Selection, Parameters and Classification of Varistors
Voltage Sensitive Resistors (VSRs) are not ordinary resistors, but a kind of components with transient voltage suppression function, the effect is the same as TVS.
This article introduces some basic knowledge of varistors, including parameters, selection, application and so on.
Modern varistor schematic symbol, which is the same as a thermistor symbol
Basic Knowledge about Varistor
Varistor with MY, MY suffix: J (household), W (voltage regulator), G (overvoltage), P (high-frequency), L (lightning), H (arc extinguishing), Z (noise), etc., which is the general naming approach, different manufacturers of the naming is not quite the same.
Voltage Dependent Resistor is a special resistor with non-linear volt-ampere characteristics, called Voltage Dependent Resistor, abbreviated as VDR, or called Varistor.
Voltage Dependent Resistor is not a real resistor, but a component with transient voltage suppression function, generally no positive and negative poles, which is different from TVS, the same is connected in parallel with the protected IC or circuit, the response time of VDR will be a little slower than TVS.
When the voltage added to the varistor on the voltage exceeds a certain value, its resistance will fall rapidly to conduct a large current to protect the back-end circuit; when lower than its operating voltage, the varistor resistance is extremely high, equivalent to an open circuit, does not affect the operating state of the back-end circuit.
Although varistors can absorb large amounts of surge energy, they cannot withstand sustained currents above the milliamperes level, which must be taken into account when they are used for overvoltage protection.
Let's take a look at the parameters of the varistor.
The following are the electrical parameters of a particular varistor.
Varistor current vs voltage for zinc oxide (ZnO) and silicon carbide (SiC) device
Electrical parameters of a particular varistor
▉ Voltage
The breakdown voltage or threshold voltage is the voltage across the varistor measured at a specific current, usually 1mA DC current flowing through the varistor.
The general symbol is Vb@1mA(DC) or V1mA.
▉ Maximum Operating Voltage
There are two types: AC and DC. For AC, it is generally expressed by AC RMS, which means that the AC RMS value applied to the varistor cannot exceed this value, and Vwac in SPEC above means that the AC voltage RMS value applied to the varistor cannot exceed 4 V. For DC, the maximum voltage of the protected signal or circuit cannot exceed this value.
The Vdc in SPEC is 5.5V, i.e., this varistor is not suitable for DC circuits above 5.5V.
▉ Maximum Clamp Voltage
This refers to the voltage across the varistor when a specified pulse energy waveform such as (8/20µs) is applied. From SPEC, the maximum clamping voltage is 18V when an 8/20µs pulse waveform is applied to the varistor. 8/20us pulse means that 100% Ipp is reached at 8us and 50% Ipp is reached at 20us.
▉ Energy tolerance
The maximum energy absorbed by a varistor when a specified pulse energy waveform is applied (e.g., 10/1000µs waveform), symbolized by E, and the unit is J. The larger the chip diameter of a varistor, the larger the energy withstand and the larger the withstand current.
The formula for calculating energy resistance is: W=KIVT, I is the peak current flowing through the varistor, K is the waveform coefficient of current I. Different pulse waveform coefficients K are not the same (K=1 for a square wave of 2ms, K=1.4 for a waveform of 8/20μs, K=1.4 for a waveform of 10/1000μs), T is the duration of the current, and V is the voltage across the varistor at the time of current I. The 10/1000µs pulse energy is calculated by the following formula: W = KIVT.
▉ Inrush current
This is the maximum current at which the electrical characteristics of the varistor will not be degraded when a specified pulse energy waveform (e.g., 8/20µs) is applied. As can be seen from the SPEC, the maximum inrush current of this varistor is up to 3A at an 8/20µs pulse waveform.
There is such a definition: 1 time, with 8/20µs standard waveform current as the maximum current value of an inrush, at this time the rate of change of voltage sensitivity is still within ±10%; 2 times, with 8/20µs standard waveform current as the maximum current value of two inrush, and the interval between two inrush time is 5 minutes, at this time the rate of change of voltage sensitivity is still within ±10%.
In some information, there will be the concept of throughput capacity, can also be regarded as the maximum surge current throughput capacity, throughput capacity is more difficult to calculate, a number of piezoresistors in parallel, the pressure-sensitive voltage is unchanged, throughput is equal to the sum of a few, the requirements of parallel piezoresistor volt-ampere characteristics as far as possible the same, otherwise it is easy to cause the shunt is not uniform and damage the piezoresistor on the selection of the general references to the SPEC that can be.
▉ Static capacitance
It refers to the inherent capacitance of the varistor itself, the general test conditions are oscillator frequency of 1KHz or 1MHz, oscillator voltage 1Vrms.
▉ Leakage Current
Nowadays, SPEC does not give the leakage current parameter of varistor. Leakage current, also known as waiting current, refers to the current flowing through the varistor at the specified temperature and maximum DC voltage.
Selection Points
The selection of varistors, I summarize the following points.
First, in AC circuit, V1mA(min)≥(2.2~2.5)*Vac, Vac is the effective value of the working voltage of the protected AC circuit.
Second,in DC circuit, V1mA(min)≥(1.6~2)*Vdc, Vdc is the maximum working voltage of DC circuit.
Third,if the working voltage or withstand voltage of the protected circuit is low, and the surge energy is relatively large, the voltage-sensitive resistor with lower voltage and larger chip diameter can be selected.
Fourth, if the operating voltage or withstand voltage is higher, you can choose a higher voltage-sensitive piezoresistor, both to protect the circuit, but also to extend the life of the piezoresistor.
Fifth, the capacitance of the varistor is generally tens to hundreds of pF, can not be used in high-frequency signals, generally used in low-frequency signals, DC power supply and AC power supply.
Classification of varistors
According to the structure: can be divided into junction type varistors, body type varistors, single particle layer varistors and film varistors.
According to the use of materials: can be divided into zinc oxide varistors, silicon carbide varistors, metal oxide varistors, germanium (silicon) varistors, barium titanate varistors.
According to the volt-ampere characteristics: can be divided into symmetrical varistors (without polarity) and asymmetrical varistors (with polarity), we generally use varistors are not polarized.
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UF Capacitors Varistor Cross to
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Description
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UF Capacitors |
Vishay |
Littelfuse |
MAIDA |
NIC |
Chemi-Con |
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Metal Oxide |
VAR Series |
VDR Series |
LA Varistor Series |
D Series |
NVR Series |
V Series |
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Joyin-JVR |
Panasonic |
TDK/EPCOS |
CNR |
Thinking-TVR |
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JVR**U***K Series |
ERZE**D***K Series |
B722**S***K Series |
CNR**D***K Series |
TVR Series |
