Practical case: Check reverse bias and leakage

Materials

• 1 × D1: Silicon diode (1N4007 recommended)
• 1 × R1: 1 MΩ resistor, ≥0.25 W, 1% or 5%
• 1 × DC supply: 12 V (or 9 V battery). If using a bench supply, enable current limit ≤ 10 mA
• 1 × Digital multimeter (DMM) with mV and µA ranges
• 1 × Breadboard and several jumper wires
• Optional: 10 MΩ resistor (for higher sensitivity), hair dryer or warm air (to see temperature effect)

Wiring guide

• Abbreviations used:
• V_R: Voltage across R1, measured between dots V_R+ and V_R-
• V_D: Voltage across D1, measured between dots V_D+ and V_D-
• I_R: Reverse leakage current of the diode, computed as I_R = V_R / R1
• +V: Positive terminal of the DC supply; GND: return/negative terminal
• Steps:
• Connect +V to the top of R1.
• Connect the bottom of R1 to the cathode of D1 (diode in reverse). Identify the cathode by the stripe; it must face R1.
• Connect the anode of D1 to GND.
• Do not power yet. Double-check polarity: +V → R1 → diode cathode (stripe) → diode anode → GND.
• Prepare your DMM in DC voltage mode:
  • For V_R: measure between the dots labeled V_R+ and V_R-.
  • For V_D: measure between the dots labeled V_D+ and V_D-.
• Power the circuit and proceed to the measurements.

Schematic

       +V (9 V)
        │
        ├─●V_R+
        │
       ┌┴┐
       │ │  R1 = 100 kΩ (limitador)
       │ │
       └┬┘
        │
        ├─●V_R−
        │
        ├─●V_D+
        │
       ┌┴┐
       │ │  D1 = 1N5819 (cátodo arriba)
       │ │
       └┬┘
        │
        ├─●V_D−
        │
       GND

Measurements and tests

• Reverse-bias setup check:
  • Ensure the diode stripe (cathode, K) is connected to R1 and its anode (A) goes to GND.
  • With power ON, confirm V_D is close to the supply voltage (for 12 V supply, expect V_D ≈ 11.9–12.0 V if leakage is very small).
• Leakage estimation (I_R):
  • Measure V_R between V_R+ and V_R-.
  • Compute I_R = V_R / R1. Example: if V_R = 1.2 V and R1 = 1 MΩ, then I_R ≈ 1.2 µA.
  • Expect microamp (µA) or sub-µA values for general-purpose diodes at room temperature and ~12 V reverse bias.
• Forward-bias comparison (sanity check):
  • Power OFF. Flip D1 so its anode is at R1 and cathode to GND (now forward-biased).
  • Power ON. Measure V_D again: expect ~0.6–0.8 V (silicon).
  • Measure V_R and compute current: I ≈ (V_supply − V_D) / R1. With 12 V, V_D = 0.7 V, R1 = 1 MΩ → I ≈ 11.3 µA.
  • Power OFF and restore the reverse-bias orientation to continue.
• Temperature effect (optional):
  • In reverse bias, gently warm the diode (no overheating). Monitor V_R: as the diode warms, I_R increases, so V_R rises.
  • Record V_R at room temperature and after warming to quantify the change in I_R.

Common mistakes and tips

• Reversed labels: K (cathode) must face R1 in the reverse-bias test; the stripe on the diode marks K.
• Floating measurements: Always reference V_R and V_D to the exact dots shown; avoid guessing nodes.
• Too small R1: With low R1, V_R may be too small to read for tiny leakage. Increase to 10 MΩ for higher sensitivity, but be mindful that meter input impedance (typically 10 MΩ) will then influence readings.
• Supply limits: Keep current limit enabled on a bench supply. Do not exceed the diode’s maximum reverse voltage (1N4007 is very tolerant; if using small-signal diodes like 1N4148, keep reverse voltage well under its rating, typically ≤ 75–100 V).
• Noise and resolution: Use the mV range and average multiple readings if your DMM supports it. Shield long leads to reduce noise when measuring sub-µA leakage.

Possible improvements

• Use a second DMM: one across R1 (for V_R) and one across D1 (for V_D) to observe changes simultaneously.
• Add a SPDT switch to flip between reverse and forward bias without rewiring.
• Calibrate R1: measure its exact resistance and use that value in I_R = V_R / R1 for better accuracy.
• For very small leakage, build a transimpedance amplifier (op-amp) to convert nA of leakage into measurable volts.

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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