Introduce to Voltage Dependent Resistor :
The Varistor may be a passive two-terminal solid-state semiconductor that’s accustomed give protection to electrical and electronic circuits. associate degree electrical device are often outlined as a basic element of electrical and natural philosophy circuits. Resistors area unit primarily used for dominant the electrical parameters (voltage and current) in an exceeding circuit victimization the property of electrical device termed as resistance.
Based on electrical device worth at sure voltages to spot that varistor (if exists) within the datasheets of the makers. within the datasheets or catalogues or web content of varistor area unit all reasonably parameters, however a worth of the electrical device Ta given voltages, a curve. they need the advantage over transient suppressor diodes in abundant|the maximum amount} as they’ll absorb much higher transient energies and might suppress positive and negative transients.
What is Varistor :
A special variety of electrical device whose resistance is varied by varied the applied voltage is termed as a voltage-dependent electrical device (VDR) and is additionally merely known as-as a varistor. it’s a non-linear semiconductor component and its name is obtained from the words resistance. Varistors are factory-made from a non-homogeneous material, giving a rectifying action at the contact points of 2 particles. several series and parallel connections
determine the voltage rating and also the current capability of the varistor.
But not like a resistance whose resistance worth is manually varied between its minimum and most values, the varistor modifications its resistance worth mechanically with the change in voltage across it creating it a voltage-dependant, non-linear electrical device or VDR for a brief. therefore, however, will the modification in Associate in Nursing applied voltage modification its resistance? Well, the solution lies in its composition. Since it’s made from semiconductor material, its resistance dips because the voltage across it will increase. once there’s Associate in Nursing excessive increase in voltage, the resistance across it decreases manifold. This behaviour makes them a decent selection for over-voltage protection in sensitive circuits.
Varistor Operation :
The metal compound and carbide varistors operate thanks to the grain boundaries between the grains of the fabric act as little PN junctions. the complete element acts as if it’s an outsized mass of little diodes nonparallel and parallel. once an occasional voltage is applied, little or no current flows as a result of the junctions are reverse biased and also the solely current is that the escape current. once a surge seems across the device that exceeds the clamping voltage, the diodes expertise avalanche breakdown and an outsized current are ready to flow through the device.
Varistors have several benefits and might be utilized in many various sorts of applications for the suppression of mains borne transients from domestic appliances and lighting to industrial instrumentation on each AC or DC power lines. Varistors are often connected directly across mains provides and across semiconductor switches for a cover of transistors, MOSFET and thyristor bridges.
Varistor Specification :
- Clamping voltage: this can be the voltage at that the varistor starts to indicate vital physical phenomenon.
- Rated voltage: This voltage, either expressed as AC or DC is that the most voltage at that the device will be used. it’s commonly best to own a decent margin between the rated voltage and also the in operation voltage, though this may be balanced against the clamping voltage and also the level of protection needed.
- Maximum pulse energy: this can be the most energy of a pulse, expressed in Joules that the device will dissipate. The energy rating for the varistor is usually outlined exploitation standardized transients. The transient is expressed within the format x/y wherever x is that the time for the transient rise and y is that the time to succeed in its peak worth. Typical formats square measure 8/20 and 10/1000. Response time This is that the time for the varistor to start out physical phenomenon when the heartbeat is applied. In several instances, this can be not a difficulty. Typical values square measure sub 100nS.
- Capacitance: The metal chemical compound varistor contains a comparatively high capacitance across the device. though for low-frequency applications, this could not be a difficulty, it’s going to gift issues once it’s used with lines carrying knowledge, etc. It is, therefore, necessary to ascertain the worth of the capacitance across the device for any circuit wherever this might be a difficulty. Typical metal chemical compound varistors might have capacitance levels between one hundred and one thousand pF, though low capacitance versions square measure obtainable.
- Standby current: The standby current is that the level of current that’s drawn by the varistor once it’s in operation below the clamping voltage. commonly this current is going to be fixed at a given in operation voltage across the device.
- Peak current: this is often the most current that the device can handle. it’s going to be expressed as a current for a given time.
Varistor Characteristic :
The clamping voltage or the Varistor voltage shown within the figure is outlined because the voltage up to that the present through the Varistor is incredibly low, most of the order of few milliamperes. This current is usually known as the escape current. This price of escape current is thanks to the high resistance of the Varistor once the clamping voltage is applied across the Varistor. Varistor The varistor V-I characteristics shown within the higher than figure represent that little modification in applied voltage causes a major quantity of modification in a current. As shown within the V-I characteristics, it acts as 2 Zener diodes connected back to back and operates in each quadrant one and 3 (both directions).
This rated or clamping voltage is that the voltage across the varistor measured with the desired DC current of 1mA. That is, the DC voltage level applied across its terminals that permits a current of 1mA to flow through the varistors resistive body that itself is dependant upon the materials employed in its construction. At this voltage level, the varistor begins to vary from its insulating state into its conducting state.
The V-I graphical record of Varistor is analogous thereto of a Zener diode. it’s bi-directional in nature since we tend to see that it operates in each initial and third quadrant. This feature makes it appropriate to attach it during a circuit with either AC or DC supply. For Associate in Nursing AC supply, it’s apt since it will operate in either directions or polarities of an undulation. the varistor self-regulates the transient voltage across it by permitting a lot of currents to flow through it and sense of its steep non-linear I-V curve, it will pass wide varied currents over a slim voltage vary clipping-off any voltage spikes.