A Metal Oxide Varistor, or MOV, is the blue or orange circular component you can usually detect on the AC input side of allelectric circuit. HeMetalloxid-varistorcan be seen as a different kind ofvariable resistorwhich can vary its resistance as a function of the applied voltage. When a high current flows through an MOV, its resistance value decreases and it acts like a short circuit. Therefore, MOVs are usually used in parallel with a fuse to protect circuits from high voltage spikes. In this article we will learn more about it.MOV worksand how to use it in your projectsProtect your circuits against voltage spikes. We'll also learn about the electrical properties of MOVs and how to choose an MOV for your project needs, so let's get started.
What is a MOV (Metal Oxide Varistor)?
MOV is simply a variable resistor, but differentPotentiometer, MOVs canchanges its resistance depending on the applied voltage.As voltage increases, resistance decreases and vice versa. This property is useful for protecting circuits from high voltage spikes; therefore youhey are mostly used assurge protectionin an electronic network. A simple MOV is shown in the image below
How does MOV work?
Under normal operating conditions, the resistance of the MOV is high and consumes very little current, but when there is a grid surge, the voltage across the MOVKnee strain or pinchand draw more current, this dissipates the surge and protects the equipment. MOVs can only be used forshort-term surge protection, they cannot handle sustained surges. If MOVs are subjected to repeated surges, their characteristics can easily deteriorate. In the event of an overvoltage, the clamping voltage drops slightly, which can lead to its destruction after some time. To avoid this type of risk, MOVs are usually placed in series with a thermal switch/fuse that can trip if a high current is drawn. Let's discuss more about how MOV works in a loop.
How to use a MOV in your loop?
MOVs, also known as varistors, are commonly used in conjunction with a fuse in parallel with the circuit to be protected. The following image shows how to use it.MOV in electronicsthe circuit.
When the voltage is within the rated limits, the resistance of the MOV is very high and therefore all current flows through the circuit and no current flows through the MOV. However, if there is a spike in the utility voltage, it will show up directly on the MOV as it is paralleled with the AC utility. This high voltage reduces the resistance of the MOV to a very low value, making it look like a short circuit.
This will cause a large current to flow through the MOV, which will blow the fuse and disconnect the circuit from main voltage. During voltage spikes, the faulty high voltage returns to normal very soon; in these cases, the duration of current flow is not high enough to blow the fuse and the circuit returns to normal operation when the voltage normalizes. But every time a spike is detected, the MOV temporarily disconnects the circuit, causing a short circuit and damaging itself with high current each time. So if you find that an MOV is damaged in a circuit, it could be because the circuit has experienced too many voltage spikes.
Construction MOV
The metal oxide varistor is avoltage dependent resistorwhich is made with ceramic powders of metallic oxides like zinc oxide and some other metallic oxides like cobalt, manganese, bismuth, etc. An MOV consists of approximately 90% zinc oxide and a small amount of other metal oxides. Metal oxide ceramic powders are held intact between two metal plates called electrodes.
The metal oxide grains create a diode junction between each immediate neighbor. Therefore, a MOV is a large number ofdiodesconnected in series. If you apply a small voltage to the electrodes, areverse currentappears above the intersections. Initially, the generated current is small, but when a large voltage is applied to the MOV, the diode junctions collapse due to electron tunneling and avalanche breakdown. Heinternal structure of a MOVshown in the image below.
HeMOV-varistorbegins to conduct when a certain voltage is applied to the connecting lines and stops conducting when the voltage drops below thevoltage limit. MOVs are available in various form factors such as disc form, axial tap devices, block and screw terminals, and radial tap devices. HeMOVs should always be connected in parallelFor greater power handling capability and if you want a higher voltage rating, connect them in series.
Electrical properties of MOV
Let's look at the different electrical properties of MOV to understand them better.MOV properties.
static resistance
The static resistance curve of an MOV is plotted with the resistance value of the MOV on the x-axis and the voltage value on the y-axis.
The above curve is the voltage and resistance curve of an MOV, at normal voltage the resistance is at maximum, but as the voltage increases the resistance of the varistor decreases. This curve can be used to understand how much resistance your MOV will have at different voltage levels.
VI Properties
According to Ohm's Law, the V-I characteristic of a linear resistor is always a straight line, but we cannot expect this with a variable resistor. As you can see in the image below, even with a small change in voltage, there is also a significant change in current.
MOV can work in both directions and therefore has symmetrical bidirectional properties. The curve resembles the characteristic curve of twoZenerdiodenconnected back to back. When the MOV is not conducting, it even has a certain voltage, e.g. B. 0-200 volts, heavy duty. The curve has a linear relationship where the current flowing through the varistor is almost zero. If we increase the applied voltage in the 200-250V range, the resistance decreases and the varistor starts to conduct and a few microamps of current start to flow, which doesn't make much difference in the curve.
As soon as the voltage rise reaches the nominal or blocking voltage (250 V), the varistor becomes highly conductive, approximately 1 mA of current begins to flow through the varistor. When the transient voltage across the varistor is equal to or greater than the clamping voltage, the resistance of the varistor becomes small, making it a conductor due to the avalanche effect of the semiconductor material.
ability to move
As we already know that MOV is built with two electrodes, it acts as a dielectric medium and has capacitor effects, which can affect the system operation if not taken into account. Each semiconductor varistor has a capacitance value as a function of area, which is also inversely dependent on its thickness.
The capacitance value does not matter much in a DC circuit because the capacitance remains almost constant until the device voltage reaches the clamping voltage. There is no capacitance effect when the voltage reaches the clamp voltage when the varistor starts its normal function.
When dealing with AC circuits, the capacitance of the MOV can affect the overall resistance of the MOV body, causingleakage current. Since the varistor is connected in parallel with the device to be protected, the bleed resistance of the varistor drops rapidly with increasing frequency. HeMOV reactance valuecan be calculated by the formula
Xc=1/2πfC
where Xc is the capacitive reactance and f is the frequency of the AC power supply. As the frequency increases, the leakage current also increases, as shownin the non-conducting vanishing region of the V-I characteristic curve discussed above.
Choosing the right MOV for protection
You must know the different parameters of a MOV to choose the right device for your computer. HeSpecifying an MOVdepends on the following
- Maximum operating voltage:It is the steady-state DC voltage at which the typical leakage current is less than the specified value.
- voltage suppressed:It is the voltage at which the MOV starts to conduct and dissipate the overcurrent.
- about current:It is the maximum peak current that can be supplied to the device without damaging it; it is mainly expressed in "current for a certain time". Although the device can handle the spike, manufacturers recommend replacing the device when a spike occurs.
- Overvoltage change:Every time the device experiences a surge, the rated clamping voltage drops, the voltage change after the surge is called the surge offset.
- Energy consumption:The maximum amount of energy the MOV can dissipate during a given peak pulse time of a given waveform during a burst. This value can be determined by operating all devices within a specific control loop with specific values. Energy is usually expressed in standard x/y transients, where x is the slope of the transient and y is the time to reach half-level.
- Reaction time:It is the time in which the varistor starts to conduct after the occurrence of overvoltage, in many cases there is no exact response time. Typical response time is always set to 100 ns.
- Max AC Voltage:It is the maximum RMS line voltage that can be constantly supplied to the varistor, the maximum RMS value should be chosen slightly above the actual RMS line voltage. The maximum voltage of the sine wave must not overlap with the minimum varistor as this can reduce component life. In the product description, the manufacturers themselves specify the maximum AC voltage that we can supply to the device.
- leakage current:It is the amount of current consumed by the varistor when it operates below the clamping voltage, that is, h if there is no overvoltage in the network. Usually, the leakage current is specified at a certain operating voltage in the device.
MOV apps
MOVs can be used to protect different types of devices from different types of errors. They can be used for single-phase phase-to-phase protection and single-phase phase-to-phase and phase-to-earth protection in AC/DC circuits. They can be used as solid state switching protection in transistors, MOSFETs or thyristors and as arcing protection in motor driven equipment.
In terms of application, MOVs can be used in circuits where there is a risk of surges or voltage spikes. MOVs are mainly used in surge-protected adapters and strips, power supplies connected to the mains, telephone lines and other communication lines, industrial high-power AC line protection, data systems or power systems, general electronic devices such as cell phones, digital cameras, personal digital assistants, MP3 players and laptops.
MOVs are also used as microwave mixers for modulation, detection and frequency conversion in some cases, which are not the most famous MOV applications.
MOV protection circuit design tips
Now that we've discussed what an MOV is and how it's used to protect your circuit from surges, let's wrap up the article with some design tips that will come in handy when designing your circuit.
- The first step in selecting an MOV is to determine the steady-state operating voltage that will be supplied across the varistor. You must select the varistor with a maximum AC or DC voltage equal to or slightly greater than the applied voltage. It is common to choose the varistor that has a maximum rated voltage of 10 to 15% higher than the actual grid voltage, as power lines always have a voltage fluctuation tolerance. This ratio is partially included in the voltage values, if you want to get an extremely low leakage current despite the lowest possible protection level, you can use the varistor with a higher operating voltage.
- Find out how much energy the varistor absorbs in case of a surge. This can be determined from the absolute maximum load of the varistor during an ambient overvoltage and the specifications given in the data sheet. You should choose the varistor that can dissipate more power equal to or slightly more than the required dissipated power during the overvoltage that the circuit can generate.
- Calculate the peak transient current or transient current through the varistor. You must select the varistor whose surge current rating is equal to or slightly higher than the current rating required by an event that the circuit may generate to ensure proper operation.
- Similar to all of the above features, you must also determine the required power dissipation and select the varistor that has a wattage rating that ideally matches or exceeds the wattage required by the circuit.
- The power, peak current, and rated power are always chosen to be greater than the expected event. If you are unsure about the event factors, it is recommended that you choose the device with the highest power and the power and overcurrent metrics.
- The final and most important step of all is selecting the model that can provide the required clamping tension. You can choose the clamp voltage based on the approximate maximum voltage your circuit will allow to enter or exit during an event. You need to make sure your circuit can handle this stress, this is the biggest stress your circuit will experience on the downline.