Steps to select a varistor

• Steps to select a varistor

The basic steps for selecting a metal oxide varistor are shown in Figure 1. The following follows the basic steps to track the selected process through specific examples.

Fig.1 Basic steps for selecting a varistor

Case) Lightning surge absorption measures between power lines

Fig.2 Circuit example

voltage VE=200(Vr.m.s.)±10%
Surge voltage Vs=5(kV)
Equivalent surge impedance ZS=100(Ω)
Surge pulse width tT=50(μs)
Load withstand voltage VP=800(V)
Number of surges N=104times

①Determining the varistor voltage

First, select the appropriate varistor voltage based on the circuit voltage.

1) According to the circuit voltage VE(V), the minimum value of the varistor voltage to be selected is determined by the equation 1.
V_E≦V_V（min.）（１－α）　…①
V_E：The peak of the circuit voltage
V_V（min.）：Varistor minimum voltage value
α：Safety factor（α＝0.1）
In the example, the power supply voltage is 200V_r.m.s.±10％，
Determine the varistor voltage with Equation 1
V_V（min.）≧（200√2 ×1.1）/（1－0.1）
≧346（V）

In the example, it is necessary to select a varistor with a lower limit of the standard range of the varistor voltage range recorded in the catalogue above 346V.

In addition, there is a standard for varistor, that is, the upper limit of the voltage that can be continuously applied, that is, the maximum allowable circuit voltage.

If a voltage exceeding this voltage is continuously applied, the varistor may be degraded, which also needs to be considered.

2) According to the circuit voltage VE(V), the required maximum allowable circuit voltage is obtained by the equation ②.
V_E≦V_A（１－α）　…②
V_E：Circuit voltage
V_A：Maximum allowable circuit voltage
α：Design margin（α＝0.2）
In the example, the supply voltage is 200V_r.m.s.±10％，
Determine the varistor voltage with Equation ①
V_V（min.）≧（200×1.1）/（1－0.2）
≧275（V） 

The nominal varistor voltage of the varistor needs to be selected according to the following two conditions: the standard range of the varistor voltage range is above 346V, and the maximum allowable circuit voltage is above 275V.

For the product catalog, products with varistor voltages above NVDxxUCD390 should be selected.

But this alone is not enough.

It is also necessary to investigate whether the voltage suppression range of the varistor is appropriate when subjected to a surge voltage surge.

If it is not appropriate, it may not be possible to obtain an appropriate voltage suppression effect at the critical moment of the surge voltage surge.

②Calculate the inrush current through the varistor

Assume that the circuit to be protected is Fig. 3, and the surge current Ip through the metal oxide varistor can be obtained by the equation ③.

Fig. 3 Surge equivalent circuit

Ip＝（Vs－Vc）/ Zs　…③
Ip ：Inrush current 

Vs ：Surge voltage
Vc ：Varistor suppress voltage

Zs ：Equivalent surge impedance

Vc is less than Vs in most cases, so you can also ignore Vc for simple calculations.

In the case, VS＝5（kV）、ZS＝100（Ω）、VP＝800（V），
According to type ③

Ip＝5000/100
＝50（A）　※Pulse Width tT＝50（μs）
It can be seen that the current value through the varistor is affected by the surge voltage 50(A)。

③Determine the suppression voltage (limit voltage)

The limiting voltage is selected according to the voltage-current characteristic curve of the product catalog, and the limiting voltage of the metal oxide varistor corresponding to the current IP obtained by the equation ③ should be VP or less with respect to the withstand voltage VP of the protection target (refer to Fig. 5).

Fig.3  Relationship between voltage-current characteristic curve and IP and VP

In the example circuit, the nominal voltage of the varistor is above 390 (V),The maximum allowable circuit voltage is above 275 (Vr.m.s.), according to the voltage-current characteristic curve of the catalog,A metal oxide varistor having a limit voltage of 800 (V) or less corresponding to Ip=50 (A) is tentatively selected.The result obtained against this condition is...
φ10 product：NVD10UCD430、NVD10UCD470
φ14 product：NVD14UCD430、NVD14UCD470

Need to choose from the above four products.

④Determine the size of the disk diameter

According to the surge current IP obtained by the transmission 3 and the surge pulse width tT(s) and the number of repetitions,

Use a surge resistance and surge life characteristics (refer to individual standards) to select a metal oxide varistor within the allowable range.

IP=50(A), tT=50(μs) calculated according to ③, repeated 104 times

Select the surge tolerance and surge life characteristics of the product catalog.

The result of comparing the product catalog is...
NVD10UCDxxx：80（A）、NVD14UCDxxx：120（A）

As can be seen from the results, these products are all larger than 50 (A), and the corresponding varieties of ③ can be used.

However, the number of surges 104 times may contain uncertainties.

If there is no problem with the placement, the design should leave enough margin.

Select the φ14 product of the NVD14UCD type.

The above is the general flow of selecting a varistor.

I hope to provide a reference when you research varistor to cope with surges.