What happens when a resistor fails?

When a resistor fails, its electrical properties deviate significantly from its intended specifications, most commonly leading to an open circuit, a short circuit, or a drastic change in its resistance value. This can severely disrupt the normal operation of an electronic circuit or device.

Common Resistor Failure Modes

Resistors can fail in several ways, often as a direct result of electrical or thermal stress. Understanding these modes is crucial for diagnosing circuit issues.

Open Circuit Failure

This is the most common and often desired failure mode for a resistor designed to act as a fuse in certain conditions. If an overload condition continues, such as excessive current or voltage, the resistive material will deteriorate and burn out. This causes the resistance value to increase drastically, eventually becoming infinite (an open circuit), which effectively breaks the electrical path.

• Consequence: The flow of current through that part of the circuit ceases. Any component downstream that relies on this path will stop receiving power or signal, rendering it inoperative.

Short Circuit Failure

Less common for standard resistors but more damaging, a short circuit failure occurs when the resistive material or its insulation breaks down and forms a conductive path, reducing the resistance to near zero ohms. This can happen due to internal arcing or a catastrophic material breakdown.

• Consequence: An uncontrolled surge of current can flow through the shorted path. This can lead to rapid overheating of other components, damage to the power supply, or even fire.

Resistance Value Drift

Sometimes, a resistor doesn't completely fail but its resistance value changes significantly from its specified value. This drift can occur due to prolonged exposure to stress, such as constant overheating or aging of the material. The resistance value may increase over time due to deterioration, impacting circuit performance.

• Consequence: The circuit may function incorrectly, leading to inaccurate voltage or current division, instability, reduced efficiency, or intermittent failures.

Primary Causes of Resistor Failure

Understanding why resistors fail helps in preventing future issues and improving circuit reliability.

• Overload: Exceeding the resistor's specified power rating (measured in watts) is the leading cause of failure. If the power dissipated by the resistor (P = I²R or P = V²/R) consistently exceeds its rating, it generates excessive heat. This heat causes the resistive material to deteriorate and burn out, leading to a significant increase in resistance, often resulting in an open circuit.
• Overheating: Beyond electrical overload, poor ventilation, high ambient temperatures, or close proximity to other heat-generating components can cause thermal stress, accelerating degradation.
• Physical Damage: Mechanical stress, impacts, vibration, or improper handling during assembly or repair can crack the resistive element or lead to broken leads.
• Environmental Factors: Exposure to moisture, corrosive chemicals, extreme humidity, or rapid temperature cycling can degrade the resistor's protective coating and resistive element over time.
• Manufacturing Defects: Although rare with reputable manufacturers, internal flaws in the resistor's construction, material impurities, or poor quality control can lead to premature failure.

Identifying a Failed Resistor

Recognizing the signs of a failed resistor is the first step in troubleshooting an electronic circuit.

Visible Symptoms

• Burn Marks: Charred or discolored areas on the resistor's body or the surrounding circuit board.
• Cracked or Bloated Casing: The protective coating may crack or swell due to internal heat and gas buildup.
• Smoke or Burning Smell: A strong, acrid odor, often accompanied by visible smoke, indicates severe overheating.

Electrical Symptoms

• The circuit or a specific part of it stops functioning entirely.
• An absence of power to certain components, indicating an open circuit.
• Excessive heat from other components or the power supply, potentially indicating a short circuit.
• Incorrect voltage readings at various points in the circuit, suggesting a resistance value drift.
• Unusual sounds (e.g., buzzing) or intermittent operation.

Troubleshooting and Solutions

Diagnosing a failed resistor typically involves a systematic approach.

1. Visual Inspection: Always begin by carefully examining the circuit board and components for any of the visible symptoms mentioned above.
2. Multimeter Testing:
   • Power Down: Always ensure the circuit is completely powered off and discharged before performing resistance measurements to prevent electric shock and damage to the multimeter.
   • Isolation: For accurate readings, it's often best to desolder at least one leg of the suspected resistor from the circuit board. This isolates the resistor and prevents parallel paths from skewing the measurement.
   • Measure Resistance: Set your multimeter to the resistance (Ohms, Ω) setting. Touch the probes to each end of the resistor.
     • An "OL" (Over Limit) or infinite reading indicates an open circuit.
     • A reading significantly different from the resistor's marked value indicates value drift.
     • A very low reading (near zero), though rare for resistors, could indicate a short circuit.
3. Replacement: If a resistor is found to be faulty, replace it with a new one that has the exact same resistance value (ohms) and an equal or higher power rating (watts). Always ensure the new component is of good quality.
4. Root Cause Analysis: To prevent recurring failures, investigate why the resistor failed. Was it an overload? Poor cooling? A fault elsewhere in the circuit causing excessive current? Addressing the root cause is critical for long-term reliability. For instance, if overload was the cause, consider using a higher-wattage resistor or redesigning the circuit to reduce current flow through that component.

Summary of Resistor Failure Effects