Varistor Metal Oxide Varistor Products Page 5 Varistor Metal-Oxide Varistor Products

2017 Littelfuse, Inc. Specifications are subject to change without notice. Revised: 09/14/17 Metal-Oxide Varistors (MOVs) Transient Voltage Scenarios ESD (Electrostatic Discharge) Electrostatic discharge is characterized by very fast rise times and very high peak voltages and currents. This energy is the result of an imbalance of positive and negative charges between objects. Below are some examples of the voltages which can be generated, depending on the relative humidity (RH): Walking across a carpet: 35kV @ RH = 20%; 1.5kV @ RH = 65% Walking across a vinyl floor: 12kV @ RH = 20%; 250V @ RH = 65% Worker at a bench: 6kV @ RH = 20%; 100V @ RH = 65% Vinyl envelopes: 7kV @ RH = 20%; 600V @ RH = 65% Poly bag picked up from desk: 20kV @ RH = 20%; 1.2kV @ RH = 65% Referring to Table 2 on the previous page, it can be seen that ESD that is generated by everyday activities can far surpass the vulnerability threshold of standard semiconductor technologies. Figure 2 shows the ESD waveform as defined in the IEC 61000-4-2 test specification. Inductive Load Switching The switching of inductive loads generates high energy transients which increase in magnitude with increasingly heavy loads. When the inductive load is switched off, the collapsing magnetic field is converted into electrical energy which takes the form of a double exponential transient. Depending on the source, these transients can be as large as hundreds of volts and hundreds of Amps, with duration times of 400ms. Typical sources of inductive transients are: Generator Motor Relay Transformer These examples are extremely common in electrical and electronic systems. Because the sizes of the loads vary according to the application, the wave shape, duration, peak current and peak voltage are all variables which exist in real world transients. Once these variables can be approximated, a suitable suppressor technology can be selected. T 1 V B V B = 25V to 125V V B = 14V T= 40ms to 400ms V S 90% 10% t V T 1 = 5ms to 10ms R = 0.5 to 4 Figure 3. Automotive Load Dump Figure 3, shows a transient which is the result of stored energy within the alternator of an automobile charging system. A similar transient can also be caused by other DC motors in a vehicle. For example, DC motors power amenities such as power locks, seats and windows. These various applications of a DC motor can produce transients that are just as harmful to the sensitive electronic components as transients created in the external environment. Lightning Induced Transients Even though a direct strike is clearly destructive, transients induced by lightning are not the result of a direct strike. When a lightning strike occurs, the event creates a magnetic field which can induce transients of large magnitude in nearby electrical cables. Figure 4, shows how a cloud-to-cloud strike will effect not only ove RHead cables, but also buried cables. Even a strike 1 mile distant (1.6km) can generate 70V in electrical cables. Buried Line Transient Generated: 70 V at 1.6km (1 mile) 10 kV at 150m (160 yards) Figure 4. Cloud-to-Cloud Lightning Strike Introduction to Overvoltage Suppression (continued)

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