LessonThe voltage divider · 20 min
Practice set: voltage dividers
Real design questions, from quick checks to full designs. For each one: sketch the divider, label R1 (top) and R2 (bottom), write the formula, then the numbers.
Practice 1R1 = 1.8 kΩ, R2 = 3.3 kΩ, Vin = 5 V. Vout?
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Answer: 3.235 V
5 × 3.3 / 5.1 = 3.24 V - a popular 5 V → 3.3 V pair.
Practice 2A divider gives 3 V from 12 V. R2 = 1 kΩ. What is R1?
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Answer: 3000 Ω
Ratio 1/4: R1 = 3 × R2 = 3 kΩ.
Practice 3How much current flows through a 47 k / 4.7 k divider on 24 V?
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Answer: 0.4642 mA
24 / 51.7 kΩ = 0.464 mA.
Practice 4A pot's wiper on 3.3 V reads 1.1 V. What fraction of the way up from 0 V is the wiper?
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Answer: 33.3 %
1.1 / 3.3 = 1/3 = 33 %.
Practice 5In the dark an LDR is 200 kΩ. Below a 10 kΩ top resistor on 5 V, what does it give?
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Answer: 4.762 V
5 × 200 / 210 = 4.76 V.
Challenge 6A street light should switch on when an LDR (at the bottom, 10 kΩ top resistor, 5 V) reaches 30 kΩ. A comparator compares it with a reference from a second divider whose top resistor is 10 kΩ. What bottom resistor makes the reference equal to the LDR voltage at 30 kΩ?
Hint
Two dividers on the same supply with the same top resistor give the same voltage when their bottom resistors are equal.
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Answer: 30000 Ω
30 kΩ: both give 5 × 30 / 40 = 3.75 V. Because both dividers use the same 5 V, the switching point does not move if the supply drifts - a "ratiometric" design.
Challenge 7A 3.3 V microcontroller reads a 0 - 10 V industrial sensor signal. With R1 = 22 kΩ, what R2 makes 10 V read exactly 3.0 V?
Hint
R2 / (R1 + R2) = 0.3 → R2 = 0.3 R1 / 0.7.
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Answer: 9429 Ω
R2 = 0.3 × 22 k / 0.7 = 9.43 kΩ (use 9.1 k + 330 Ω in series, or 9.53 k from E96).
Challenge 8A 12 V, 1 kΩ / 1 kΩ divider is connected to a 1 kΩ load. How much power does R1 dissipate?
Hint
R2 ∥ RL = 500 Ω. Find Vout, then R1's voltage.
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Answer: 64 mW
Vout = 12 × 0.5 / 1.5 = 4 V. R1 has 8 V across it: P = 8² / 1000 = 64 mW.
Challenge 9Using 5 % resistors, a 10 k / 10 k divider on 10 V can give anything between a minimum and a maximum. What is the maximum?
Hint
Maximum: R2 at its highest (10.5 k), R1 at its lowest (9.5 k).
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Answer: 5.25 V
10 × 10.5 / (9.5 + 10.5) = 5.25 V (minimum 4.75 V): ±5 %.