Practical case: Voltage divider with two resistors

Materials

• 1 × DC source (+5 to +9 V) or a 9 V battery with clip
• 1 × Breadboard
• 1 × Resistor R1 = 10 kΩ (top resistor)
• 1 × Resistor R2 = 10 kΩ (bottom resistor)
• 1 × Digital multimeter (DMM)
• 4–6 × Jumper wires

Wiring guide

• Place R1 and R2 in series: connect one end of R1 to the +V rail, the other end of R1 to one end of R2, and the other end of R2 to the GND rail.
• Connect the DC source: + terminal to +V rail, − terminal to GND rail.
• The midpoint between R1 and R2 is the divider output node.
• Abbreviations used in the schematic:
• VM+IN: Red probe point to measure the input voltage (+V) against ground.
• VM+OUT: Red probe point to measure the divider output voltage against ground.
• VM−: Black probe point (ground reference).

Schematic

                 +9 V (fuente DC)
                      │
                      ├──● VIN
                      │
                     ┌┴┐
                     │ │
                     │ │
                     └┬┘    R1 = 10 kΩ (R1)
                      │
                      ├──● VOUT
                      │
                     ┌┴┐
                     │ │
                     │ │
                     └┬┘    R2 = 10 kΩ (R2)
                      │
                      ├──● GND
                      │
                     GND

Measurements and tests

• Verify the DC supply:

  • Set the DMM to DC volts.
  • Place the black probe on the VM− dot (GND).
  • Place the red probe on the VM+IN dot (+V).
  • Read Vin; it should match your source (e.g., ~9.0 V for a 9 V battery).

• Measure the divider output:

  • Keep the black probe on VM−.
  • Move the red probe to VM+OUT.
  • Record Vout. Theoretical value: Vout = Vin × R2 / (R1 + R2). With R1 = R2 = 10 kΩ, Vout ≈ 0.5 × Vin.

• Estimate current through the chain (no ammeter needed):

  • Compute I = Vin / (R1 + R2). For Vin = 9 V and total 20 kΩ, I ≈ 0.45 mA.
  • This is also the current through each resistor (series path).

• Account for resistor tolerance:

  • If Vout differs slightly from theory, 5% or 1% tolerance and meter accuracy can explain it.
  • Optionally measure each resistor with the DMM (power off) to see actual values and recompute Vout.

Theory in a minute

• A voltage divider scales a voltage using series resistors: Vout = Vin × R2 / (R1 + R2).
• Using equal resistors halves the input; choosing R2 smaller than R1 yields a lower fraction, and larger yields a higher fraction (but always below Vin).

Common mistakes

• Using too-low resistor values, causing unnecessary current draw and heating.
• Forgetting the common ground when measuring; always put the black probe on VM−.
• Misplacing the midpoint node so R1 and R2 are not truly in series.

Safety and good practices

• Do not short +V to GND; double-check connections before powering.
• Use kΩ-range resistors (e.g., 4.7 kΩ–100 kΩ) to keep current reasonable.
• Power off before changing resistor placements or measuring resistance.

Useful extensions

• Try R1 = 15 kΩ and R2 = 10 kΩ; predict and measure Vout = Vin × 10/(15+10) = 0.4 × Vin.
• Add a load resistor from Vout to GND (e.g., 10 kΩ) and observe how Vout drops due to loading; compute the equivalent R2 || Rload.

More Practical Cases on Prometeo.blog

Carlos Núñez Zorrilla

Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

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