Varistors
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Useful information about varistors
Varistors are often used in the electronics sector to protect sensitive circuits against overvoltage. They absorb dangerously high voltage pulses, suppress switching sparks on electrical contacts, act as voltage limiters and can also be used for voltage stabilization and signal deformation. Read our guide to learn more about .
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What was a varistor?
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What types and types are there?
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Purchase Criteria for Varistors – What's it all about?
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Our practical tip: Operating voltage
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FAQ – Frequently Asked Questions about Varistors
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Conclusion: How to buy the appropriate varistor
What was a varistor?
Varistors are passive electrical components whose resistance depends on the voltage applied. They are also referred to as VDR (Voltage Dependant Resistor) and MOV (Metal Oxide Varistor).
A varistor consists mainly of zinc oxides with admixtures of other metal oxides. Pressed into the desired shape and sintered, a metal layer is applied to this blank on both ends of the head and, if necessary, the connecting wires are soldered on. In the case of disk varistors, a coating of paint or epoxy resin protects the housing from environmental influences.
The voltage-dependant resistors have a pronounced non-linear characteristic. As of a defined threshold voltage, they move quickly from the high-impedance to the low-impedance state. This physical process takes place extremely quickly in the nanosecond range, so that a varistor as a voltage-dependant resistor is ideally suited to effectively intercept and limit overvoltage peaks with a very short rise time (transients).
The voltage-dependant change in the resistance value is due to the fact that the existing blocking layers in the semiconductor zones of the varistor are increasingly degraded by a applied voltage and its electrical field. In the range of the respective threshold voltage, this process increases exponentially, the resistance decreases rapidly and a further voltage increase is prevented.
What types and types are there?
Varistors are available for conventional through-hole mounting (THT) as well as SMD designs. SMD types have a rectangular design similar to chip capacitors, varistors for conventional assembly have a disc design with radial wiring.
Versions are produced for different operating voltages ranging from single-digit voltage range to over 1000 volts. The thickness of the varistor disk is oriented to a certain extent to the varistor voltage, because at higher voltages a more blocking zone is needed. Special car varistors are available for use in the automotive sector, which are tailored to the 12-volt on-board power supply. Depending on the required load capacity with regard to the leakage or energy absorption capacity, the size also varies.
Purchase Criteria for Varistors – What's it all about?
The selection is made after prior determination of the operating parameters of electronic circuits. The maximum tolerable overvoltage determines the type of varistor to be used. Normal voltage tolerances of the power supply upwards must be taken into account; the varistor must not yet respond here. Otherwise, a continuously flowing current will cause harmful heating.
All necessary safety standards must be taken into account, and the varistor must also, if necessary, comply with the relevant standards. In order to ensure a long-term reliable circuit function and protection against overvoltage events, it should not be dimensioned too sparingly with regard to its maximum peak current and the energy leakage capacity. Varistors are relatively inexpensive mass products, so in case of doubt it is advisable to select a more robust version.
Our practical tip: Operating voltage
Never select a lower maximum permissible operating voltage than the one specified in the data sheet. Varistors age quite quickly if they are already partially operated in the conductive area. After a relatively short period of time, they no longer fulfill their function as effective overvoltage protection or only fulfill it inadequately. The same applies to the design of the nominal leakage current. It is better to plan a reserve here than to make a tight dimensioning.
FAQ – Frequently Asked Questions about Varistors
How is a varistor connected?
Varistors are connected in parallel to the voltage source or to the consumer. In the case of particularly sensitive electronic modules, the varistor should be as close as possible to the circuit to be protected in order to keep the line resistances low and thus to optimize the protective function. In special applications, such as the conscious deformation of voltage and current curves, series resistors can also be used.
Do varistors have a polarity?
Varistors are basically non-polarized components, so there is no polarity indication on the housing. In addition to direct voltage operation, they are also suitable for alternating voltage operation.
How do ich calculate which varistor voltage ich need?
In principle, the varistor voltage should be about 20% higher than the supply voltage for direct voltages. At a device operating voltage of 12 V DC, the 14.4 V would be, however, since no varistor is produced with this voltage, the next higher voltage value must be selected; in this case, 18 V. The varistor voltages differ from this when operating at alternating voltages, because at sinusoidal voltages the peak value of the voltage is always higher than the effective value. Please refer to the respective data sheets for detailed information.
Can ich test or measure a varistor for function?
With a multimeter it is difficult to determine whether the varistor is still in order. Varistors for relatively low operating voltages can be checked for compliance with the varistor voltage with an adjustable laboratory power supply by setting the current limitation to a few mA and slowly increasing the voltage. Only in the range of the varistor voltage should a sudden reduction in resistance occur, i.e. the current increases, otherwise the varistor is defective.
Conclusion: How to buy the appropriate varistor
Adjust the electrical values of the varistor to the given rated operating voltage and to the intensity and number of expected overvoltage events. It should be borne in mind that varistors have a limited energy absorption capacity and only endure a certain current. If necessary, it must be taken into account here that several overvoltage pulses can occur in succession – for example in the case of multiple switching operations – without the varistor having sufficient cooling in between.
As a replacement for a defective varistor, the type should be as similar as possible. Varistors are available in staggered type series, so that this is possible in practice without any problems.