Varistor Surge Testing

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by EMC-PARTNER AG, Baslerstrasse 160, 4242 Laufen +41617630111

1. Test Parameters

1.1 Introduction

Most common used varistor are disk type varistor with diameters from 5mm up to 40mm.The ceramic disk is fitted with tinned copper wire and then coated with epoxy resin. The varistor voltage depends on the thickness of the ceramic disk and the grain size. The disk diameter influences the current and energy rating. The possible current density is also a function of the grain size. Typically low-voltage varistor have a smaller grain size and can not handle the same current density as high voltage types. That explains the differences in surge current, energy absorption and mechanical dimension within the various type series.

1.2 Clamping Voltage

Peak voltage across the varistor measured under conditions of a specific peak current (Iclass) with a 8/20us surge current.

1.3 Maximum Peak current

The maximum surge current that can be handled by a varistor depends on amplitude, pulse duration and number of pulses applied over device lifetime. The maximum non-repetitive surge current is defined by an 8/20us waveform according to IEC60060. This waveform approximates a rectangular wave of 20us.

1.4 Energy surge rating

Surge currents of relatively long duration are required for testing maximum energy absorption capability. This condition is more representative of the high energy surges usually experienced from inductive discharge of motors and transformers. A rectangular wave of 2ms according to IEC60060 is commonly used for this test. Some manufacturers test also with an double exponential 10/1000us pulse.

The energy absorption in the varistor is the integral of the current flow through and the voltage across the varistor. The 2ms pulse has compared to the 10/1000us pulse the advantage of easy calculation. The energy is with good accuracy the product of peak current and clamping voltage multiplied with the pulse duration.

Example:

Varistor: 20mm disk type with nominal r.m.s voltage of 250V:

Maximum peak current: 6500A , this leads to an energy absorption of 80Joule with the 8/20us pulse.

Energy surge rating: 180Joule , measured with a 2ms pulse at a current of about 100A

Typically the maximum energy absorption is about 1.5 to 2.5 times higher than the energy absorbed with the 8/20us pulse at the maximum surge peak current.

2. EMC-PARTNER Varistor Tester

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2.2 Clamping Voltage : MIG0603CLV, MIG0603CLV2

MIG0603CLV Technical data in short form

Waveform : 8/20us +/- 20%

Current ranges : 2.5A, 25A, 200A

Varistor clamping voltage ranges : 100V, 300V, 1000V, 3000V

Measurement accuracy : +/- 3% peak voltage and current

Charging time : max. 10sec

Advantages of EMC-PARTNER clamping voltage tester:

• Built in accurate measuring system

• The class current can be entered directly together with the expected clamping voltage. After the pulse the generator measures and displays the peak current and the clamping voltage

• short charging times for efficient testing.

2.3 Maximum Peak current : MIG0612, MIG0624, MIG1248

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MIG0624 Technical data in short form

Waveform : 8/20us +/- 10% (!!)

Current range : (four independent outputs) 6kA, 12kA, 18kA, 24kA

Varistor clamping voltage ranges : 100V, 300V, 1000V, 3000V

Measurement accuracy : +/- 3% peak voltage and current

Charging time : max. 30sec

Advantages of EMC-PARTNER current generators:

• Four independent outputs which each 1ohm allow ideal adaptation to the varistor impedance.

• As nominal value the desired current can be entered directly together with the expected clamping voltage. After the pulse the generator measures and displays the peak current and the clamping voltage

• Short charging times for efficient testing.

2.4 Energy surge rating : MIG0612EA, MIG0636EA, MIG0624LP1

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MIG0612EA Technical data in short form

Waveform : 2ms (90%-90%) -0%/+20%

Current range : 5A..150A

Varistor clamping voltage ranges : 100V, 300V, 1000V, 3000V

Varistor energy range : 2J..700J depending on the clamping voltage

Measurement accuracy : +/- 3% peak voltage and current, +/- 10% energy in joule

Charging time : max. 15sec

Advantages of EMC-PARTNER Energy absorption generators:

• 2ms rectangular wave-form

• As nominal value the desired energy can be entered directly in joule together with the expected clamping voltage. After the pulse the generator measures and displays the peak current, the clamping voltage and the absorbed energy (only with 2 ms rectangular pulses).

• Short charging times for efficient testing.

e.g. Varistor S10K130 10mm size Energy = 10Joule Vcl = 340V measured on CRO monitor outputs 1V = 20A 1V = 100V E = 13.6A * 266V = 10.4Joule

2.5 General

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Recommended Equipment for disk type varistor :

up to 20mm : MIG0603CLV + MIG0624 + MIG0612EA

up to 40mm : MIG0603CLV2 + MIG1248 + MIG0636EA

Energy calculation :

• 10/1000us : through integration of ( Vcl(t) * I(t) * dt ),
  or as an estimation : E = 1.4 * Vcl * Ipeak * 1000us

• 2ms : with good accuracy : E = 1.25 * Vcl * Ipeak * 2ms

Maximum available energy with 8/20us pulse :

For most varistor the specified energy rating can not be reached without exceeding the maximum allowable peak current. Therefore a generator with 2 ms rectangular or 10/1000 µs waveform must be used for energy tests.

Measurement :

All generators have an internal voltage and current measurement and a peak detector with an accuracy of +/-3%.

The MIG0612EA and MIG0636EA have furthermore an integrated energy measurement and display.

Charging time :

The charging time depends upon the selected charging voltage setting. For maximum voltage it is :

MIG0603CLV 10sec

MIG0612 15sec

MIG0624 30sec

MIG1248 30sec

MIG0612EA 15sec

MIG0636EA 30sec

Other Varistor :

For your specific investigations we can supply you with PSPICE models of our generators.

Other related Tester :

MIG0603CLP for SMD varistor

MIG0612SPD and MIG 2412SPD for Surge Protective Devices according IEC61643-1

MIG0603UL for Transient Voltage Surge Suppressers according UL1449

2.6 Overview : Varistor types and appropriate surge generators

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Notes: (*1) : only with MIG0603CLV2 (*2) : Test can be performed with MIG0603CLV

The table covers samples of the following manufacturers : G.E. / HARRIS , MAIDA , Panasonic , FUJI , SANKEN , Philips , Siemens/EPCOS , Thomson / AVX