Practical case: Measure resistance with a multimeter

Materials

• 1× Resistor, e.g., 1 kΩ, 1% or 5% (R1, the RUT)
• 1× Digital multimeter with Ω (resistance) mode (DMM)
• 2× Multimeter test leads (red and black)
• 1× Breadboard or small insulating surface (optional)
• 2× Mini grabber or alligator clips (optional, for steadier contact)

Wiring guide

• Ensure the resistor is isolated: remove it from any powered circuit or lift at least one lead from the board so it is not connected to other components (in-circuit readings can be wrong).
• Place R1 (the resistor under test, RUT) on a breadboard row or on an insulating surface so both leads are accessible.
• Set the DMM to resistance (Ω). If manual-ranging, choose a range higher than the expected value (e.g., 20 kΩ to measure ~1 kΩ).
• Connect the red lead to the DMM’s Ω/V jack and the black lead to COM.
• Touch the red probe to the top terminal labeled Ω+ in the schematic, and the black probe to the bottom terminal labeled Ω−.
• Hold steady contact until the reading stabilizes. If the display shows “OL” or over-range, select a higher range.
• Abbreviations used:
• DMM = Digital Multimeter.
• RUT = Resistor Under Test (R1).
• Ω+ = Point where the red (positive) ohmmeter lead touches.
• Ω− = Point where the black (common) ohmmeter lead touches.

Schematic

            +V
            │
           ┌┴┐                 S1 = interruptor SPST (serie)
           │ │
           │ │
           └┬┘
            │● R_med+
            │
           ┌┴┐                 R1 = 1 kΩ (resistor)
           │ │
           │ │
           └┬┘
            │● R_med−
            │
           GND

Measurements and tests

• Preparation:
  • Zero/leads check: briefly short red and black probes together in Ω mode; the DMM should read near 0 Ω.
  • Visual estimate: read the resistor color code (e.g., brown–black–red = 1 kΩ; gold = ±5%) to predict the expected value.
• Main measurement:
  • Measure between Ω+ and Ω− as shown. Record R_meas (the displayed resistance).
  • Compare R_meas to the nominal value and tolerance band. For a 1 kΩ ±5% resistor, acceptable span is 950–1050 Ω.
• Cross-checks:
  • If using clip leads, remeasure with direct probe contact to rule out clip contact resistance.
  • Reverse probes (swap Ω+ and Ω−); resistance should be identical (resistors are non-polarized).
• In-circuit caution (only if you must measure without removing R1):
  • Power OFF and discharge capacitors first.
  • Measure; if R_meas is unexpectedly low, parallel paths in the circuit are affecting the reading. Isolate by lifting one lead of R1 and remeasure.

Common mistakes

• Measuring while the circuit is powered: the DMM’s ohmmeter injects a small test current; external voltages can damage the meter or give false readings.
• Not isolating the resistor: parallel components make the apparent resistance lower than the true value.
• Wrong DMM jack or range: leads in the current jack or too-low range cause “OL” or nonsense values.
• Poor probe contact: touching paint, oxidation, or the resistor body rather than the metal lead increases measured resistance.

Safety tips

• Always power down and disconnect power sources before switching the DMM to Ω mode.
• Discharge capacitors; stored charge can bias the measurement or harm the meter.
• Do not measure resistance on live mains circuits.

Extensions and improvements

• Tolerance verification: measure multiple units of the same value to observe distribution within the tolerance band.
• Temperature effect: warm the resistor gently between fingers and watch small changes in R_meas (temperature coefficient).
• Series/parallel sanity check: connect two known resistors and verify expected combined resistance before returning to in-circuit work.

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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