What Is Metal Oxide Varistor
Metal oxide varistors are voltage dependent, nonlinear devices which have an electrical behavior similar to back-to-back zener diodes. The symmetrical, sharp breakdown characteristics enable the metal oxide varistor to provide excellent transient suppression performance. When exposed to high voltage transients the metal oxide varistor impedance changes many orders of magnitude from a near open circuit to a highly conductive level, thus clamping the transient voltage to a safe level. The potentially destructive energy of the incoming transient pulse is absorbed by the metal oxide varistor, thereby protecting vulnerable circuit components.
Advantages of Metal Oxide Varistor
Provide effective voltage clamping
Metal oxide varistor provide effective voltage clamping, diverting excess voltage away from sensitive electronic components and preventing them from being damaged by transient events.
Fast response
Metal oxide varistor have a fast response time, reacting quickly to voltage spikes and limiting the duration of overvoltage conditions.
Cost-effective
Metal oxide varistor are relatively simple and cost-effective components.
Durable
Under normal operating conditions, metal oxide varistor have a long lifespan.
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We have built a modern standard workshop with an area of 9000 square meters, hired more than 100 skillful employees, and set up three modern production lines with annual output of 15 million 34S’s standard MOVs and 3 million onboard SPDs.
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Our products have been approved by UL and TUV safety certifications. Moreover, our company has obtained ISO 9001:2018 quality management system certification.
Advanced equipment
We also own a complete list of testing equipment, including 120kA 8/20μs and 15kA 10/350μs lightning tester, 2 ms rectangular surge tester, power frequency temporary overvoltage (TOV) tester, high temperature aging test chamber, constant temperature and humidity chamber, which effectively guarantees the safety and reliability of our product performance.
Our products
We have deliberately focused on metal oxide varistor (MOV), surge protected devices (SPD), and related technology services for more than 20 years.Our main product is MOV that be used in SPD and SPD.
Electrical Characteristics of Metal Oxide Varistor
Static resistance
The metal oxide varistor static resistance curve is plotted with the resistance value of the X-axis metal oxide varistor and the Y-axis voltage value. It is a metal oxide varistor's voltage and resistance curve; the resistance is at its highest at the standard voltage, but the varistor's resistance decreases as the voltage rises. This curve can be used to understand how much resistance at various voltage levels would be in your metal oxide varistor.
V-I characteristics
The V-I characteristic curve of a linear resistor is always a straight line, according to Ohms law, but in terms of a variable resistor, we can not assume the same. The metal oxide varistor can work in both directions, so it has bi-directional, symmetrical characteristics. The curve would look identical to the characteristic curve of two back-to-back connected Zener diodes. The curve has a linear relationship when the metal oxide varistor does not work, where the current flowing through the varistor is almost zero, with a high resistance up to a certain voltage, say 0-200Volts. The resistance decreases as we increase the applied voltage in the range of 200-250V, and the varistor starts conducting and a few current micro-amperes begin to flow, which does not make much difference in the curve. The varistor becomes highly conductive once the rising voltage exceeds the rated or clamping voltage (250V), about 1mA of current begins to flow through the varistor. The resistance of the varistor becomes small when the transient voltage across the varistor is equal to or greater than the clamping voltage, which transforms it into a conductor due to the semiconductor material's avalanche effect.
Capacitance of metal oxide varistor
As we have already recognized that the metal oxide varistor is built with two electrodes, it operates as a dielectric medium and has capacitor effects that, if not taken into account, could influence the system's functioning. Depending on the region that is also inversely dependent on its thickness, each semiconductor varistor will have a capacitance value. When it comes to a DC circuit, the capacitance value is not a big deal, because the capacitance will remain almost constant until the device's voltage exceeds the clamping voltage. When the voltage exceeds the clamping voltage, there will be no capacitance effect as the varistor begins its normal work. The capacitance of the metal oxide varistor could affect the overall body resistance of the metal oxide varistor that causes the leakage current when it comes to AC circuits. The leakage resistance of the varistor decreases rapidly as the frequency increases as the varistor is connected parallel to the system to be covered. The metal oxide varistor reactance value can be determined using the formula.
How to Choose the Right Metal Oxide Varistor for Protection
Maximum working voltage
This is the DC steady-state voltage at which the typical leakage current is lower than the value you specify.
Clamping voltage
It is the voltage at which the surge current begins to be conducted and dissipated by the metal oxide varistor.
Surge current
It is the maximum peak current that can be given to the device without causing any harm to the device; it is often expressed for a given time in 'current'. The manufacturers suggest remetal oxide varistoring the system if there is an event of surge current, although the device can handle the surge current.
Surge shift
If the system experiences a spike, the rated clamping voltage decreases, the surge shift is called the variation in the voltage after the surge.
Energy absorption
The maximum amount of energy that can be dissipated during a surge by the metal oxide varistor for a given peak pulse period of a particular waveform. You may evaluate this value by running all the devices with unique values inside a particular regulated circuit. In standard transient x/y, the energy is normally expressed where x is the transient rise and y is the time to reach its half-peak value.
Response time
It is the time at which the varistor begins to conduct after the surge occurs, there is no exact response time in certain cases. The standard time of response is always set as 100nS.
Maximum AC voltage
It is the maximum RMS line voltage that can be given to the varistor constantly, the maximum RMS value should be chosen to be slightly above the actual RMS line voltage. The peak voltage of the sine wave should not overlap with the minimum varistor, if it does, it might reduce the lifetime of the components. In the product description itself, the manufacturers can define the maximum AC voltage we can supply to the system.
Leakage current
When the varistor works below the clamping voltage, it is the amount of current that the varistor draws when there is no surge in the network. The leakage current will usually be defined across the system at a given operating voltage.
Introduction to Basic Information About Metal Oxide Varistor
Metal oxide varistors use a mixture of oxide powders such as bismuth, cobalt, manganese, and zinc oxides to form a ceramic disk-shaped mass which is a semi conductive (not always conductive), thus creating a semiconductor known as a varistor. The ceramic oxide mass mixture contains a larger or greater proportional amount of zinc oxide. The unique semi conductive properties of the zinc oxide based varistor determine what level or amount voltage is required to make the varistor conductive. Normally, or under normal voltage, the varistor is an open non-conductive circuit. The varistor is not conductive until the voltage on the load side surpasses the maximum continuous operating voltage (mcov) determined by the design and mixture of the oxide varistor. Once the metal oxide varistor is subjected to voltage above the mcov, the metal oxide varistor becomes conductive, passes the voltage through the oxide matrix, and discharges the abnormal current out to the ground conductor.
Commonly on standard 120vac power supplies, the mcov will be around or slightly above 15 or 20 amps. That is, it will not conduct voltage from the load electrode across the varistor mass to the ground electrode until voltage above the maximum continuous operating voltage (mcov) is induced into the metal oxide varistor oxide matrix. When abnormal higher voltage is induced from the load electrode into the normally non-conductive oxide varistor, the oxides in the varistor are stimulated to form diode-junctions that close the normally open gaps between the oxide crystals. The diode-junctions become circuit bridges that allow voltage current to pass from one oxide crystal to the other. This makes the varistor conductive allowing voltage to be conducted from one side of the varistor to the other. This passage of voltage across the varistor shunts or bleeds-off a portion of the abnormal voltage from the load circuit through the now (active) conductive varistor into the ground conductor. The now energized ground conducts that abnormal voltage back to the power strip surge breaker switch, causing that breaker switch to heat and trip the switch to open / off.
Depending on the amount of abnormal voltage conducted into and through the metal oxide varistor, the metal oxide varistor will shunt the abnormal voltage to the ground without issue, or the metal oxide varistor will self-destruct, self-sacrifice, or burn-out as abnormal voltage conduction leads to increased uncompensated electrical residence that cannot be easily conducted or discharged from one side of the metal oxide varistor to the other. This is known as surpassing the voltage discharge threshold of the metal oxide varistor. Like a balloon, if the air pressure and relative air volume are too great, the balloon will fill and then burst. Lightning is all to often the culprit that caused a power strip surge suppressor to fail and ignite causing a fire. We often see homeowner modifications to power strip surge suppressors where the homeowner or consumer remetal oxide varistored the power strip surge suppressor grounding pin form the plug. By remetal oxide varistoring the grounding pin, you have disabled the suppressor by taking away the prime avenue used by the metal oxide varistor protection to conduct the excessive voltage and the subsequent increased electrical resistance heating needed to trip the suppressor. The ground is always a closed conductive circuit. If the plug ground pin is remetal oxide varistored the ground circuit is now open and non-conductive.
The Manufacturing Process of Metal Oxide Varistor
Material selection
The foundation of a quality metal oxide varistor lies in the choice of materials. Manufacturers carefully select the metal oxide composition and other materials to meet specifc voltage and energy absorption requirements.
Electrode application
Metallic electrodes are applied to the ceramic discs to provide electrical connections. The placement and design of these electrodes are critical to the metal oxide varistor's performance.
Ceramic processing
The selected materials are mixed, shaped, and processed to create the ceramic-like discs that form the core of the metal oxide varistor. These discs are then sintered at high temperatures to achieve the desired electrical properties.
Testing and quality control
Every metal oxide varistor undergoes rigorous testing during and after manufacturing. This includes electrical testing to ensure the varistor meets its specifed clamping voltage and energy absorption capabilities. Measures are applied to identify and reject any varistors that do not meet the required standards.
You have to choose the varistor with the highest AC or DC voltage that matches or slightly higher than the applied voltage. The first step in selecting a metal oxide varistor is to decide the continuous working voltage that will be given through the varistor. It is normal to choose a varistor that has a maximum rated voltage 10-15 percent higher than the actual line voltage as the supply lines often have tolerance of voltage variance. In some situations, if you prefer to achieve an exceptionally low leakage current despite the lowest safety level available, you can use the varistor with a higher operating voltage. This ratio would be included in their voltage values.
Find out the amount of energy that the varistor consumes in the event of a wave. This can be calculated by using all of the varistor's absolute maximum load during an environmental surge and the requirements given in the datasheet. You can choose the varistor that can dissipate more energy that is equal to or slightly greater than the energy dissipation that the circuit can generate during the surge. Using the varistor to measure the peak transient current or the surge current. In order to ensure that it works properly, you should pick the varistor that has the surge current rating equal to or slightly greater than the current rating required by an event that the circuit will cause.
You can also decide the power dissipation needed and select the varistor that has a power rating equal to or preferably exceeds the power handling required by the event that the circuit can generate. Similar to all the above properties. If you are unsure about the factors of the case, the prudent thing to do is to choose the system with higher strength, surge current and energy ratings. The power, surge current and energy ratings are often chosen in a way that is greater than the predicted event. Selecting the model that can provide the necessary clamping voltage is the final and most significant step of all. Based on the estimated peak voltage value, you can choose the clamping voltage that will allow the input or output of your circuit to be seen during a case. This will be the maximum voltage your circuit down line will feel, you should make sure that your circuit will be able to withstand this voltage.
Our factory
GuiYang High-Tech YiGe Electronic Co., Ltd has been founded in October 2010 by four natural persons. We have deliberately focused on metal oxide varistor (MOV), surge protected devices (SPD), and related technology services for more than 20 years. As a national high-tech enterprise, we have a strong research and development (R&D) ability and competitive technological strength. We also have a long-term close cooperation with Guizhou University and obtained patents for many extraordinary inventions. Mr. Zihao Fei is our company’s legal person. He is a well-known Chinese expert in varistor technology and served in the International Electrotechnical Commission (IEC) SC37B for 5 years and Chinese Institute of Electronics (CIE) for 15 years. He participated in drawing up the international standard: IEC61643-331.2017: Components for Low-Voltage Surge Protective Devices Part 331: Performance Requirements and Test Methods for Metal Oxide Varistors (MOV). He is also the main drafter of the national standard: GB18802.331.2020: Components for low-voltage surge protective devices Part 331: Performance requirements and test methods for metal oxide varistors (MOV).
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