AVX Zinc Oxide Varistors Catalog - AVX

Catalogue AVX Zinc Oxide Varistors Catalog

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ZINC OXIDE VARISTORS

Introduction

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Table III gives the energies calculated according to waveform in Figure 19.

4.6 - Average dissipated power

a) Average power dissipated In the "rest" state

Considering the high values of the coefficient «, a special attention is required concerning the dissipated power value in case of possible changes in the operating voltage.

Indeed, starting with the equation:

I = KV«

the average power dissipated by the varistor is given by the equation:

PC = KV«+¹when a direct current voltage is applied, and Pa = APc

in the case of a sinusoidal voltage having the same peak value and direct current voltage value.


Current]	i
Ip Ip/2		j
	I	i
	J	■ T	\__
0	^ti ^t2 Time

Figure 19

P/Po

io⁵

Table III


	a = 5oi	J
	/	/
J		f	a = io
I	/


Vp	Ip	Waveform		Energy
(V)	(A)	(Ms)		(J)
500	300	1.2	50	10
500	300	8	20	3
500	300	10	1000	210

io⁴

io³

10²

The following changes are found when the varlstor absorbs an energy greater than the maximum permissible value:

• Higher leakage current.

• Decrease in the voltage at 1 mA.

• Decrease in coefficient «.

If the energy increases well beyond the maximum value, the characteristics degrade to such an extent that, even at the rated voltage, the varistor has a very low resistance value.

The permissible energy for a given varistor is mainly related to the size of the part. For example, Table IV gives the permissible energy for different varistors sizes with an operating voltage of 250 V.

Table IV

V/Vo

i.i

i.2

i.3

Figure 20

The A value as a function of a was given in Figure 8. A small change of the operating voltage can induce a dissipated power variation which is all the more greater since the value of exponent a is high (Figure 20).

It can be seen that a 10% change in the rated voltage increases the dissipated power by a factor of 20 when coefficient a equals 30, and by a factor of 150 when the coefficient equals 50.

Table V gives the power P dissipated at values of the applied direct current voltage when the value of a equals 30.

b) Average power dissipated during the transient state

If the transients to which the varistor is subjected are repeated at a sufficiently high frequency, there will be an increase AT in the average temperature of the part given by the expression: AT = P/5

in which P represents the average dissipated power which depends on the energy of the pulse and its repetition frequency and 8 the dissipation factor in air of the unit.

This temperature rise should stay below the threshold indicated by the manufacturer or it may damage the component coating resin or even cause thermal runaway of the ceramic.


Operating	Uncoated
Voltage	Disc	Energy
(V)	0 (mm)	(J)
250	5	10
250	7	21
250	10	40
250	14	72
250	20	130

Table V


	V-	P
	(V)	(mW)
	180	0.5
	220	0.2
	230	0.75

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