What do the numbers mean on a varistor?

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.

How do you read numbers on varistor?

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.

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 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 you read a MOV value?

The current rating of the MOV could be twice that of the SMPS rating, meaning if the SMPS wattage is rated at 24 watts at the secondary, then the primary could be calculated as 24/285 = 0.084 amps, therefore the MOV current could be anywhere above 0.084 x 2 = 0.168 amps or 200mA.

How do I choose a MOV varistor?

The higher the surge rating requirement, the larger the MOV diameter needs to be. In a square MOV, the surge rating is a little higher. For example, a 10 mm square MOV will be rated at about 3.5 kA, a 14 mm square MOV is rated about 8 kA and so on. The square MOV can handle more current as it has a larger area.

How do you calculate MOV?

Estimate the MOV’s energy rating in units of joules. This is the electrical energy that the MOV can safely absorb at least once before failing. You can calculate the energy by multiplying peak surge volts by amps, and then by seconds of time. For example, 1,000 volts x .

How is varistor connected?

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 application of varistor?

Varistor Applications Varistors are used in almost all heavy electrical circuits to small electronic designs. Varistors provide high voltage surge protection for both AC and DC circuits. To protect electrical circuits from over voltages.

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.

What is function of MOV?

MOV Performance The main function of MOV is to work as a surge suppressor. When the voltage across the varistor is below the clamping voltage then varistor will not conduct. The performance of the varistor gets slow down based on time even if tiny surges flow throughout it.

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.

What is the clamping voltage of a varistor?

This rated or clamping voltage is the voltage across the varistor measured with the specified DC current of 1mA. That is, the DC voltage level applied across its terminals that allows a current of 1mA to flow through the varistors resistive body which itself is dependant upon the materials used in its construction.

Where is MOV used?

A Metal Oxide Varistor (MOV) is a protection component used in power supply circuits that is powered directly from AC mains. It is used to protect the circuit from high voltage spikes by varying its resistance.

What is the usage of 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. However, this capacitor cannot suppress voltage surges.

What happens when varistor fails?

Why does a varistor burn out?

1) The surrounding working temperature is too high or too low: A common problem is that the ambient temperature of the varistor is too high or too low, which will eventually cause the varistor to be damaged.

Do varistors wear out?

Varistors can be destroyed by too many surges. They wear out a little with each surge above the threshold and some day they are completely destroyed. Over-voltage is also a common problem.

What happens when MOV fails?

When subjected to stresses above its ratings, an MOV can fail as a short circuit. If applied conditions significantly exceed the energy rating of the device, and current is not limited, the MOV may be completely destroyed. For this reason, the use of current-limiting fuses is suggested.

How do I choose a varistor?

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.

What is the protection level of the varistor?

The protection level is the voltage drop across the varistor for surge currents > 1 mA. The V/I characteristics show the maximum protection level as a function of surge current (8/20 µs waveform). In the product tables the protection level for surge currents according to the R10 series (ISO 497) is additionally specified.

What is a typical feature of metal oxide varistors?

A typical feature of metal oxide varistors is the dependence of the maximum permissible ratings for surge current, and thus for energy absorption, on the pulse shape, pulse duration, and the number of times this load is repeated during the overall lifetime of the varistor.

What is the maximum non-repetitive surge current for varistors?

The ability of a varistor to with- stand a single pulse of defined shape is characterized by the maximum non-repetitive surge cur- rent specified in the product tables (single pulse, tr≤20 µs). If pulses of longer duration or multiple pulses are applied, the surge current must be derated as described in section 1.8.

What is the voltage drop across the varistor?

The varistor voltage is the voltage drop across the varistor when a current of 1 mA is applied to the device. It has no particular electrophysical significance but is often used as a practical stan- dard reference in specifying varistors.