Practical case: Verify forward bias of a diode

Materials

• 1 × Silicon diode D1 (1N4148 or 1N4007)
• 1 × Resistor R1 = 1 kΩ, 1/4 W
• 1 × DC power supply, 5 V regulated
• 1 × Breadboard
• 4 × Jumper wires
• 1 × Digital multimeter (DMM); optional: a second DMM for simultaneous readings

Wiring guide

• Place R1 in series from the +V rail (5 V) to a free node on the breadboard.
• Place D1 from that node down to the GND rail. Orient D1 for forward bias:
• Anode (unmarked lead) to the node coming from R1.
• Cathode (striped end) to GND.
• Connect power supply: +5 V to the breadboard +V rail, and supply ground to the GND rail.
• Abbreviations for measurements:
• V_R: Voltage across R1. Measure with DMM red probe at V_R+ and black probe at V_R−.
• V_D: Voltage across D1. Measure with DMM red probe at V_D+ and black probe at V_D−.
• I_D: Diode current, computed as I_D = V_R / R1.
• Keep the DMM on DC volts for V_R and V_D. Do not switch to current range unless you know how to insert the meter in series.

Schematic

           +5 V (Fuente DC)
             │
            ● V_R+
             │
            ┌┴┐
            │ │
            │ │
            └┬┘   R1 = 1 kΩ (resistencia serie)
             │
            ● V_R-
            ● V_D+
             │
            ┌┴┐
            │ │
            │ │
            └┬┘   D1 = Diodo 1N4148 (A arriba, K abajo)
             │
            ● V_D-
             │
            GND

Measurements and tests

• Power-on checks:

  • Set the supply to 5.0 V before connecting to the circuit.
  • With power OFF, verify D1 orientation: anode to R1, striped cathode to GND.
  • Power ON and ensure the supply current is a few milliamps (no overload indication).

• Measure V_R (voltage across R1):

  • Place DMM red probe at ● V_R+ and black probe at ● V_R−.
  • Expect V_R ≈ 4.2 V to 4.4 V (typical if V_D ≈ 0.6–0.8 V).

• Measure V_D (voltage across D1):

  • Place DMM red probe at ● V_D+ and black probe at ● V_D−.
  • Expect V_D ≈ 0.6–0.8 V for a silicon diode at a few mA.

• Compute I_D (diode current):

  • Use I_D = V_R / R1. With R1 = 1 kΩ, expect I_D ≈ (5 V − V_D)/1 kΩ ≈ 3.5–4.4 mA.
  • Cross-check: V_R + V_D ≈ 5.0 V (Kirchhoff’s Voltage Law).

• Forward-bias verification criteria:

  • V_D within ~0.6–0.8 V and I_D in the low-mA range confirms forward bias.
  • If V_D ≈ 0 V and V_R ≈ 0 V, the circuit may be open (bad connection).
  • If V_D ≈ 5 V and V_R ≈ 0 V, D1 may be reversed or open.

• Optional extension (contrast with reverse bias):

  • Power OFF, flip D1 so the cathode is up (toward R1) and anode to GND.
  • Power ON: V_R should drop near 0 V, V_D should be near 5 V, and I_D ≈ 0 mA (normal leakage only).

Common mistakes

• Reversing the diode (stripe not to GND in forward-bias test).
• Using too small a resistor (excessive current); keep ≥ 1 kΩ at 5 V for this basic test.
• Measuring current with the DMM in parallel like a voltmeter (this can blow the meter fuse). Prefer computing I_D from V_R and R1.

Safety and good practice

• Double-check supply polarity before powering the breadboard.
• Start with power OFF while wiring or changing components.
• If using a different supply voltage, recalculate R1 to keep I_D in the 2–10 mA range: R1 ≈ (V_SUPPLY − 0.7 V) / I_D.

Possible improvements

• Try different diode types (Schottky vs. silicon) and compare V_D; Schottky typically ≈ 0.2–0.4 V.
• Sweep the supply from 3–9 V and plot V_D and I_D to visualize the diode I–V behavior.

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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