Understanding of Varistor

1. Origin of word

Varistor is a compound word of variable and resistor. Varistor is a two pole element which varies the resistance between two poles from infinite to zero (insulator to ideal conductor). A varistor is known as voltage₋dependent resistor (VDR). In a physical aspect, non₋ohmic variable resistor is called varistor.

2. Usage

Varistor is used as non₋conductive shunting device when the voltage across it remains well below its clamping voltage (sometimes it is calld as threshold voltage or varistor voltage) but when the voltage is over the clamping voltage then it acts as varistor, (non₋ohmic variable resistor), thus varistors are used for suppressing voltage surges, in other words it used as SPD (surge protective device). In the region of it action as varistor (voltage clamping area), the relation of voltage and current can be expressed as following,

I = kVα₋₋₋₋₋₋₋ (*)

Where ‘α’ is ‘called non₋linearity coefficient’ or ‘varistor effect’, and the value is varied from 3 to 35 which varies by the kind of varistor element.

3. Kinds of element

Raw material of varistor is generally the metal oxide dust ceramics, so varistor can be verified by the kind of metal oxide. Typically there are two kinds of varistor, zinc₋oxide and silicon₋carbide.

⒜ Zinc₋oxide varistor has the value range of α is bigger than 25 and it used as low voltage line SPD and as SPD inside the electrical and electronic equipment.

⒝ Silicone₋carbide varistor has low range of α, 3 to 7, but the range of treating power and voltage is somewhat higher so it can be used in large generator or on electrical sub₋station. It cannot be found in consumer appliances.

⒞ Semiconductor diode also works as varistor in low energy level, but generally speaking these are called as transient voltage suppressor (TVS) diode or sometimes as ‘avalanche’ diode, not vaistor. Such a diode has ‘α’ range bigger than 35 in its operating range.

4. Manufacturing of Zinc₋oxide Varistors

Zinc₋oxide varistor is introduced in 1970 by Japanese physicians Mastoka M, et al. (Non₋ohmic Properties of Zinc Oxide Ceramics, Japan Journal of Physics 10(1970) p736~742). Zinc₋oxide ceramic varistor is composed of mainly 99.9% ZnO and ZnO grains mixed with one or two percent of doping metal₋oxide such as Bi2O3, Sb2O3, Mn3O4, Co3O4,Cr2O3 or other metal oxide, and little quantity of rare earth oxides such as Pr6O11 in grain. Metal oxide ceramic is made through complicated and long series of processes, such as dissolving, revealing, pre₋sintering, crashing and grinding, revealing in amorphous mortar, mold formation and final₋sintering etc. from grain compounds. Varistor is finished by making electrodes on both slab sides metalizing with silver coating and soldering terminals.

5. Characteristic Parameters of Varistor

ZnO varistor can be treated as is composed with multi₋layer of n₋type semiconductor. Each layer has approximately 3.3 volt of potential barrier (3~3.6V) when the semiconductor flows current of 100μA. The voltage versus current curve of ZnO varistor is symmetrical for their potential polarity. If we treat the potential barrier as 3.3 volt per layer, then we can approximate the knee voltage of thickness 1 mm as following,

Knee voltage of 1mm thickness=3.3×Number of layer=3.3×(1/0.02)=165(V),

where the thickness of a layer is 0.02 mm.

⒜ VaristorVoltage (V1mA): The voltage that the varistor current flows 1mA, when the voltage is increased slowly. Varistor voltage are different as applying voltage with DC or AC. Ingeneral varistor voltage indicates as with DC.

⒝ Leakage Current (LI): The current of varistor flows when the potential is 80% of varistor voltage.

⒞ Nonlinearity coefficient (α): In the of varistor acting voltage area, the ratio of logarithmic of current ratio over logarithmic of voltage ratio, which can be calculated as follow,

α = log(I2/I1)/log(V2/V1)=(logI2–logI1)/(logV2–logV1).

In general, this parameter is used for the laboratory purpose, not for the electrical specification purpose.

⒟ Maximum applied Continuous Voltage, or Maximum allowable Voltage (Vmax): The voltage which do not change the varistor characteristics and do not make the varistor tired. That is the maximum voltage of which the current flowing through varistor is negligible. In general the Vmax value is around two of third of V1mA.

⒠ Clamping Voltage (Vc): Suppressing voltage tested when the current flows through certain values depending on the varistor dimension (area), for examples, 150A for the disc ceramic of 25 mm diameter and 300A for 40 mm diameter.

⒡ Maximum Peak Current (Ipeak): For the suge current of wave form 8/20μs(8μs of rising time and 20μs of sustaining time of half amplitude), the maximum dischargeable peak surge current. It is permitted only one time discharge. If the current is bigger than this value, the varistor probably get tired and the capability of varistor would be deteriorated. Typical Ipeak is 20kA for 25mm diameter disc varistor and 40kA for 40mm diameter disc varistor.

⒢ Maximum permissible Energy (Wmax): The energy dissipated through a variator can be presented by following because the power P can be presented as V(t)I(t),

The energy of the wave form of the current T1/T2 (rising time/sustaining time) can be approximated by,

W = VpIp(1.4T2–0.88T1)10₋6.

Where the VP and IP are peak value of voltage and current respectively.

Many maker of varistor indicates the peak energy for 10/1000μs current wave and the typical value for 25mm diameter V1mA=510V is 440 joule and for 40mm diameter V1mA is 640joule. (1J=1wattsecond)

⒣ Average Dissipated Power (Pc): Power is denoted by Pc=IV and according to equation (*), I=KVα, power can be expressed as follow,

Pc=KVα+1.

Because the varistor coefficient α is varying by applying voltage, the average power is differ by voltage also. In general, Pc is dependent of the heat capacity of varistor body. Some makers denote Pc, as 1W for 25mm diameter disc varistor, 1.4W for 34mm diameter and1.8Wfor40mmdiameter.

⒤ Response Time: The time to make the varistor from insulator state to conductor state, that is, the time to make the current increase suddenly. The response time of varistor itself is very short as nanosecond order (ns is one over billions second = 10₋9 s). So the varistor can catch up an electrostatic transient of 50A/ns. But the terminal lead of varistor makes the speeds lower as up to 15ns around, because the lead has a little inductance. This parameter is used in laboratory only. None of makers indicates this parameter in their specification sheet.

6. Applications of Varistor

⒜ Equivalent Circuit of Varistor: The ZnO varistor can be expressed by an equivalent circuit as flow,

“on” resistance of ZnO “r” is connected to the parallel connection of total capacitances “C” and the non₋linear variable resistor “ρ”. That is “r + (C // ρ)”.

Because of the “C” in this expression and “C” absorbs high frequency signals, varistor application is limited only use on low frequency signal, like a commercial electrical or some control data lines.

⒝ In every semiconductor, current flow in proportion to their temperature increasing, and currents make temperature increase too. When the temperature increase is exceeding a certain temperature, the current is increasing suddenly and very quickly. In the case of currents increase thermally, a certain layer of ceramic varistor lost probably its property and be a simple metal layer. Temperature rise and current rise make the other layer heated and eventually the varistor destroyed to shorted metal state. We know this phenomenon as “thermal runaway”. If there is no protective element which will interrupt the current by thermal coupling with variator, an explosion of varistor or an interruption the electrical system cannot be avoid.

⒞ As we know well, the current varies sensitively around the varistor knee voltage. In the case of several varistors have been connected in parallel and the applied voltage is around the knee voltage, the current flow concentrated in most sensitive one. But the applied voltage is quite high from the knee voltage and varistor characteristics are almost same, then the current flow of varistor might be almost same for each.