Varistor

 
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 composed of parts of the words “variable resistor”. Their resistance decreases when the voltage increases. In case of excessive voltage increases, their resistance drops dramatically. This behavior makes them suitable to protect circuits during voltage surges. Causes of a surge can include lightning strikes and electrostatic discharges. The most common type of VDR is the metal oxide varistor or MOV. Definition Varistors are nonlinear two-element semiconductors that drop in resistance as voltage increases. Voltage dependent resistors are often used as surge suppressors for sensitive circuits. Packages Here are some examples of different packages which are often encountered. The block packages are used for higher power ratings. Characteristics A voltage dependent resistor has a nonlinear varying resistance, dependent on the voltage applied. The impedance is high under nominal load conditions, but will sharply decrease to a low value when a voltage threshold,  the breakdown voltage, is exceeded. They are often used to protect circuits against excessive transient voltages. When the circuit is exposed to a high voltage transient, the varistor starts to conduct and clamps the transient voltage to a safe level. The energy of the incoming surge is partially conducted and partially absorbed, protecting the circuit. The most common type is the MOV, or metal oxide varistor. They are constructed of a sintered matrix of zinc oxide (ZnO) grains. The grain boundaries provide P-N junction semiconductor characteristics, similar to a diode junction. The matrix of randomly oriented grains can be compared to a large network of diodes in series and parallel. When a low voltage is applied, only very little current flows, caused by the reverse leakage through the junctions. However when a [… read more]

Metal oxide film resistor

 
Metal oxide film resistor

What are metal oxide film resistors? Metal-oxide film resistors are fixed form, axial resistors.  They are made of ceramic rod that is coated with a thin film of metal oxides, such as tin oxide. Metal oxide film resistors must not be confused with metal oxide varistors, made of zinc oxide or silicon carbide. Properties Metal oxide film resistors exceed the performance of metal film and carbon film for the following properties: power rating, voltage rating, overload capabilities, surges and high temperatures. Designers choose often the metal oxide film resistor for high endurance applications. For an overview of resistor types with their properties, look here. Stability properties are less good than for the metal film resistor. The metal oxide film resistors have poor properties for low values and tolerance. The temperature coefficient is around 300 ppm/°C, which is higher than for metal film types. The resistance material for metal oxide resistors is tin oxide that is contaminated with antimony oxide, this is to increase the resistivity. Metal oxide resistors can withstand higher temperatures than carbon or metal film resistors. The noise properties are similar to carbon resistors. Maximum operating temperature comparison Material Carbon film Metal film Metal oxide Temperature 200 °C / 390 °F 250-300 °C / 480-570 °F 450 °C / 840 °F Typical Applications Many properties of metal oxide film resistors are similar to metal film resistors. For basic use, metal film and metal oxide film are currently the predominant resistor types. Compared to carbon film, the prices are just as low. Only for dissipation values above 1 watt combined with reasonable stability, the carbon film resistors are still more cost efficient. Construction The metal oxide film is mostly produced with chemical deposition methods. Almost always a ceramic carrier is used as substrate. The deposition process involves the reaction [… read more]