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Original fan voltage: 12V Original fan current: 0.4A V=IR (Ohm's Law), so 12 = 0.4R => R = 12 / 0.4 = 30 Fan resistance: 30 Ohms Target voltage: 7V V=IR (Ohm's Law), so 7 = 30I => I = 7 / 30 = 0.233 Target current: 0.233A Voltage drop across resistor: 12V - 7V = 5V V=IR (Ohm's Law), so 5 = 0.233R => R = 5 / 0.233 = 21.429 Target resistance: 21.429 Ohms Power dissipated by resistor: 5V * 0.233A = 1.167W |