How do I know if my component is dead (diode, transistor, capacitor)?

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Short Answer: Use a multimeter’s diode/continuity mode.
Detailed:

  • Diode – Should show ~0.6V one way, open circuit (OL) the other.

  • BJT transistor – Measure base-emitter and base-collector like two diodes.

  • Capacitor – Set to ohms; it should start low (charging) then go to infinite. If it reads short (0Ω) or stays low, it’s dead. For electrolytics, look for bulging top or leaked electrolyte.


Figuring out if a component is dead comes down to measuring how it behaves vs. how it should behave. A multimeter is usually enough for quick checks.

🔧 1. Diode test

✅ What a good diode does

  • Conducts in one direction (forward)
  • Blocks in the other (reverse)

🧪 How to test

  • Set multimeter to diode mode
  • Measure both directions

📊 Results

  • Forward direction: ~0.6–0.7V (silicon diode)
  • Reverse direction: OL / no conduction

❌ Bad diode signs

  • 0V both ways → shorted
  • OL both ways → open (dead)

⚡ 2. Transistor (BJT) test

Think of it as two diodes back-to-back.

🧪 How to test (NPN example)

  • Base → Emitter: ~0.6–0.7V
  • Base → Collector: ~0.6–0.7V
  • Collector ↔ Emitter: no conduction

❌ Bad transistor signs

  • Short between any pins
  • No junction behavior (no diode drops)
  • Same reading in all directions

👉 For PNP, polarities are reversed.

🔋 3. Capacitor test

This one is trickier because behavior changes over time.

🧪 Quick multimeter test (resistance mode)

  • Connect probes → value should:
    • Start low
    • Gradually increase (charging effect)

❌ Bad capacitor signs

  • Always 0Ω → shorted
  • Always OL → open
  • No “charging” behavior

🧪 Better methods

  • Use capacitance mode (if available)
  • Use ESR meter for electrolytics

⚠️ Important rules (people skip these and get wrong results)

  • Always disconnect at least one leg from the circuit
  • Power OFF the circuit
  • Discharge capacitors before testing
  • Watch polarity (especially electrolytics)

🧠 Quick mental model

  • Diode: one-way valve
  • Transistor: two diodes + control
  • Capacitor: stores and releases charge

🚨 When results are misleading

In-circuit measurements can fool you because:

  • Parallel paths exist
  • Other components affect readings

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