Practical case: Identify the cathode and anode in diodes

Materials

• 1 × Breadboard
• 1 × Adjustable DC power supply (set to +5 V)
• 1 × Resistor R1 = 1 kΩ, 0.25 W
• 1 × Small-signal diode D1 (e.g., 1N4148) — Device Under Test
• 1 × Red LED D2 (5 mm) — Device Under Test
• 6 × Jumper wires
• 1 × Digital multimeter (with diode-test mode)
• 2 × Multimeter probes (red/black)

Wiring guide

• Place R1 vertically on the breadboard so its two leads are on different rows.
• Place the diode under test (DX) vertically below R1 so it will be in series with R1.
• For the first run, insert D1 (1N4148) as DX.
• For the second run, replace DX with D2 (LED).
• Wire the top of R1 to +V from the power supply.
• Wire the bottom of DX to the ground (GND) rail of the breadboard.
• Ensure a single series path: +V → R1 → DX → GND. There must be no parallel paths.
• Prepare multimeter measurements:
• Voltage across resistor: V_R is measured between nodes labeled V_R+ and V_R-.
• Voltage across diode: V_D is measured between nodes labeled V_D+ and V_D-.
• Meanings:
  • V_R+: node at +V side of R1.
  • V_R-: node between R1 and DX (top of DX).
  • V_D+: node between R1 and DX (top of DX).
  • V_D-: node at the bottom of DX, near GND.

Schematic

                      +5 V (Fuente DC 5 V, Material 1)
                       │
                       ● VR+
                       │
                      ┌┴┐
                      │ │   R1 = 1 kΩ (serie, Material 3)
                      │ │
                      └┬┘
                       │
                       ● VR-
                       │───● VD+
                       │
                      ┌┴┐
                      │ │   D1 = diodo bajo prueba (Material 2)
                      │ │        (p. ej., 1N4148 / 1N4007)
                      └┬┘
                       │
                       ● VD-
                       │
                      GND

Measurements and tests

• Visual identification:

  • For most glass/plastic diodes (e.g., 1N4148), the cathode is marked with a painted band; the opposite end is the anode.
  • For LEDs, the shorter lead is typically the cathode; the flat edge on the LED body also marks the cathode.

• Multimeter diode-test (no power supply):

  • Set the multimeter to diode-test mode.
  • Place the red probe on the suspected anode and the black probe on the suspected cathode.
  • A forward reading typically shows ~0.55–0.75 V for silicon diodes (D1) and ~1.6–2.2 V for a red LED (D2). Reverse leads should show OL or no conduction.
  • Conclusion: The terminal touched by the black probe when a valid forward reading appears is the cathode.

• Powered series test (using the schematic):

  • Confirm supply is set to +5 V before powering.
  • With D1 as DX:
    • Measure V_R between V_R+ and V_R-. Expect ~4.2–4.5 V.
    • Measure V_D between V_D+ and V_D-. Expect ~0.55–0.75 V (forward-biased).
    • Compute current: I_D ≈ V_R / R1. Expect ~4–5 mA.
    • If you reverse DX and repeat, expect V_R ≈ 0 V and V_D ≈ +5 V (no current).
  • With D2 as DX:
    • If LED is forward-biased, it should glow dimly with R1 = 1 kΩ at 5 V.
    • Measure V_D. Expect ~1.6–2.0 V for a red LED; V_R ≈ 3–3.4 V; I_D ≈ 3–3.4 mA.
    • If LED is reversed, V_R ≈ 0 V and V_D ≈ +5 V; no light.
  • Polarity rule from this test: In the forward-bias condition (current flows), the upper terminal (V_D+) is the anode and the lower terminal (V_D-) is the cathode.

• Decision checklist:

  • Forward reading in diode-test or glowing LED with series resistor present → the end toward +V is the anode; the end toward GND is the cathode.
  • No conduction both ways (in diode-test and powered test) → possible damaged diode or incorrect meter range/wiring.

Common mistakes

• Forgetting the series resistor: applying +5 V directly to an LED can destroy it.
• Misreading the band on D1: the band marks the cathode, not the anode.
• Measuring current by shorting the meter across the diode: use V_R and Ohm’s law instead, or place the meter in series properly.
• Using too low a supply: with 3 V and a 1 kΩ resistor, some LEDs may be very dim, making the test inconclusive.

Safety notes

• Always power off before swapping DX between D1 and D2 or changing orientation.
• Start with +5 V and a 1 kΩ resistor to limit current to a safe few milliamps.
• Do not exceed the multimeter’s limits if you choose to measure current directly.

Extensions and improvements

• Try other diode types (e.g., 1N4007, Schottky) and record their forward voltages.
• Sweep the supply from 3–9 V and observe how V_R and brightness (for LEDs) change; compute I_D each time.
• Build a small “polarity tester” jig with fixed R1 and a two-pin socket so you can quickly identify anode/cathode of unknown diodes.

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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