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A 5 V source is connected across three series resistors R1, R2 and R3 (in that order from the supply to ground) to form a potential divider. The junction between R1 and R2 is to output 3 V, and the junction between R2 and R3 is to output 2 V (measured with respect to ground, no load current drawn). Which set of resistances gives these voltages?

1. R1 = 1 kohm, R2 = 1 kohm, R3 = 2 kohm
2. R1 = 2 kohm, R2 = 1 kohm, R3 = 2 kohm
3. R1 = 3 kohm, R2 = 2 kohm, R3 = 2 kohm
4. R1 = 3 kohm, R2 = 2 kohm, R3 = 3 kohm

Correct answer: R1 = 2 kohm, R2 = 1 kohm, R3 = 2 kohm

Solution

With R1 = 2, R2 = 1, R3 = 2 (kohm), total = 5 kohm; the lower tap gives 5*(2/5) = 2 V and the upper tap gives 5*(3/5) = 3 V.

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