Varistor type

Varistor measurement: varistors are generally used in parallel in a circuit. When the voltage across a resistor changes abruptly, a short-circuit of the resistor causes the current fuse to blow and protect it. Varistors are often used in power circuits for overvoltage protection and regulation. When measuring, set the multimeter to 10k file. The test leads are connected to both ends of the resistor. The resistance value marked on the varistor should be displayed on the multimeter. If the value exceeds this value, it indicates that the varistor has damaged the varistor nominal parameter pressure. Sensitive resistance is indicated by the letter “MY”. If J is added for home use, the following letters W, G, P, L, H, Z, B, C, N, and K are used for voltage regulation, overvoltage protection, and high-frequency circuits, respectively. , lightning protection, arc extinguishing, noise elimination, compensation, degaussing, high energy or high reliability. Although varistors can absorb large surges of electrical energy, they cannot withstand continuous currents of more than milliamperes, and must be taken into account when used as overvoltage protection. Varistor selection, the general choice of nominal varistor voltage V1mA and flow capacity two parameters.

1, the so-called varistor voltage, the breakdown voltage or threshold voltage. Refers to the voltage value under the specified current, in most cases when using 1mA DC current into the varistor measured voltage value, the product of the varistor voltage range from 10-9000V. According to the specific needs of the correct choice. Generally V1mA = 1.5Vp = 2.2VAC, where Vp is the peak value of the circuit rated voltage. VAC is the effective value of rated AC voltage. The selection of the voltage value of the ZnO varistor is of vital importance. It is related to the protection effect and service life. If the rated power supply voltage of an appliance is 220V, the varistor voltage value V1mA=1.5Vp=1.5×1.414×220V=476V, V1mA=2.2VAC=2.2×220V=484V, so the breakdown voltage of the varistor can be Choose between 470-480V.

2. The so-called flow capacity, that is, the peak value of the maximum pulse current is the maximum when the varistor voltage does not exceed ±10% for the specified inrush current waveform and the specified number of inrush currents when the ambient temperature is 25°C. Pulse current value. In order to prolong the service life of the device, the inrush current amplitude absorbed by the ZnO varistor should be smaller than the maximum flow rate of the product given in the manual. However, starting from the protective effect, it is required that the selected traffic volume is better. In many cases, the actual traffic flow is difficult to calculate accurately, so 2-20KA products are used. If the flow rate of the product at hand cannot meet the requirements of use, several single varistors can be used in parallel, and the sensible pressure after the parallel connection will remain unchanged. The flow rate is the sum of the values ​​of the individual varistors. It is required that the volt-ampere characteristics of the varistors connected in parallel should be as same as possible, otherwise, the shunt unevenness and the varistors may be damaged.

Application Principles of Varistors Varistors are transient voltage suppression components that can be used in place of transient suppression diodes, Zener diodes, and capacitor combinations. Varistors can protect the circuits of ICs and other devices from damage caused by electrostatic discharges, surges, and other transient currents (such as lightning strikes). When used, it is only required to connect the varistor to the circuit of the protected IC or device. When the voltage momentarily exceeds a certain value, the resistance of the varistor rapidly decreases, and the large current is conducted, thereby protecting the IC or the electrical appliance. Equipment; When the voltage is lower than the varistor operating voltage value, the varistor resistance is extremely high, almost open circuit, and thus will not affect the normal operation of the device or electrical equipment.

Varistor Selection Before using varistors, the following related technical parameters should be known: Nominal voltage refers to the voltage across the varistor at the specified temperature and DC current. Leakage current refers to the value of the current flowing in the varistor when the maximum continuous DC voltage is applied at 25°C. The gradation voltage refers to the voltage peak appearing across the varistor when passing through an 8/20-rated current pulse. The flow rate is the peak current when a predetermined pulse current (8/20 μs) waveform is applied. Surge environmental parameters include maximum inrush current Ipm (or maximum surge voltage Vpm and surge source impedance Zo), surge pulse width Tt, minimum time interval Tm between two adjacent surges, and a predetermined varistor The total number of surge pulses N during the service life.

3.1 Nominal voltage selection In general, varistors are often used in parallel with a protected device or device. Under normal conditions, the dc or ac voltage across the varistor should be lower than the nominal voltage, even if the power supply fluctuates. In the worst case, it should not be higher than the maximum continuous operating voltage selected in the rated value. The nominal voltage value corresponding to the maximum continuous operating voltage value is the selected value. For overvoltage protection applications, the varistor voltage value should be greater than the actual circuit voltage value. Generally, the following formula should be used to select:

VmA=av/bc

Where: a is the circuit voltage fluctuation coefficient, generally take 1.2; v is the circuit DC operating voltage (AC is the effective value); b is the varistor voltage error, generally take 0.85; c is the aging factor of the component, generally take 0.9;

The actual value of VmA calculated in this way is 1.5 times the DC operating voltage. Considering the peak value in the AC state, the calculation result should be expanded by 1.414 times. In addition, you must also pay attention to:

(1) It must be ensured that when the voltage fluctuation is maximum, the continuous operating voltage will not exceed the maximum allowable value; otherwise, the service life of the varistor will be shortened;

(2) When a varistor is used between the power line and the earth, the line-to-ground voltage may rise due to a poor grounding. Therefore, a varistor with a higher nominal voltage than that between the line and the line is usually used.

The surge current absorbed by the varistor should be less than the maximum through-flow of the product.

Circuits that apply circuit surge and transient protection. For the application of varistor connections can be roughly divided into four types:

The first type is the connection between the power lines or between the power line and the earth. As a varistor, the most representative use case is that lightning strikes on the power lines and long-distance transmission signal lines cause waves Impulse and other protection of electronic products. In general, accessing varistors between lines can be effective for sensing pulses between lines, while varistors connected between ground and ground are effective for sensing pulses between transmission lines and the earth. If the two forms of line-to-line connection and line-to-ground connection are further combined, surge pulses can be better absorbed.

The second type is the connection in the load, which is mainly used to absorb the induction pulse caused by the sudden opening and closing of the inductive load to prevent the component from being damaged. In general, as long as it is connected in parallel to the inductive load, depending on the type of current and the amount of energy, it can be considered that it can be used together with the R-C series absorption circuit.

The third type is the connection between the contacts. This connection is mainly used to prevent the occurrence of the arcing of the induced charge switch contacts. Generally, the varistors can be connected in parallel with the contacts.

The fourth type is mainly used for the protection of semiconductor devices, this connection is mainly used for thyristors, high-power transistor and other semiconductor devices, generally used in parallel with the protection device to limit the voltage is lower than the protection of the device Pressure rating, which is an effective protection for semiconductor devices.

4 Problems with zinc oxide varistors Existing varistors are divided into two categories that cannot be substituted for each other in terms of formulation and performance:

4.1 High-voltage varistor High-voltage varistors have the advantages of high voltage gradient (100-250V/mm) and good large-current characteristics (V10kA/V1mA ≤ 1.4) but only for narrow pulse widths (2 ≤ ms). Pressure and surge have the ideal protection, energy density, (50 ~ 300) J/cm3.