Practical case: Measure current with a shunt resistor

Materials

• 1 × DC power supply, +5 V (current limit ≥ 200 mA)
• 1 × RS: Shunt resistor 0.10 Ω, 1% tolerance, ≥ 1 W
• 1 × RL: Load resistor 100 Ω, 1/4 W (or higher)
• 1 × Digital multimeter (DMM) with millivolt range
• 1 × Breadboard or alligator clip leads
• 4 × Hookup wires (minimum)

Wiring guide

• Set the DC supply to +5 V. Enable a current limit (e.g., 200 mA).
• Place RS (0.10 Ω) in series between +V and RL.
• Connect RL from the lower node of RS to GND.
• Ensure all connections are secure and no leads are shorting adjacent nodes.
• Abbreviations used:
  • V_SHUNT: Voltage across RS. Measured between the top tap V_SHUNT+ (node at +V side of RS) and the bottom tap V_SHUNT− (node between RS and RL).
  • I_LOAD: Load current. Compute as I_LOAD = V_SHUNT / RS.
• To measure V_SHUNT with the DMM: set the DMM to DC mV, connect the black probe to V_SHUNT− and the red probe to V_SHUNT+.

Schematic

                   F1: Fuente DC +5 V
                        +5 V
                         │
                         │
                        ┌┴┐
                        │ │         R1 = 47 Ω (carga)
                        │ │
                        └┬┘
                         │
                         ├─● VSH+
                         │
                        ┌┴┐
                        │ │         RSH = 0.1 Ω (shunt, 1%)
                        │ │
                        └┬┘
                         │
                         ├─● VSH−
                         │
                        GND

Measurements and tests

• Safety checks:

  • Verify the supply is off while wiring.
  • Confirm polarity: +V to the top of RS; bottom of RL to GND.
  • Set a current limit on the supply (e.g., 200 mA).

• Power on and baseline:

  • Turn on the supply.
  • Ensure nothing smells hot; RS should be low-value and may warm slightly but not burn.

• Measure V_SHUNT:

  • Set the DMM to DC millivolts.
  • Place the red probe at the dot labeled V_SHUNT+ and the black probe at V_SHUNT−.
  • Read V_SHUNT in millivolts.

• Compute I_LOAD:

  • Use I_LOAD = V_SHUNT / RS.
  • Example: if V_SHUNT = 10 mV and RS = 0.10 Ω, then I_LOAD = 0.010 V / 0.10 Ω = 0.10 A (100 mA).

• Check expected values:

  • Theoretical load current if RS is small: I ≈ +V / (RL + RS).
  • With +5 V, RL = 100 Ω, RS = 0.10 Ω: I ≈ 5 V / 100.1 Ω ≈ 49.95 mA.
  • Expected V_SHUNT ≈ I × RS ≈ 0.04995 A × 0.10 Ω ≈ 5.0 mV.

• Optional cross-check (series ammeter):

  • Briefly move the DMM to current mode and insert it in series in place of a wire between RS and RL to read current directly.
  • Compare with I_LOAD from the shunt method; they should match within meter and resistor tolerances.

• Uncertainty notes:

  • Main error sources are RS tolerance, DMM accuracy at low mV, and contact resistance.
  • Averaging multiple readings can reduce noise; ensure solid probe contact on the test dots.

Common mistakes and tips

• Using too large a shunt: It will cause excessive voltage drop and alter the circuit current. Keep RS small relative to RL.
• Measuring in the wrong mode: Do not place a DMM in current mode across RS—it will short the node. Only measure V_SHUNT in voltage mode.
• Insufficient power rating for RS: P_RS = I^2 × RS. For 100 mA and 0.10 Ω, P ≈ 1 mW, which is safe; higher currents require higher wattage shunts.
• Poor probe contact: Touch exactly at the black dots; wobbly contacts produce unstable readings.

Improvements

• Use a 4‑wire (Kelvin) connection to RS to eliminate lead and contact resistance from the measurement.
• Add a rail-to-rail op-amp in differential configuration to amplify V_SHUNT for better resolution at very small currents.
• Try different RL values and supply voltages to see how V_SHUNT scales linearly with current.

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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