How to Choose a Varistor_What is the Function of a Varistor?
How to Choose a Varistor_What is the Function of a Varistor?
A varistor is a voltage-limiting protection device. Utilizing its non-linear characteristics, when an overvoltage occurs between its terminals, the varistor clamps the voltage to a relatively fixed value, thus protecting downstream circuits. The main parameters of a varistor include: varistor voltage, current carrying capacity, junction capacitance, and response time. The response time of a varistor is in the nanosecond range, faster than a gas discharge tube but slightly slower than a TVS diode. Generally, its response speed is sufficient for overvoltage protection in electronic circuits. The junction capacitance of a varistor is typically in the range of several hundred to several thousand pF. In many cases, it is not suitable for direct application in the protection of high-frequency signal lines. When used in AC circuit protection, its large junction capacitance increases leakage current, which needs to be fully considered when designing the protection circuit. The current carrying capacity of a varistor is relatively large, but smaller than that of a gas discharge tube. A varistor, abbreviated as VDR, is a voltage-sensitive non-linear overvoltage protection semiconductor element.
(1) Protection characteristics: When the impact intensity (or impact current Isp=Usp/Zs) of the impact source does not exceed the specified value, the limiting voltage of the varistor must not exceed the impact withstand voltage (Urp) that the protected object can withstand.
(2) Impact withstand characteristics: The varistor itself should be able to withstand the specified impact current, impact energy, and average power when multiple impacts occur successively.
(3) Lifetime characteristics: There are two aspects. One is the continuous operating voltage life, which means that the varistor should be able to reliably operate for the specified time (in hours) under the specified ambient temperature and system voltage conditions. The other is the impact life, which means that it can reliably withstand the specified number of impacts.
(4) After the varistor is introduced into the system, in addition to acting as a "safety valve," it will also introduce some additional effects, which are called "secondary effects." These should not reduce the normal operating performance of the system. The main factors to consider are: the capacitance of the varistor itself (tens to tens of thousands of pF), the leakage current under the system voltage, and the influence of the nonlinear current of the varistor on other circuits through the coupling of the source impedance. Varistors can be classified according to their structure, manufacturing process, application materials, and current-voltage characteristics.
1. Classification by Structure: Varistors can be classified by their structure into junction varistors, bulk varistors, single-layer varistors, and thin-film varistors, etc. Junction varistors achieve their non-linear characteristic due to the rarefied contact between the resistive element and the metal electrode, while the non-linearity of bulk varistors is determined by the semiconductor properties of the resistive element itself.
2. Classification by Application Materials: Varistors can be classified by their application materials into zinc oxide varistors, silicon carbide varistors, metal oxide varistors, germanium (silicon) varistors, barium ferrite varistors, etc.
3. Classification by Current-Voltage Characteristics: Varistors can be classified by their current-voltage characteristics into symmetrical varistors (non-polarized) and asymmetrical varistors (polarized).
When selecting a varistor, the specific conditions of the circuit must be considered. Generally, the following principles should be followed:
1. Selection of Varistor Voltage V1mA: The varistor should be selected based on the power supply voltage. The power supply voltage continuously applied across the varistor should not exceed the "maximum continuous operating voltage" value listed in the specifications. That is, the maximum DC operating voltage of the varistor must be greater than the DC operating voltage VIN of the power line (signal line), i.e., VDC ≥ VIN. For 220V AC power supply varistor selection, the fluctuation range of the mains voltage should be fully considered, and sufficient margin should be left when selecting the varistor voltage value. The fluctuation range of the domestic power grid is generally 25%. A varistor with a varistor voltage of 470V to 620V is more suitable. Selecting a varistor with a slightly higher voltage can reduce the failure rate and extend the service life, but the residual voltage will increase slightly.
2. Selection of Current Handling Capacity: The nominal discharge current of the varistor should be greater than the surge current it is required to withstand or the maximum surge current that may occur during equipment operation. The nominal discharge current should be calculated based on the value of more than 10 surges in the surge life rating curve of the varistor, approximately 30% of the maximum surge current (i.e., 0.3 IP).
3. Selection of Clamping Voltage: The clamping voltage of the varistor must be less than the maximum voltage (i.e., safe voltage) that the protected component or equipment can withstand.
4. Selection of Capacitor Cp: For high-frequency transmission signals, the capacitor Cp should be smaller, and vice versa. 5. Internal Resistance Matching: The relationship between the internal resistance R (R≥2Ω) of the protected component (circuit) and the transient internal resistance Rv of the varistor: R≥5Rv; for protected components with low internal resistance, a large-capacitance varistor should be used as much as possible without affecting the signal transmission rate.
The main function of a varistor is to provide transient voltage protection in circuits. Due to its working principle as described above, a varistor acts like a switch. Only when the voltage exceeds a threshold does its resistance become infinitesimally small, closing the switch and causing a surge in current with minimal impact on other circuits, thus reducing the effect of overvoltage on downstream sensitive circuits. This protective function of the varistor allows for repeated use and can also be implemented as a one-time protection device similar to a current fuse.
Varistor protection is widely used. For example, the power supply circuit of home color televisions uses varistors for overvoltage protection. When the voltage exceeds a threshold, the varistor exhibits its clamping characteristic, pulling down the excessive voltage and ensuring that subsequent circuits operate within a safe voltage range.
Varistors are primarily used for transient overvoltage protection in circuits, but due to their voltage-current characteristics similar to those of a semiconductor Zener diode, they also possess various other circuit element functions. For example, a varistor is a DC high-voltage, low-current voltage regulator that can stabilize voltages up to several kilovolts, a capability unattainable by silicon Zener diodes. Varistors can be used as voltage fluctuation detection elements, DC level shifting elements, fluorescent initiation elements, voltage equalization elements, and so on.
Lightning strikes can cause atmospheric overvoltages. Most are induced overvoltages. Overvoltages generated by lightning discharges onto transmission lines are called direct lightning overvoltages, which are extremely high, reaching 10² to 10⁴ V, causing immense damage. Therefore, measures must be taken to prevent overvoltages in outdoor power systems and electrical equipment.
