Practical case: Identify value by color code

Level: Basic — Read resistor color bands and verify the value with a multimeter using a simple divider.

Materials

• 1x Resistor under test (R_UT), 4- or 5-band, unknown value
• 1x Reference resistor (R_REF), 10 kΩ ±1% (or any known, 4.7 kΩ–100 kΩ range)
• 1x Breadboard
• 1x DC power supply, 5 V (battery + regulator is fine)
• 1x Digital multimeter (DMM) with DC voltage and resistance modes
• 6x Jumper wires
• 1x Resistor color code chart (printed or on phone)

Wiring guide

• Abbreviations used in the schematic:
• V_IN: Supply voltage measurement point.
• V_MID: Midpoint between R_UT and R_REF.
• GND: Ground reference point.
• Steps:
• Plug R_UT and R_REF in series on the breadboard.
• Connect the top of R_UT to +V from the power supply.
• Connect the bottom of R_REF to GND of the power supply.
• Ensure the junction between R_UT and R_REF is accessible for probing (this is V_MID).
• Keep the DMM black probe on GND when measuring voltages; touch the red probe to V_IN or V_MID as needed.
• Double-check polarity before powering up.

Schematic

+5 V
        │              ● V_IN
        │──────────────●
        │
       ┌┴┐
       │ │  R_ref = 1 kΩ (resistor de referencia)
       │ │
       └┬┘
        │
        │──────────────● V_OUT
        │
       ┌┴┐
       │ │  R_UT = Resistor bajo prueba
       │ │
       └┬┘
        │
       GND

Measurements and tests

• Visual decoding (color bands):

  • Identify the tolerance band (gold/silver) at the end to set reading direction.
  • For 4-band: first two bands = digits, third = multiplier, fourth = tolerance.
  • For 5-band: first three bands = digits, fourth = multiplier, fifth = tolerance.
  • Convert colors to digits/multiplier using your chart and compute R_code and tolerance.

• Voltage divider verification (powered):

  • Set DMM to DC volts. Black probe on ● GND, red on ● V_IN; record V_IN.
  • Keeping black on ● GND, move red to ● V_MID; record V_MID.
  • Compute estimated resistance of R_UT (top resistor):
    • R_UT_est = R_REF × (V_IN / V_MID − 1)
  • Compare R_UT_est with R_code and check if the difference is within the tolerance band.

• Optional ohmmeter cross-check (unpowered):

  • Power off and disconnect the supply.
  • Remove R_UT from the breadboard and measure it with the DMM in resistance mode.
  • Compare with both R_code and R_UT_est.

Common mistakes

• Reading bands from the wrong end; always start from the end opposite the tolerance band.
• Using too small or too large R_REF, causing V_MID to be too close to 0 V or V_IN (reduces accuracy).
• Measuring resistance in-circuit while powered; remove power and lift one lead for a true ohms reading.
• Assuming supply is exactly 5.00 V; always measure V_IN before calculations.

Safety

• Keep supply ≤ 9 V for this exercise; higher voltages are unnecessary and risky on a breadboard.
• Never switch the DMM to ohms or current while the circuit is powered.
• Avoid touching bare conductors while probing to prevent slipping and shorting nodes.

Improvements

• Repeat with several R_UT values to practice quick mental checks of the divider formula.
• Use a tighter-tolerance R_REF (0.1%) to reduce calculation uncertainty.
• Automate calculations with a simple spreadsheet: inputs V_IN, V_MID, R_REF; output R_UT_est and percent error versus R_code.

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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