Practical case: Replacement of a damaged capacitor

Materials

• 1x DC supply, +5 V (bench supply or USB 5 V)
• 1x Electrolytic capacitor, C1 = 100 µF, ≥10 V (new replacement)
• 1x Faulty electrolytic capacitor (original C1 to be removed)
• 1x Resistor, R1 = 330 Ω, 1/4 W
• 1x Red LED, D1 (standard 5 mm or similar)
• 1x Breadboard and jumper wires (for test setup) or an existing PCB to repair
• 1x Digital multimeter with DC voltage and continuity/capacitance modes
• Optional (for PCB repair): soldering iron, solder, solder wick (or pump), flux, isopropyl alcohol, ESD wrist strap, safety glasses

Wiring guide

• Power off the circuit and disconnect the +5 V source before touching components.
• Identify the damaged capacitor C1 (look for bulging top, leakage, or burnt smell). Mark the board’s polarity: the pad connected to GND and the pad connected to +V.
• If you are repairing a PCB:
• Desolder the faulty C1. Heat each lead, remove solder with wick or pump, and gently pull the capacitor out. Clean the pads.
• Insert the new C1: long lead (+) to the +V pad; short lead (−, stripe) to the GND pad. Solder and trim leads.
• If you are building the test circuit on a breadboard:
• Create a +V rail and a GND rail.
• Place C1 between +V and GND with correct polarity (− to GND).
• From +V, go through R1 to a node; from that node, go through D1 to GND (LED cathode to GND).
• Double-check polarity of C1 and D1 before powering.
• Abbreviations used in the schematic:
• VC: voltage at the +V node (top of C1) measured relative to GND.
• VLED: voltage at the LED anode node (between R1 and D1) measured relative to GND.

Schematic

              +5 V
               │
              ┌┴┐  
              │ │   R1 = 100 kΩ (limitador/carga)
              │ │
              └┬┘
               │ ● V_C
               │
        ┌──────┴───────────┐
        │                  │
       ┌┴┐                ┌┴┐
       │ │                │ │   R_LED = 220 Ω (serie LED)
       │ │ C1 = 47 µF     │ │
       └┬┘ 16 V (a        └┬┘  ● V_RLED+
        │   reemplazar)     │
        │                   │
        │                  ┌┴┐
        │                  │ │   LED1 = LED rojo (ánodo arriba)
        │                  │ │
        │                  └┬┘  ● V_RLED−
        │                   │
        └───────────┬───────┘
                    │
                   GND

Measurements and tests

• Safety checks:

  • Verify power is OFF before touching or replacing components.
  • Discharge C1 by briefly connecting a 1 kΩ resistor between +V and GND (do not short it directly).

• Pre-replacement diagnostics (power OFF):

  • Use continuity mode across the removed, faulty C1. A steady beep suggests a shorted capacitor; open or very low capacitance suggests it dried out.
  • If your meter has capacitance mode, measure the old C1; values far from 100 µF indicate failure.

• Voltage tests after replacement (power ON):

  • Place the black probe at GND and the red probe on the ● VC node. VC should be close to the supply (about +5.0 V).
  • Move the red probe to ● VLED. VLED should be near the LED forward voltage (about 1.8–2.2 V for a red LED) when lit.

• Ripple and stability (optional):

  • If you have a multimeter with a “AC on DC” function, measure AC at ● VC with respect to GND. It should be very small (typically <50 mV) in this simple setup. Large ripple suggests poor capacitance or wrong installation.

• Pass/fail criteria:

  • LED is steadily ON (no flicker).
  • VC ≈ supply voltage; VLED ≈ LED forward drop; no abnormal heating or smell.
  • No continuity short between +V and GND after installation.

Common mistakes

• Reversing the electrolytic capacitor polarity (− stripe must go to GND). This can cause heating or venting.
• Using too low a voltage rating (choose at least 2× the supply voltage; here ≥10 V for a 5 V rail).
• Cold or bridged solder joints on PCB repairs.
• Measuring voltages without referencing GND; always place the black probe at GND for VC and VLED.

Safety

• Always power down and discharge capacitors before handling.
• Wear safety glasses when soldering or when first powering up after replacement.
• Do not exceed capacitor voltage rating; electrolytics can vent if overstressed.

Improvements

• Add a 100 nF ceramic capacitor in parallel with C1 (from +V to GND) to better filter high-frequency noise.
• Choose low-ESR electrolytics for power rails.
• Label polarity on the board for easier future maintenance.

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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