How to read the varistor data sheet? From related terms to component selection

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How to read the varistor data sheet? From related terms to component selection [Copy link]

Q: Varistor specifications

Varistors are non-linear, bidirectional, voltage-dependent protection devices with relatively high transient current and energy ratings (reaction times in the nanosecond to millisecond range). The fast reaction time of varistors is used to protect electronic circuits from voltage transients, voltage surges, voltage spikes, overvoltage events, and ESD. Varistors are typically used on input lines at the front end of a circuit and sometimes on output lines at the back end of a circuit. Varistors are normally-on devices until an overvoltage occurs, in which case the varistor clamps the voltage by exponentially decreasing its resistance.

Because the voltage level of most overvoltage situations is unknown, it is difficult to know if a varistor is being used within its limits. Varistors will not typically show any degradation over time if operated within their specifications; however, they will become "more resistive" (age) over time after a voltage event. Varistor aging failure will initially appear as an almost short circuit, but will eventually become an open circuit if the varistor does not have a thermistor or resistor wire in series to limit the current.

Some varistors have a built-in thermal element, which not only allows for quick detection of excessive heat, but also saves board space. These varistors are available in 2- or 3-pin packages, where the third pin is usually an "output indicator" that shows the state of the varistor internally, usually to some external indicator circuit. Multiple varistors can also be contained within each package.

Most varistors also include pulse rate graphs or repetitive surge capability graphs in their data sheets to show the specifications for the varistor. Exceeding these specifications may result in the device not meeting the originally published specifications. If the specifications are not selected correctly, it may result in the varistor not functioning properly, having a shortened life cycle, or failing completely. Because different test methods can be used to determine the varistor value for each varistor, it is important to check the data sheet before determining whether the correct varistor has been selected.

Varistor related terms

Varistor Voltage (minimum): The approximate minimum voltage at which a varistor's resistance value changes, or the "start-to-conduct" voltage, usually determined by operating the varistor under specified controlled circuit or electrical values.

Varistor Voltage (Typical): This value is the typical surge voltage or approximate “midpoint” voltage between the minimum and maximum varistor voltages. This is usually a specific common value derived from the midpoint voltage between the minimum and maximum voltages.

Varistor Voltage (Max): Sometimes called Clamping Voltage (Max), is the approximate maximum voltage that a varistor can conduct (or pass through a circuit) for a specified peak pulse duration without causing device failure if operated within specifications. This value is usually determined by operating the varistor in a specified controlled circuit, or at an electrical value such as Maximum Clamping Voltage (at Class Current).

Current-Surge: The maximum peak current within a specified peak pulse duration of a given waveform that can be applied without causing equipment failure. While varistors can handle such surges, most manufacturers agree that surge current should only occur once before recommending replacement of the varistor.

Energy: The maximum joules (or watt-seconds) that a varistor can dissipate for a specified peak pulse duration of a given waveform when an event occurs. This value is usually determined by operating the varistor under specified controlled circuits and electrical values. A varistor will only react to this maximum magnitude once, after which replacement is recommended. Although a varistor may still function after an event, it may not function properly.

Maximum AC Voltage: This value is the maximum RMS line voltage that can be continuously applied to the varistor. This value can be chosen slightly higher than the actual RMS line voltage. The peak voltage of the sine wave should not overlap the minimum varistor voltage, which will reduce the life of the varistor. Thankfully, this is common practice for manufacturers to calculate within their specifications.

Maximum DC voltage: This value is the maximum DC line voltage that can be continuously applied to the varistor. The value chosen can be slightly higher than the actual DC line voltage, but manufacturers often include this margin in their specifications.

How to choose the right varistor

1. Determine the continuous working voltage that will allow the varistor to operate normally, and select a varistor with a maximum AC or DC voltage equal to or slightly higher than this continuous working voltage. Since the power supply line usually has a voltage variation tolerance, it is common for the maximum rated voltage to be 10-15% higher than the actual line voltage. Typically, the varistor has this ratio factored into its voltage value. When extremely low leakage current is more important than the lowest possible protection level, consider using a varistor with a higher working voltage.

2. Determine the energy absorbed by the varistor when the event occurs. While this may sometimes be an unknown value, it can be determined using all of the absolute maximum load values of the varistor during the event defined by the environment and datasheet specifications. It is important to select a varistor with an energy dissipation rating whose minimum value equals or ideally exceeds the energy dissipation required by the circuit that may produce the event. A varistor can only react once to the highest energy level it is rated for, after which it is recommended to be replaced. Although a varistor may still function after an event, it may not function properly.

3. Calculate the peak transient current through the varistor, commonly referred to as the surge current. Surge current is the maximum current that can pass through the varistor for a specified duration and waveform. It is important to select a varistor with a surge current rating that equals or, ideally, exceeds the current rating required by the event that the circuit may produce to ensure proper operation. While varistors can handle this surge, most manufacturers assume that surge current should only occur once before recommending replacement of the varistor.

4. Determine power dissipation requirements. It is important to select a varistor with a power rating that equals or, ideally, exceeds the power capacity required for the event that the circuit may produce. It is common for power, surge current, and energy levels to be much higher than expected for the event. These derating factors are often at least 50 times greater than the common stress tolerances required. If you are unsure of the factors for the event, it is safer to select a device with higher power, surge current, and energy ratings.

5. Choose a model that provides the maximum value of the desired varistor voltage, sometimes called the clamping voltage (max). The clamping voltage must be chosen based on the approximate maximum voltage that the circuit input or output can carry during the event. You must ensure that the circuit can carry this voltage. In short, this voltage is approximately the highest voltage that the circuit can carry on the lower line. However, when the voltage approaches the minimum varistor voltage, the varistor begins to conduct and may produce some weaker clamping effect before the actual clamping voltage is reached.

langtuodianzi

Varistors are widely used in lightning protection of electronic equipment.

suncat

Thank you for sharing the information