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Catalogue AVX Zinc Oxide Varistors

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Zinc Oxide Varistors

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Introduction

3 - Temperature influence on the I = f(V) characteristic

A typical I = f(V) curve is given in Figure 6.

Different distinct regions can be observed:

• The first one depends on the temperature and corresponds to low applied voltages (corresponding currents are in the range of the uA). Consequently, a higher leakage current is noticeable when temperature is increasing.

• The second one shows less variation and corresponds to the nominal varistor voltage region (Figure 7). The temperature coefficient of the varistor voltage at 1 mA is:

The A versus « curve

Figure 8

For usual values of « (30 to 40), the continuously dissipated power is about 7 times greater than that dissipated by a sinusoidal signal having the same peak value. For example, a protective varistor operating at RMS voltage of 250 V has a power dissipation of a few mW.

4.2 - Non-linearity coefficient

The peak current and voltage values basically depend on the I = f(V) characteristic or, to be more precise, on the value of the coefficient defined by:

AVA/ AT

and has a negative value with | K| < 9.10^-4/°C

K =

As the temperature coefficient decreases with increasing current density, this curve also depends on the type of the varistor.

• For higher voltages, the temperature has no significant influence. Practically the clamping voltages of the varistors are not affected by a temperature change.

k A V 1 mA (%) V 1 mA

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				I
				À
			y
		y	s /	y
100	°C l_	y 75°C		25° C

log

a =

log (V1/V2)

In which I1 and I2 are the current values corresponding to voltage values V1 and V2.

The value of a depends on the technology used (chemical composition, heat synthesis, etc.). Nevertheless, the value is not constant over the entire current range (several decades). For example, Figure 9 shows the variation of this coefficient for currents ranging from 100 nA to 100 A. It can be seen that a passes through a maximum value and always stays at high values, even at high levels of current.

10^-410^-5

10^-810^-9

H-h

- 25 0 25 50 75 100 125

-2

-9.10^-4 / °C

(V)

10² 10³

Figure 6

rigure 7

4 - Varistor characteristics

The choice of a varistor for a specific application should be guided by the following major characteristics:

1) Working or operating voltage (alternating or direct).

2) Leakage current at the working voltage.

3) Max. clamping voltage for a given current.

4) Maximum current passing through the varistor.

5) Energy of the pulse to be dissipated in the varistor.

6) Average power to be dissipated.

4.1 - Max. operating voltage and leakage current

The maximum operating voltage corresponds to the "rest" state of the varistor. This "rest" voltage offers a low leakage current in order to limit the power consumption of the protective device and not to disturb the circuit to be protected. The leakage currents usually have values in the range of a few micro-amperes.

Pa = AV .lp = AKVp^a+1

Figure 9

Figure 10

The non-lineary of the varistor can be expressed in another way by the ratio of the voltages corresponding to 2 current values.

_Vl.

v₂

with

= A

Where:

V1 voltage for current I1

V2 voltage for current I2

The curve giving versus the value of a is shown in Figure10 for 2 ratios of I1 /I2 =10³ and 10⁶.

in which: A = a constant f(a) K = a constant (I = KV").

Pc = dissipated power for a DC voltage Vp.

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