Aumov LV Ultramov Varistor Design Guide Page 16 Varistor Design Guide For DC & Automotive Applications

2015 Littelfuse DC Application Varistor Design Guide DC Application Varistor Design Guide Transient Suppression Techniques There are two different approaches to suppressing transients: attenuation and diversion. Attenuation techniques are based on filtering the transient, thus preventing their propagation into the sensitive circuit; diversion techniques redirect the transient away from sensitive loads and thereby limit the residual voltages. Clamping- and crowbar-type devices are often used to divert a transient: Crowbar devices, primarily gas tubes or protection thyristors, are widely used in the communication field where power-follow current is less of a problem than in power circuits. These types of devices employ a switching action to divert the transient and reduce voltage below line condition by starving the circuit of power. These devices require auto resetting. Clamping devices are components with a variable impedance that depends on the voltage across the terminal. These devices exhibit a nonlinear impedance characteristic. The variation of the impedance is continuous. A clamping device is designed to maintain "normal" line conditions. It typically dissipates some energy within the body of the device. Overvoltage Suppression Comparison The most suitable type of transient suppressor depends on the intended application; in addition, some applications require the use of both primary and secondary protection devices. The function of the transient suppressor is to limit the maximum instantaneous voltage that can develop across the protected loads in one way or another. The choice depends on various factors but ultimately comes down to a trade-off between the cost of the suppressor and the level of protection needed. When it's used to protect sensitive circuits, the length of time a transient suppressor requires to begin functioning is extremely important. If the suppressor is slow acting and a fast-rising transient spike appears on the system, the voltage across the protected load can rise to damaging levels before suppression kicks in. On power lines, a Metal Oxide Varistor is usually the best type of suppression device. TVS Diodes and Gas Discharge Tubes are also used occasionally. Transient Suppression Techniques 15

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