470v 10k471 Varistor is non-linear resistor utilize a semiconductor electronic ceramic element mainly compose of Zinc Oxide and its resistor change as a function of the apply voltage. It s called Varistor or Transient surge absorber.
What is a varistor?
A varistor is a voltage-dependent resistor (VDR). Its resistance is variable and depends on the voltage applied. Hence the word is compos 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 resistor are often use as surge suppressor for sensitive circuits.
Here are some examples of different packages that are often encounter. The block packages are use for higher power ratings.
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Disc |
Block |
Radial leaded |
Axial leaded |
Characteristics of 10k471 Varistor:
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 exceed. They are often use to protect circuits against excessive transient voltage.
When the circuit is exposed to a high voltage transient, the varistor starts to conduct and clamps the voltage to a safe level. The energy of the incoming surge is partially conduct and partially absorb, protecting the circuit.
The most common type is the MOV, or metal oxide varistor. They are construct of a sinter matrix of zinc oxide (ZnO) grains. The grain boundaries provide P-N junction semiconductor characteristic, similar to a diode junction. The matrix of randomly oriented grains can be compare to a large network of diodes in series and parallel.
When a low voltage is apply, only very little current flows, caused by the reverse leakage through the junctions. However, when a high voltage is apply, which exceeds the breakdown-voltage, the junction experience an breakdown and a large current can flow.
The relationship between the current (I) through and the voltage (V) across the terminal, is typically described by:
The term α describes the degree of non-linearity. Figure 1 shows the characteristic curves of a MOV(high α).
Important selection parameters are the clamping voltage, peak current, maximum pulse energy, rated AC/DC voltage and standby current. When used on communications lines, stray capacitance is also an important parameter. A high capacitance can act as a filter for high-frequency signals or induce crosstalk, limiting the available bandwidth of the communications line.
470v 10k471 Varistor is non-linear resistors utilize a semiconductor electronic ceramic element mainly compose of Zinc Oxide and its resistor change as a function of the apply voltage. are useful for short duration protection in case of high transient voltage surges in the order of 1-1000 microseconds.
They are however not suit to handle sustain surges. If transient pulse energy in joules (J) is too high and significantly exceeds the absolute maximum ratings, they can melt, burn or explode.
MOVs degrade when exposed to repeated surges. After each surge the MOVs voltage moves a little lower, how much depends on the joule rating of the MOV in relation to the pulse. As the clamping voltage falls lower and lower, a possible failure mode is a partial or complete short circuit, when the clamping voltage falls below the protected line voltage. This situation could lead to a fire hazard.
To prevent fire hazards, they are often connected in series with a thermal fuse which disconnect the MOV in case of overheating. To limit the degradation, it is advise to use an as high voltage as the protected circuit allows, to limit the amount of exposure to surges.
Applications of 470v 10k471 Varistor:
The nonlinear characteristic of the varistor makes them ideal for use as surge protector devices. Sources of high voltage transients can, for example, be lightning strikes, electrostatic discharges or inductive discharge from motors or transformers. They are for example often used in surge protector power strips. Special types with low capacitance protect communication lines. These VDRs are useful for a wide variety of applications that can include:
- Telephone and other communication line protection
- Radio communication equipment transient suppression
- Surge protector power strips
- Cable TV system surge protector
- Power supply protection
- Microprocessor protection
- Electronics equipment protection
- Low voltage board-level protection
- Transient voltage surge suppressor (TVSS)
- Car electronic protection
- Industrial high energy AC protection
Types:
The most important types are:
- Metal oxide varistor – Described above, the MOV is composed of zinc oxide (ZnO)
- Silicon carbide varistor – At one time this was the most common type before the MOV came into the market. These components utilize silicon carbide (SiC). They have been intensively used in high power, high voltage applications. The disadvantage of these devices is that they draw a significant standby current, therefore a series gap is required to limit the standby power consumption.
Alternative types of surge suppressing devices include:
- Selenium cells – These suppressors use selenium rectifiers allowing a high-energy reverse breakdown current. Some selenium cells have seal-healing properties that allow them to withstand high-energy discharges. They, however, do not have the clamping ability of modern MOVs.
- Zener diodes – A transient suppression device that utilizes silicon rectifier technology. They have a very constant voltage-clamp ability. The major drawback of these components is that they have a limited energy dissipation capability.
- Crowbar devices – A crowbar device short-circuits a surge to ground, this short-circuit will continue until the current is below a certain very low level. Creating a lagging or power-follow effect. Examples of crowbar devices are:
- Gas discharge tube (GDT) or spark gap – These devices conduct after a conducting spark is created, the disadvantage is that they take a relatively long time to trigger, the advantage is the large current carrying capabilities.
- Thyristor surge protection device (TSPD) – have similar characteristics as a GDT, but can act much faster.
Varistor symbol
The following symbol is used for a varistor. It is depicted as a variable resistor which is dependent on voltage, U
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Package Includes for 470v 10k471 Varistor:
1 x 470v 10k471 Varistor