Practical case: Use a diode as polarity protection

Materials

• 1 × DC source (5–12 V bench supply or 9 V battery with clip)
• 1 × Breadboard and jumper wires
• 1 × D1: 1N5819 Schottky diode (polarity protection)
• 1 × D2: 5 mm red LED
• 1 × R1: 470 Ω, 1/4 W resistor (LED current limit)
• 1 × Digital multimeter (DMM)

Wiring guide

• Keep power OFF while wiring.
• Create a ground (GND) rail on the breadboard and a positive rail (+V) from your supply.
• Place D1 in series between +V and the protected rail (Vout):
• D1 anode to +V; D1 cathode (striped end) to the protected rail Vout.
• Build the indicator load from Vout to GND:
• From Vout, connect R1 to the LED anode (long lead).
• Connect the LED cathode (short lead/flat side) to GND.
• Connect the supply ground to GND.
• Abbreviation map for measurement dots in the schematic:
• Vin: voltage at the supply side of D1 (before the diode).
• Vout: voltage at the protected rail (after D1, before R1).
• VL: node between R1 and the LED.
• V_D: diode drop = Vin − Vout (measure with red probe on Vin dot, black on Vout dot).
• V_R: resistor drop = Vout − VL (red on Vout dot, black on VL dot).
• I_L: load current = V_R / R1.

Schematic

           +V IN
            │
            ├──● VIN
            │
           ┌┴┐         D1 = 1N5819 (protección en serie)
           │ │
           │ │
           └┬┘
            │
            ├──● VOUT
            │
           ┌┴┐         R1 = 470 Ω (limitadora)
           │ │
           │ │
           └┬┘
            │
            ├──● VR-
            │
           ┌┴┐         LED1 = LED rojo 5 mm
           │ │
           │ │
           └┬┘
            │
            ├──● VLED-
            │
           GND

Measurements and tests

• Safety pre-check:

  • Verify D1 orientation (anode to +V, striped cathode toward Vout).
  • Confirm LED orientation (anode to R1/Vout, cathode to GND).
  • Set the supply current limit to a safe value (e.g., 100–200 mA).

• Forward-polarity test (normal use):

  • Power ON with correct polarity: supply + to +V, − to GND.
  • Measure Vin (red probe on Vin dot, black on GND): expect ≈ supply voltage.
  • Measure Vout (red probe on Vout dot, black on GND): slightly lower than Vin.
  • Compute V_D = Vin − Vout: expect ≈ 0.2–0.4 V for 1N5819.
  • LED should be ON. Measure VL (red probe on VL dot, black on GND).
  • Measure V_R = Vout − VL (red on Vout dot, black on VL dot).
  • Compute I_L = V_R / 470 Ω; typical 5–15 mA depending on supply.

• Reverse-polarity test (protection behavior):

  • Power OFF. Reverse the supply leads (swap + and −).
  • Power ON. Measure Vout (red on Vout dot, black on GND): expect ≈ 0 V (LED OFF).
  • Measure Vin (red on Vin dot, black on GND): will be negative relative to GND if using a DMM; the series diode blocks current so the load is protected.
  • Optional: Put the DMM in current mode in series with +V; expect ≈ 0 mA (only leakage).

• Voltage-drop comparison (optional):

  • Replace D1 with a standard silicon diode (e.g., 1N4007) and repeat forward test.
  • Observe larger V_D (≈ 0.6–0.8 V) and slightly dimmer LED due to reduced Vout.

Common mistakes

• D1 installed backwards (will block in normal operation, LED stays OFF).
• Using no current-limiting resistor for the LED (can damage the LED).
• Expecting zero drop across a diode; all real diodes have forward voltage.
• Measuring I_L by shorting across R1 with the ammeter (this creates a short; instead, measure V_R and compute I_L or insert the meter in series properly).

Safety

• Do not exceed the diode’s reverse voltage rating with your supply.
• Limit current on a bench supply; avoid shorting the source when probing.
• Disconnect power before rearranging any wiring.

Improvements

• Reduce loss: replace the series diode with a “ideal diode” MOSFET reverse‑polarity circuit (P‑channel high side) for <100 mV drop.
• Add a fuse and a TVS diode for robust overcurrent and surge protection on the input.

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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