the rheostat with a nominal voltage higher than the voltage between the lines is usually used< Span style="font size: 18px;">SMD chip varistorThe surge current absorbed should be less than the large magnetic flux of the product

When a higher power frequency temporary overvoltage acts on the rheostat, it may cause the rheostat to break down and immediately short circuit (low impedance short circuit), and the temperature fuse will blow too late, which may cause a fire

In order to avoid this phenomenon, an impact resistant power frequency fuse can be connected in series on each rheostat, and the rheostat can also be used in series with the ceramic gas discharge tube. During normal operation, the ceramic gas discharge tube is not connected, and the varistor has no leakage current, which can greatly extend the service life; When being impacted by the surge, the ceramic gas discharge tube first breaks down, and then the surge voltage is limited by the rheostat