What do the numbers on a varistor mean?
k = a constant for the component. V = applied voltage. n = value for the device style. Typically for silicon carbide the value of n is between about 3 – 7, but for zinc oxide device the value can be in the region of 20 – 50 making the characteristic much sharper. Varistor characteristic.
What is the purpose of a varistor?
Varistors are used to protect a circuit from high voltage surges. When a high voltage surge is applied to a circuit, the outcome is usually catastrophic to the circuit. A capacitor may be installed across the signal lines.
How is varistor connected in circuit?
Varistors are connected in circuits across a mains supply either phase-to-neutral, phase-to-phase for AC operation, or positive-to-negative for DC operation and have a voltage rating to suit their application.
What is the working principle of varistor?
A varistor does not obey Ohm’s law and hence it is not like an ohmic resistor. It is basically a non-ohmic resistor that does not follow Ohm’s law and hence it is also called a non-linear resistor or a voltage-dependent resistor.
How do you read varistors?
Touch one meter probe to the free varistor lead and the other probe to the connected lead. Read the resistance on the meter. If it reads nearly infinite resistance, the varistor is still good. If it reads very low resistance, the varistor is blown.
How do I select a varistor?
Select a model to provide the required voltage-clamping characteristic A final consideration is to choose the appropriate package style to suit the application. Consider the maximum continuous voltage that will be applied to the varistor including any high line conditions (i.e., 110% or more of nominal voltage).
Is varistor a resistor?
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.
Does varistor have polarity?
Varistor is equivalent with Zener diode of two series connection. Therefore, do not have polarity. No polarity, due to symmetrical current-voltage characteristics.
How do I select varistor value?
Therefore, in general, when we select a varistor, the flow capacity should be calculated by pressing the 10 to 100 impacts in the rating curve of the varistor surge lifetime. The actual surge current passed is about 30% of the maximum impact current. (Ie 0.3I).
What is maximum allowable voltage in varistor?
Littelfuse Varistors are available with AC operating voltages from 2.5V to 6000V. Higher voltages are limited only by packaging ability. Peak current handling exceeds 70,000A and energy capability extends beyond 10,000J for the larger units.
How is varistor value calculated?
The simplest ways of determining the current duration are simulation or measurement (τ = t*r). The time constant can also be calculated to an approximation with equation 13. Here the varistor resistance of voltage class K20 is calculated for 1 A.
How do I check my varistor?
How do you check if a varistor is working?
How do you choose varistor voltage?
Choose a varistor with an energy absorption rating that equals or slightly exceeds the energy values associated with the event the circuit may experience. If, however, the transient voltage is due to an external event, the magnitude of the source energy is unknown.
How does a varistor fail?
Under different current conditions, failure modes include electrical puncture (see Figure 4), physical cracking (see Figure 5), and thermal runaway. Cracking happens because varistors are basically a ceramic material, and hitting them with a sharp high-amplitude surge is like hitting a dinner plate with a hammer.
Why would a varistor burn out?
The surge energy is too large and exceeds the absorbed power; 8, the pressure is not enough; 9, current and surge are too large and so on. The safety of the use of varistors has always been a problem that needs attention.
What happens when varistor fails?
What causes a varistor to fail?
How does a varistor protect a circuit?
Varistors are used as control or compensation elements in circuits either to provide optimal operating conditions or to protect against excessive transient voltages. When used as protection devices, they shunt the current created by the excessive voltage away from sensitive components when triggered.