The Advantages of High Voltage Ceramic Capacitors compared with film capacitor

 High voltage proof. Voltage Proof Test is to test whether the voltage at both ends of the capacitor lead can reach its corresponding withstand voltage standard and specified standard. Normally for Film capacitors, the voltage proof is 1.2UR, while high voltage ceramic capacitors can reach 2UR.

Smaller capacity. Compared with a film capacitor, a high voltage ceramic capacitor can achieve a small capacity. For regular models of film capacitor CBB21, the smallest capacity is 0.1uf, while high voltage ceramic capacitors can achieve 0.5pf and 1pf.

Smaller pitch. High voltage ceramic voltage can achieve a small pitch. For film capacitors, the pitches for regular models are as follow, 7.5mm, 10mm, 15mm, 20mm, 22.5mm, 27.5mm. The pitch is normally quite larger, while the pitch of high voltage ceramic capacitors can be 2.5mm, 5mm, 7.5mm.

JEC is a professional manufacturer in China for more than 30 years. Our safety capacitors receive all safety certifications we mentioned above from industry powers. We have a strict quality control system, please be rest assured. So if you’re interested in our products, welcome to contact us, we’re here 24 hours online waiting for you. 

What Is a Varistor?

 A "varistor" is a resistance device with non-linear volt-ampere characteristics. It is mainly used to clamp the voltage when the circuit is subjected to overvoltage and absorb excess current to protect sensitive devices. The resistor body material of the varistor is a semiconductor, which is a voltage-limiting protection device. Using the non-linear characteristics of the varistor, when an overvoltage occurs between the two poles of the varistor, the varistor can clamp the voltage to a relatively fixed voltage value, thereby realizing the protection of the subsequent circuit. The main parameters of the varistor are varistor voltage, current capacity, junction capacitance, response time, etc.

1.Working principle of the varistor

When the voltage applied to the varistor is lower than its threshold, the current flowing through it is extremely small, which is equivalent to a resistor with infinite resistance. In other words, when the voltage applied to it is lower than its threshold, it is equivalent to an off-state switch.

When the voltage applied to the varistor exceeds its threshold, the current flowing through it increases sharply, which is equivalent to a resistor with infinite resistance. In other words, when the voltage applied to it is higher than its threshold, it is equivalent to a closed switch

2.Precautions for varistor

(1) It must be ensured that the continuous working voltage will not exceed the maximum allowable value when the voltage fluctuation is the largest, otherwise the service life of the varistor will be shortened;

(2) When a varistor is used between the power line and the ground, sometimes the voltage between the line and the ground rises due to poor grounding. Therefore, a varistor with a higher nominal voltage than the line-to-line use is usually used.

The failure mode of the varistor is usually a short circuit. In order to prevent the failure of the varistor from causing a short circuit of the power supply and catching fire, a temperature fuse or thermal release mechanism can be connected in series with each varistor. The temperature fuse should have a good thermal coupling with the varistor. When the varistor fails (high impedance short circuit), the heat generated by it will fuse the temperature fuse, thereby separating the failed varistor from the circuit and ensuring the equipment Security. When a higher power frequency temporary overvoltage acts on the varistor, it may cause the varistor to break down and short-circuit (low impedance short circuit) instantaneously, and the temperature fuse is too late to fuse, or it may catch fire. In order to avoid this phenomenon, an impact-resistant power frequency fuse can be connected in series with each varistor (single-use power frequency fuse may not blow when aging failure). The varistor can also be used in series with the ceramic gas discharge tube. The ceramic gas discharge tube does not conduct during normal operation, and the varistor has no leakage current, which can greatly extend the service life; when it is impacted by a surge, the ceramic gas discharge tube is the first to hit Then, the surge voltage is limited by the varistor. The total residual voltage is the sum of the two, which increases slightly (tens of volts); after the impact, the discharge tube cannot maintain conduction due to the varistor limiting the current The arc is extinguished and the normal working state is restored; when the varistor fails in a short circuit, the ceramic gas discharge tube will quickly fail due to the large power frequency current flowing through the ceramic gas discharge tube, but most of its failure modes are open circuits, so it is not easy to cause a fire.

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Varistor

 

What is a varistor?

A varistor is a voltage-dependent resistor(VDR). The resistance of a varistor is variable and depends on the voltage applied. The word is derived from the "variable resistor. As the voltage increases, their resistance decreases. If the voltage is too high, its resistance will drop sharply. This behavior makes them suitable for protecting circuits during surges. The causes of the surge may include lightning and electrostatic discharge. The most common type of VDR is metal oxide varistor or MOV.

Definition

The varistor is a nonlinear binary semiconductor whose resistance decreases with the increase of voltage. Voltage-dependent resistors are commonly used as surge suppressors for sensitive circuits.

Characteristics

A resistor having a nonlinear change, depending on the voltage applied. Under nominal load conditions, the impedance is very high, but when the voltage threshold (the breakdown voltage) is exceeded, the impedance drops sharply to a low value. They are usually used to protect circuits from excessive transient voltages. When the circuit is exposed to high voltage transients, the varistor initiates conduction and the transient voltage clamp is brought to a safe level. The energy from the incoming surge is partially transmitted and partially absorbed, thereby protecting the circuit.

The most common type is MOV or metal oxide varistor. They consist of sintered substrates of zinc oxide (ZnO) grains. Grain boundaries provide PN junction semiconductor characteristics, similar to diode junctions. The matrix of randomly oriented grains can be compared with large diode networks in series and parallel. When a low voltage is applied, the current due to the reverse leakage through the junction is very small. However, when a high voltage is applied that exceeds the breakdown voltage, an avalanche breakdown occurs and a large current flows through the junction. This behavior leads to nonlinear current-voltage characteristics.

The construction of the varistor

A varistor is formed when a crystal of silicon carbide or metal oxide is pressed into a ceramic material.

The material is then dried and sintered at a high temperature. The electrical properties of the equipment depend on the temperature and atmospheric conditions provided.

For good electrical contact, the contact point of the material is metalized with silver or copper. Then lead is then welded to the contact and the varistor is supplied and coded.

Application field

The nonlinear characteristics of varistors make them ideal for use as surge protectors. The source of high voltage transients may be an electrostatic or inductive discharge from a motor or transformer, such as a lightning strike. For example, they are commonly used for surge protectors on power panels. A special type of low capacitance protects communication lines. These VDRS are available for a variety of applications, including:

Telephone and other communication line protection

Transient suppression of radio communication equipment

Surge protector power panel

Surge protector for the cable TV system

Power protection

Microprocessor protection

Electronic equipment protection

Low-pressure plate level protection

Transient voltage surge suppressor (TVSS)

Automobile electronic protection

Industrial high energy exchange protection

Advantages of varistor

It provides excellent overpressure protection.

Because it does not show polarity effects, it is easy to achieve bidirectionally.

Disadvantages of varistor

This is expensive equipment.

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