Practical case: Limit current with a resistor on an LED

Materials

• 1 × Red LED (forward voltage ≈ 2.0 V) [Materials #1]
• 1 × Resistor 330 Ω, 1/4 W (series limiter) [Materials #2]
• 1 × DC power supply, 5 V (current‑limited if available) [Materials #3]
• 1 × Digital multimeter (DMM) [Materials #4]
• 1 × Breadboard and a few jumper wires [Materials #5]

Wiring guide

• Disconnect power while wiring.
• Place the LED on the breadboard. Identify polarity: the anode is the longer lead; the cathode side usually has a flat edge on the LED body.
• Connect the resistor (R1) in series with the LED:
• +V from the DC supply goes to one end of R1.
• The other end of R1 goes to the LED anode.
• The LED cathode goes to GND.
• Set the power supply to +5 V. If it has current limit, set ~20 mA max.
• Abbreviations used in the schematic and measurements:
• V_R+, V_R−: positive and negative probe points to measure the voltage across the resistor R1.
• V_LED+, V_LED−: positive and negative probe points to measure the voltage across the LED.
• I_LED: LED current; compute as I_LED = V_R / R1, where V_R = V_R+ − V_R− and R1 = 330 Ω.
• Do not insert the ammeter in series unless instructed; you can compute current from V_R to avoid opening the circuit.

Schematic

         +5 V
          │
          ├─●V_R+
          │
        ┌┴┐           R1: 330 Ω, 1/4 W (resistor en serie)
        │ │
        │ │
        └┬┘
          │
          ├─●V_R-
          ├─●V_LED+
          │
        ┌┴┐           D1: LED rojo 5 mm (ánodo arriba)
        │ │
        │ │
        └┬┘
          │
          ├─●V_LED-
          │
         GND

Measurements and tests

• Before powering on:

  • Verify series order: +V → R1 → LED (anode) → LED (cathode) → GND.
  • Confirm the resistor value is 330 Ω (measure with DMM in resistance mode if unsure).
  • Ensure the LED polarity is correct.

• Power up and basic check:

  • Apply +5 V. The LED should light steadily (not flicker).

• Measure V_R (resistor voltage):

  • Place DMM in DC voltage mode.
  • Touch the red probe to the dot labeled V_R+ and the black probe to V_R−.
  • Note V_R. Expected ≈ 3.0 V (since 5 V − ~2.0 V_LED ≈ 3.0 V).

• Measure V_LED (LED voltage):

  • Keep DMM in DC voltage mode.
  • Touch the red probe to V_LED+ and the black probe to V_LED−.
  • Note V_LED. Expected ≈ 1.8–2.2 V for a typical red LED.

• Compute I_LED (LED current):

  • Use I_LED = V_R / R1. With V_R ≈ 3.0 V and R1 = 330 Ω, I_LED ≈ 9.1 mA.
  • Check resistor power: P_R = I_LED² × R1 ≈ (0.0091 A)² × 330 Ω ≈ 0.027 W (well below 1/4 W).

• Optional direct current check (advanced):

  • Power off first. Move the DMM to current mode and into the correct jack.
  • Break the connection between R1 and LED anode; insert the ammeter in series at that point.
  • Power on and read I_LED directly; it should match the computed value within meter tolerance.
  • Power off and restore the original wiring after the test.

How to choose the resistor (rule of thumb)

• Target current for standard 5 mm red LED: 5–15 mA for long life.
• Use R = (V_SUPPLY − V_F_LED) / I_TARGET.
• Example at 5 V: R ≈ (5 V − 2.0 V) / 0.010 A = 300 Ω; the next standard value up is 330 Ω.

Common mistakes

• Reversing LED polarity (no light or very dim).
• Omitting the series resistor (can damage the LED and supply).
• Using too small a resistor (excessive current, LED overheats).
• DMM still in current mode when measuring voltage (blows meter fuse).

Safety notes

• Set current limiting on the bench supply if available.
• Never touch probes together while across the supply; avoid short circuits.
• Power off before changing wiring or moving the ammeter into the circuit.

Possible improvements

• Add a switch in series to turn the LED on/off cleanly.
• Try other LED colors and update R accordingly (green/blue LEDs have higher V_F).
• Use a trimmer potentiometer in series to experiment with brightness vs. 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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