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Original fan voltage: 12V Original fan current: 0.2A V=IR (Ohm's Law), so 12 = 0.2R => R = 12 / 0.2 = 60 Fan resistance: 60 Ohms Target voltage: 7V V=IR (Ohm's Law), so 7 = 60I => I = 7 / 60 = 0.117 Target current: 0.117A Voltage drop across resistor: 12V - 7V = 5V V=IR (Ohm's Law), so 5 = 0.117R => R = 5 / 0.117 = 42.857 Target resistance: 42.857 Ohms Power dissipated by resistor: 5V * 0.117A = 0.583W |