The selection of the varistor

  Varistor (Metal Oxide Varistor) is a voltage-limiting protection devices. Using the nonlinear characteristic of the varistor when the voltage appearing between the two poles of the varistor, the varistor can be voltage-clamped to a relatively fixed voltage value, thereby achieving the protection of the rear stage circuit. The main parameters of the varistor: varistor voltage, flow capacity, junction capacitance and response time.

  Varistor response time for the ns level, faster than the air discharge tube a tube slightly slower than the TVS overvoltage protection for electronic circuits in general its responsiveness to meet the requirements. The junction capacitance of the varistor is generally in the range of magnitude of several hundred to several thousand pF, in many cases should not be applied directly in the protection of the high-frequency signal line, the application in the protection of the AC circuit, because the junction capacitance would increase leakage current, need to be fully considered in the design of the protective circuit. Through-flow capacity of the varistor, but smaller than gas discharge tube.

   Varistor varistor voltage (min (U1mA)), the through-flow capacity of the circuit design should be the key consideration. DC circuit, it should be: the to min (U1mA) ≥ (1.6 to 2) Udc, where Udc loop DC rated working voltage. AC circuit, it should be: min (U1mA) ≥ (2.2 ~ 2.5) Uac, where the rms value of the AC voltage loop Uac. The principle of the above values ​​in order to ensure that the varistor in the power circuit applications, appropriate safety margin. Signal circuit should be: min (U1mA) ≥ (1.2 to 1.5) Umax, where Umax signal circuit peak voltage. Through-flow capacity of the varistor should be based on the design of lightning protection circuit indicators. In general, the through-flow capacity of the varistor is greater than the through-flow capacity equal lightning protection circuit design.

Choice varistors ago, you should familiarize yourself with the following technical parameters:

   ● the nominal voltage (i.e., the varistor voltage) refers to the stream at a predetermined temperature and the DC voltage value of the both ends of the varistor(Metal Oxide Varistor).

   ● leakage current: refers to the value under conditions of 25 ° C, when the maximum continuous DC voltage is applied, the pressure-sensitive resistor flowing current.

   ● Level voltage refers to the varistor by 8/20 grade current pulse when the peak value of the voltage appearing at its ends.

   ● through traffic is said to impose requirements pulse current (8/20μs) peak current waveform.

   ● Surge environmental parameters, including maximum surge current Ipm (or maximum surge voltage Vpm and surge source impedance Zo), the surge pulse width Tt, the the two adjacent Surge minimum interval Tm, as well as in varistors scheduled working life of the period, the total number of surge pulse N.

   a. Varistor voltage selection

    Generally, the varistor is often used, device or apparatus being protected parallel in normal circumstances, a DC or AC voltage across the varistor should be less than the nominal voltage, even in the worst case of power fluctuations, but also selected should not be higher than the rating of the maximum continuous operating voltage, the maximum continuous operating voltage values ​​corresponding to the nominal voltage value shall be chosen. For overvoltage protection aspects of the application, the pressure-sensitive voltage value should be greater than the voltage value of the actual circuit, generally should be selected using the following formula:

VmA = av / bc

    : A circuit voltage fluctuation coefficient, generally take 1.2; v circuit DC operating voltage (AC valid value); b varistor voltage error the General of 0.85; the c component aging coefficient 0.9 general admission;

    The actual values ​​of the thus calculated VmA DC working voltage of 1.5 times, in the exchange of state should also consider the peak, so the results should be expanded to 1.414 times. In addition, the choice must also pay attention to:

  (1) must ensure that the voltage fluctuation maximum continuous operating voltage does not exceed the maximum allowed value, and would otherwise shorten the service life of the varistor;

  (2) the varistor is used between the power supply line and the earth, and sometimes due to poor grounding between the line and the ground voltage rise, it is generally used a higher nominal voltage ratio between the line and a line using the occasion varistor(Metal Oxide Varistor) .

   b. Selection of through traffic

     Products given through traffic usually given by product standard waveform, the impact of the number and the clearance time pulse test products can withstand the maximum current value. The number of products can withstand the impact waveform, amplitude, and the gap is a function of time, when the amplitude of the current waveform is reduced by 50%, the impact number is doubled, so in practical applications, varistors, surge current absorption should be less than the product of the maximum through-flow.

    c. Application

     Figure 1 shows the varistor voltage circuit surge and transient protection circuit connection diagram. Varistor connected, can be broadly divided into four types:

   The first type is a power supply line or the connection between the power supply line and the earth, as shown in 1 (a) below. As varistors, using the occasion of the most representative of the signal in the power cord and the long-distance transmission lines encountered lightning leaving wires exist surge pulses and other circumstances protective effect of electronic products. General online access varistors induced pulse line between online and land access varistor induction pulse transmission line and the earth. Further the connection between the lines and the line connected to the two forms of the combination may surge pulses have a better absorption effect.

   The second type is a load connection, see Figure 1 (b). It is mainly used in the absorption induced pulse inductive load caused by suddenly opening and closing to prevent component damage. In general, as long as the parallel inductive load can, according to the current type and energy of different size, you can consider the R-C series snubber circuit combination.

   The third type is the connection between the contact points, as shown in Figure 1 (C). This connection is mainly to prevent the occurrence of induced charge switch contacts arc burned, general access varistors(Metal Oxide Varistor) in parallel with the contacts.