subject: Varistor Selection And Use - The Electronics Industry [print this page] Varistor measurements: varistor generally used in parallel in the circuit, when the voltage across the resistor dramatic changes, the resistance to short-circuit Fuse Fuse, has protective effects. Varistor in the circuit, commonly used in Power supply Overvoltage protection and voltage regulator. When measured Multimeter Home 10k file, table document connected to both ends of resistor, multimeter should show marked the resistance varistor, if exceeded this value is large, shows varistor has been damaged.
Varistor nominal parameters Varistors letters "MY" that, for instance J for the home, followed by the letters W, G, P, L, H, Z, B, C, N, K, respectively for the voltage regulator, overvoltage protection , high-frequency circuits, lightning, arc, noise reduction, compensation, degaussing, high energy or high reliability and so on. Although varistor can absorb a great surge of electric energy, but can not afford mA continuous current level and above, the overvoltage protection must be taken into account for this. Varistor selection, general selection nominal varistor voltage and current capacity V1mA two parameters.
1, the so-called breakdown voltage, the breakdown voltage or threshold voltage. Refers to the voltage under specified current value, in most cases with 1mA DC into circulation Varistor When the measured voltage, the varistor voltage range of their products can range from 10-9000V. The correct choice according to specific needs. General V1mA = 1.5Vp = 2.2VAC, type in, Vp is the peak value of rated voltage circuit. VAC rated AC voltage of RMS. ZnO Varistor voltage selection is critical, it is related to the protective effect and service life. If a station with a rated electrical power supply voltage 220V, the varistor voltage V1mA = 1.5Vp = 1.5 220V = 476V, V1mA = 2.2VAC = 2.2220V = 484V, so the breakdown voltage of varistor optional between the 470-480V.
2, the so-called current capacity, the maximum peak pulse current is the ambient temperature of 25 case, the impact for the requirement under current waveform and frequency of the impact of current, the varistor voltage change of less than10% When the maximum pulse current value. In order to extend the life of the device, ZnO varistor absorbed by the surge current amplitude should be less than given in the product manual for the maximum traffic volume. However, the protective effect from the start, the request flows through a large selection of some good. In many cases, the actual place of communication flow is difficult to measure exactly, then use 2-20KA products. Products such as the hand can not meet the requirements of through traffic, can be a few individual varistor in parallel using the same parallel of Wheatstone, the communication flow for the single-only Varistor totals. Requirements parallel varistor voltage characteristics as the same, or easy to cause damage to varistors uneven segregation.
The application principle of varistors Varistor is a function of a transient voltage suppression devices, can be used to replace the transient suppression diodes, Zener diodes and Capacitor Combination. Varistor IC and other devices can protect the circuit, and prevent electrostatic discharge, surges and other transient current (such as lightning, etc.) caused them damage. Simply use varistors and connected to the protected circuit IC or equipment, when the voltage instantaneously above a certain value, the varistor resistance decreased rapidly, conduction-current, so as to protect IC or electric equipment; when the voltage is lower than the varistor voltage value, the varistor resistance high, almost to clear the way without affecting the device or electronic equipment to work properly.
Varistor selection Selected varistors, should first understand the following relevant technical data: Nominal voltage is the prescribed temperature and DC in the shed, across the varistor voltage. Leakage current is at 25, when the applied maximum continuous DC voltage, varistor current value stream. Varistor voltage level is through 8 / 20 rating when the current pulse presented in its peak voltage across. Through traffic is to impose requirements that the pulse current (8/20 s) waveform when the peak current. Surge of environmental parameters, including maximum surge current Ipm (or the maximum surge voltage Vpm and surge source impedance Zo), surge pulse width Tt, the minimum time between two intervals Tm surge and the target varistors working life period, the total number of N pulse surge and so on.
3.1 Nominal Voltage Selection Generally, varistors are often used to protect the device or devices in parallel, under normal circumstances, pressure-sensitive resistor across the DC or AC voltage should be lower than nominal voltage, even in the worst-case power fluctuations when the choice should not be higher than the maximum continuous rating of working voltage, the maximum continuous working voltage corresponding to the value of the nominal voltage shall be used. The application for over-voltage protection, varistor voltage value should be greater than the actual circuit voltage should generally be selected using the following formula:
VmA = av / bc The formula: a coefficient for the circuit voltage fluctuations, generally take 1.2; v DC voltage for the circuit (when AC RMS); b for the breakdown voltage error, usually take 0.85; c for the components of the aging factor, the general take 0.9; This calculated VmA
the actual value is 1.5 times the DC voltage, AC state, should also be considered in the peak, so the results should be expanded to 1.414 times. In addition, attention must be paid selected:
(1) must ensure that the maximum voltage fluctuation, the continuous working voltage does not exceed the maximum allowable value, otherwise it will shorten the life of varistor;
(2) in the power cord between the use of varistors with earth, sometimes as bad Ershi ground voltage between line and ground up, and are usually used between the lines than to use a higher nominal voltage of the pressure-sensitive situations resistor.
Varistor absorbed by the surge current should be less than the maximum traffic volume products.
