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Metal Oxide Varistor (MOV)
The varistor is composed of metal oxide (mainly zinc oxide) material, which is a clamp-type device. Its characteristics are very similar to those of two back-to-back Zener tubes, and it has a response speed of nanoseconds. The absorbing ability of a varistor to transient signals is proportional to its volume, its thickness is proportional to the threshold voltage, and its area is proportional to the absorption current. The P084A2003 varistor is currently the most widely used surge suppression device in electronic products.
When the voltage on the varistor exceeds a certain range (threshold), the resistance of the resistor is greatly reduced, thereby discharging the surge energy. Under the action of the surge voltage, the voltage on the varistor after it is turned on (generally called the clamping voltage) is equal to the current flowing through the varistor multiplied by the resistance of the varistor, so the peak value of the surge current The clamp voltage reaches the highest point.
Metal Oxide Varistor
The voltage range is very wide, from several volts to several thousand volts, and the absorption surge current can range from tens to several thousand amperes. The response speed is fast, there is no polarity, no freewheeling, the peak current capacity is large, and the price is low.
The clamping voltage is relatively high, which can generally reach 2 to 3 times the working voltage. The leakage current of the varistor increases with the increase in the number of surges. The response time is longer and the parasitic capacitance is larger.
DC power line, low-frequency signal line, or connected in series with gas discharge tube for AC power line.
Tips for choosing the right MOV
To select the correct MOV for a specific application, it is necessary to understand the source impedance and the possible pulse power of the transient. For input line or phase transients, the selection of the correct MOV is slightly more difficult, because the characteristics of the power supply are generally unknown. Generally speaking, the electrical protection of MOV selection circuit power transients and spikes is usually just an educated guess.
However, metal oxide varistors can be used for a variety of varistor voltages, from about 10 volts to more than 1000 volts AC or DC, so you can help you choose by knowing the power supply voltage. For example, choose MOV or silicon varistor. For voltage, its maximum continuous root mean square voltage rating should be slightly higher than the highest expected power supply voltage. For example, a 120 volt power supply is 130 volts rms, and 230 volts is a 260 volts rms supply.
The maximum inrush current value that the varistor will use depends on the transient pulse width and the number of pulse repetitions. An assumption can be made about the width of the transient pulse, which is usually 20 to 50 microseconds (μs) long. If the peak pulse current rating is insufficient, the varistor may overheat and be damaged. Therefore, for the varistor to operate without any failure or degradation, it must be able to quickly dissipate the absorbed energy of the transient pulse and safely return to its pre-pulse state.