Zinc Oxide Varistors

Introduction

5 - Response time of zinc oxide varistors5.1 - Intrinsic response time 6 - Varistor voltage (V
1mA )6.1 - Nominal varistor voltage (V
1mA )
This response time corresponds to the conduction mecha-nisms specific to semiconductors, therefore its value is quite low and is less than one nanosecond. The nominal voltage of a varistor (or “varistor” voltage) isdefined as the voltage drop across the varistor when a dc test current of 1 mA is applied to the component.It is defined at a temperature of 25°C. This parameter is used as a standard to define the varistorsbut has no particular electrical or physical significance. 5.2 - Practical response time
However, the response time will be modified for several reasons:• Parasitic capacitance of the component due to the insula-tion of the intergranular layers.• Overshoot phenomenon occurring when the varistor issubjected to a voltage with a steep leading edge (Figure 21) and causing a dynamic voltage peak greater than the static voltage by a few percent.• Impedance of the external circuit to the varistor. In conclusion, the practical response time of a zinc oxidevaristor usually stays below 50 nanoseconds. 6.2 - Tolerance on the varistor voltage
The standard tolerance is ±10%. Other tolerances may be defined on custom design products.To avoid any lack of understanding, different behaviors ofZn0 varistors should be noted when considering the mea- surement of V 1 mA.• The measurement time must not be too short to allow a“break-in” stabilization of the varistor and not too long so the measurement is not affected by warming the varistor. The limits of V 1mA for our products are given for a measure-ment time comprised between 100 ms and 300 ms. For times comprised between 30 ms and 1s, the varistor volt- age will differ typically by less than 2%.• The value of the peak varistor voltage measured with accurrent will be slightly higher than the dc value.• When the varistor has been submitted to unipolar stresses(pulses, dc life test, ...) the voltage-current characteristic becomes asymmetrical in polarity.

Ω Volts 0 20 40 60 80 10080 60 40 20 Ω + zinc oxide varistorGenerator at 50 NanosecondsGenerator at 50

Figure 21

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