Practical case: Test a Zener and measure its Z voltage

Materials

• 1× Zener diode D1 (e.g., 5.1 V, 0.5 W or 1 W)
• 1× Resistor R1 = 1 kΩ, 0.25 W (series current limiter)
• 1× Adjustable DC power supply (0–12 V) with current limit
• 1× Digital multimeter (DMM)
• 1× Breadboard (or clip leads)
• 6× Jumper wires

Wiring guide

• Place D1 as a shunt to ground: cathode to the upper node, anode to ground (GND).
• Put R1 in series from +V to the upper node (the node between R1 and D1).
• Connect the supply: +V to the top of R1; supply ground to the bottom rail (GND).
• Abbreviations used in the schematic:
• VS: the supply node at the top of R1; measure with DMM red probe at the VS dot and black probe at GND.
• VZ: the Zener node between R1 and D1; measure with DMM red probe at the VZ dot and black probe at GND.
• IZ: Zener current; compute from IZ = (VS − VZ) / R1. If you have a second DMM, you may also measure it in series with D1.
• Set the supply current limit to 20–50 mA before powering up to protect parts.
• Double-check D1 polarity: for a Zener in breakdown, cathode must face +V and anode to GND.

Schematic

     Fuente DC (+9...15 V)
           +V
            │             • VR1+
           ┌┴┐
           │ │    R1 = 1 kΩ (serie)
           │ │
           └┬┘
            │             • VZ+ | VR1-
           ┌┴┐
           │ │    D1 = Zener 5.1 V (cátodo arriba)
           │ │
           └┬┘
            │             • VZ-
           GND

Measurements and tests

• Pre-calculation (choose R1):

  • Decide a test current (e.g., 5–10 mA for small Zeners).
  • Use R1 ≈ (VS(max) − VZ_nom) / I_test. Example: with VS(max)=12 V and VZ=5.1 V at 10 mA → R1 ≈ (12−5.1)/0.01 ≈ 690 Ω. Using 1 kΩ is safer and keeps IZ lower.

• Power-up and initial checks:

  • Set the supply to 0 V and current limit to 20–50 mA.
  • Connect the DMM black lead to GND. Place the red lead on the VS dot and verify VS reads near the set supply voltage.

• Measure Zener voltage VZ (breakdown region):

  • Move the red probe to the VZ dot.
  • Slowly increase VS until the Zener enters breakdown: VZ will rise and then clamp near its nominal value.
  • Record VZ at a few VS settings (e.g., 7 V, 9 V, 12 V) to see the clamping behavior.

• Compute Zener current IZ:

  • Measure VS at the VS dot and VZ at the VZ dot.
  • Compute IZ = (VS − VZ) / R1. Example: VS=12.0 V, VZ=5.2 V, R1=1 kΩ → IZ ≈ (12.0−5.2)/1000 ≈ 6.8 mA.

• Line regulation check (optional):

  • Sweep VS over a small range (e.g., 8–12 V).
  • Observe and record VZ; a good Zener changes only slightly while IZ increases with VS.

• Power dissipation check:

  • Compute PZ = VZ × IZ and PR = (VS − VZ) × IZ.
  • Ensure PZ is below the Zener’s rated power and PR below the resistor’s rating.

Common mistakes

• Reversing the Zener orientation (anode up/cathode down): it will not clamp as intended.
• Using too small R1, causing excessive IZ and overheating the diode.
• Forgetting to set a current limit on the bench supply.
• Measuring VZ at the wrong node (always use the VZ dot with respect to GND).

Safety and good practices

• Increase VS slowly while watching current limit and temperature.
• If the Zener or resistor becomes hot, reduce VS or use a larger R1.
• Do not exceed the Zener’s power rating; brief tests only if near the limit.

Improvements

• Use a second DMM in series with D1 to directly read IZ and compare to the calculated value.
• Characterize dynamic resistance: change IZ slightly (small change in VS) and compute rZ ≈ ΔVZ/ΔIZ.

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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