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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: 9V V=IR (Ohm's Law), so 9 = 30I => I = 9 / 30 = 0.3 Target current: 0.3A Voltage drop across resistor: 12V - 9V = 3V V=IR (Ohm's Law), so 3 = 0.3R => R = 3 / 0.3 = 10 Target resistance: 10 Ohms Power dissipated by resistor: 3V * 0.3A = 0.9W |