Varistor Metal Oxide Varistor Products Page 19 Varistor Metal-Oxide Varistor Products

2017 Littelfuse, Inc. Specifications are subject to change without notice. Revised: 09/14/17 Metal-Oxide Varistors (MOVs) Varistor Selection Worksheet Step 1. Determine the circuit's operating parameters. (complete as much of the following information as possible). 1-a. Source and path of the transient ________ Source ________ Path 1-b. Normal operating voltage of protected device ________ (V AC ) , or ________ (V) RMS DC 1-c. Tolerance of normal operating voltage (1-b) ________ (V) or ________ Unknown 1-d. Max. allowable voltage of protected device ________ (V AC ) or ________ (V) RMS DC 1-e. Maximum expected surge current and number of hits (Specify 8x20s waveform equivalent of surge current) ________ (A) ________ (# of hits) 1-f. Maximum energy applied to device in surge event ________ (Joules) (E=1.4xVxIxT) 1-g. Maximum power applied to device in surge event ________ (W) (P=VxI) 1-h. Maximum allowable varistor capacitance (@1kHz; 0V DC bias) (This is the maximum capacitance of the varistor device that will not impair the functionality of the circuit) ________ (pF) 1-i. Required safety standards (Name of standards required, such as UL, CSA, VDE, etc.) Step 2. Calculate voltage value. 2-a. The required varistor voltage value should be equal to: the operating voltage of the protected equipment or device* + the tolerance of the operating voltage. If the tolerance is not known, multiply the operating voltage of protected equipment or device by 1.10 to 1.25 (i.e. 10-25% above operating voltage value). If the operating voltage is in AC (V RMS ) convert to V DC . ____ Operating voltage AC (V) x 1.414 = ______________________ Operating voltage (V) RMS DC ________Operating voltage of equipment or device (V DC ) + _________Tolerance (V) = _____________________ Required varistor voltage (V) - or - ____Operating voltage of equipment or device (V DC ) x (1.10 to 1.25) = _____________ Required varistor voltage (V) Step 3. Guidelines for Selecting a Varistor If a response to one of the requirements below is "False," refer to appropriate corrective action notes (A-F) at bottom of list: 3-a. Varistor voltage value - Tolerance of varistor Required varistor voltage value (2-a) ______True ______False (A) 3-b. Varistor Maximum clamping voltage value < Maximum allowable voltage of protected equipment or device (1-d) (Max. current should be less than or equal to the current at which maximum clamping voltage is measured). ______True______False (B) 3-c. Varistor maximum peak current value > Maximum expected surge current (1-e) ______True ______False (C) Note: If surge current waveform is not 8 x 20 s , use Pulse Lifetime Ratings curves. 3-d. Varistor maximum energy rating > Maximum energy applied to system (1-f) ______True ______False (D) 3-e. Varistor maximum rated power > Maximum power applied to system (1-g) ______True ______False (E) 3-f. Varistor capacitance < Maximum allowable system capacitance (1-h) ______True ______False (F) Corrective action notes: A. Select next varistor on the list (i.e. next varistor with increasing varistor voltage value) and then re-verify 3-a. B. Select previous varistor on the list (i.e. previous varistor with decreasing varistor voltage value) and then re-verify 3-b. C. Select next varistor diameter level and then re-verify 3-c.* D. Select next varistor diameter level and then re-verify 3-d.* E. Select next varistor diameter level and then re-verify 3-e.* F. Select lower varistor diameter level and then re-verify 3-c, 3-d, 3-e and 3-f.* * If varistor voltage is below 82V, selecting an 82V ROV may be preferable over a higher diameter part. Step 4. Verify the following system conditions. 4-a. Leakage current of the selected varistor is appropriate for the circuit ______ True ______ False 4-b. Verify the performance of the varistor under fault conditions ______ Verified Users should Independently evaluate the suitability of, and test each MOV device in their application for safety and suitability with the end application.

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