Practical case: Measure voltage drop across an LED

Materials

• 1 × Breadboard
• 1 × DC power supply, 5 V regulated
• 1 × LED (red)
• 1 × Resistor, 330 Ω, 1/4 W (R1)
• 1 × Digital multimeter (DMM), with voltage (V) and resistance (Ω) modes
• 4 × Jumper wires
• Optional: Extra resistors (220 Ω, 1 kΩ) to compare results

Wiring guide

• Abbreviations used:
• V_LED: voltage across the LED (top of LED is V_LED+, bottom is V_LED−).
• V_R: voltage across the resistor R1 (top is V_R+, bottom is V_R−).
• I_LED: current through the LED (same as current through R1); compute as I_LED = V_R / R1.
• Place R1 in series with the LED on the breadboard.
• Connect +5 V from the supply to the top of R1.
• Connect the bottom of R1 to the anode of the LED (longer lead).
• Connect the LED cathode (shorter lead, flat side) to GND.
• Double-check polarity: +5 V → R1 → LED (anode to resistor) → GND.
• Set the DMM to DC volts:
• To measure V_R: place probes across R1 (red to V_R+, black to V_R−).
• To measure V_LED: place probes across the LED (red to V_LED+, black to V_LED−).
• Do not place the DMM in current mode across the power rails. If you wish to measure current directly, insert the ammeter in series; otherwise compute I_LED from V_R.

Schematic

                 +5 V (PS1)
                     │
                     ● V_R+
                     │
                  ┌─────┐
                  │     │
                  │     │
                  └─────┘    R1 = 330 Ω (limitadora) [R1]
                     │
                     ● V_R−/V_LED+
                     ● I_LED
                     │
                  ┌─────┐
                  │     │
                  │     │
                  └─────┘    D1 = LED rojo 5 mm [D1]
                     │
                     ● V_LED−
                     │
                    GND

Measurements and tests

• Before power:

  • Verify R1 value with the DMM (Ω mode) ≈ 330 Ω.
  • Check LED orientation: anode toward R1, cathode to GND.
  • Set supply to 5.0 V and current limit (if available) to around 20 mA.

• Power on:

  • The LED should light softly. If not lit, power off and recheck polarity and connections.

• Measure V_R:

  • Measure between dots V_R+ and V_R−.
  • Expected reading: around 3.0 V (typical if LED drop ≈ 2.0 V).
  • Compute I_LED = V_R / 330 Ω. Example: 3.0 V / 330 Ω ≈ 9 mA.

• Measure V_LED:

  • Measure between dots V_LED+ and V_LED−.
  • Expected for a red LED: about 1.8–2.2 V at ~5–10 mA.

• Cross-check (sanity):

  • V_R + V_LED ≈ V_supply (close to 5.0 V). Small differences are normal due to wiring and DMM resolution.

• Optional comparisons:

  • Replace R1 with 220 Ω: I_LED increases, V_LED rises slightly.
  • Replace R1 with 1 kΩ: I_LED decreases, V_LED falls slightly; LED may dim.

Common mistakes

• Reversing the LED (no light, near 0 V across LED in reverse in this low-voltage test).
• Omitting the series resistor (risk of LED damage).
• Measuring current with the DMM across the supply rails (blows fuse in the meter).
• Using too low a resistor; currents >20 mA can overheat typical 5 mm LEDs.

Safety and good practice

• Always include a current-limiting resistor with LEDs.
• Start with a higher resistance if unsure; decrease carefully while monitoring I_LED.
• If your supply has current limiting, enable it around 20 mA.

Improvements and extensions

• Plot I_LED versus V_LED by trying several resistor values and recording V_R and V_LED.
• Repeat with different LED colors; note how V_LED depends on LED type (red ≈ 2.0 V, green ≈ 2.1–3.2 V, blue/white ≈ 2.7–3.4 V).
• Add a series switch to observe transients and measurement stability.

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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