Testing Resistors
Testing resistors is one of the most common tasks in electronic repair and circuit troubleshooting. A faulty resistor can cause incorrect voltages, excessive current, distorted signals or complete circuit failure. Fortunately, resistors are easy to test using a digital multimeter (DMM), provided the correct procedure is followed.
Tools Required
- Digital multimeter (recommended).
- Analogue multimeter (optional).
- Soldering iron (for lifting one lead if necessary).
- Tweezers.
- Circuit diagram or service manual (if available).
Safety First
- Disconnect the power supply.
- Remove batteries where possible.
- Discharge large electrolytic capacitors.
- Never measure resistance on a powered circuit.
- Allow hot components to cool before handling them.
Visual Inspection
Many faulty resistors can be identified before taking any measurements.
| Symptom | Possible Cause |
|---|---|
| Burn marks | Overload or short circuit. |
| Cracked body | Mechanical damage. |
| Discoloured coating | Excessive heat. |
| Broken lead | Mechanical stress. |
| Loose solder joint | Poor connection. |
Testing with a Digital Multimeter
- Switch the multimeter to the resistance (Ω) range.
- Select auto-ranging or the nearest manual range.
- Touch the probes together to confirm proper operation.
- Place one probe on each resistor terminal.
- Read the measured resistance.
- Compare the reading with the resistor's marked value.
Testing In-Circuit
Resistors can often be measured while still soldered to the circuit board. However, other components connected in parallel may affect the reading.
| Reading | Meaning |
|---|---|
| Correct value | Resistor is probably good. |
| Lower than expected | Parallel circuit path may be affecting the reading. |
| Higher than expected | Possible faulty resistor. |
| Open circuit (OL) | Resistor may have failed open. |
Testing Out of Circuit
For the most accurate measurement, disconnect one end of the resistor from the circuit.
- Heat one solder joint.
- Lift one resistor lead carefully.
- Measure the resistance.
- Compare the value with the colour code or printed marking.
- Resolder the lead after testing.
Checking the Tolerance
Every resistor has an allowable tolerance range.
Example:
100 Ω ±5% Minimum = 95 Ω Maximum = 105 Ω
Any reading within this range is considered acceptable.
Testing Potentiometers
- Measure between the two outer terminals.
- The reading should equal the rated resistance.
- Measure between the centre terminal (wiper) and each outer terminal.
- Rotate the shaft slowly.
- The resistance should change smoothly without sudden jumps.
Testing Trimmer Potentiometers
- Measure the total resistance across the outer terminals.
- Rotate the adjustment screw carefully.
- Check that the resistance changes smoothly.
- Look for dead spots or intermittent readings.
Common Faults
| Fault | Multimeter Reading |
|---|---|
| Open Circuit | OL or infinite resistance. |
| Short Circuit | Nearly 0 Ω. |
| Resistance Drift | Outside tolerance. |
| Intermittent Connection | Unstable readings. |
| Burnt Potentiometer | Dead spots while rotating. |
Common Mistakes
| Mistake | Result |
|---|---|
| Testing with power applied. | Incorrect readings or meter damage. |
| Ignoring resistor tolerance. | Good resistors may be replaced unnecessarily. |
| Testing only in-circuit. | Parallel components may affect measurements. |
| Using the wrong resistance range. | Reduced measurement accuracy. |
Troubleshooting Tips
- Always begin with a visual inspection.
- Measure suspicious resistors out of circuit if necessary.
- Compare readings with the circuit diagram.
- Replace resistors showing signs of overheating.
- Investigate the cause of failure before replacing the resistor.
When Should a Resistor Be Replaced?
- Open circuit.
- Short circuit.
- Outside its specified tolerance.
- Burnt or cracked.
- Loose or damaged leads.
- Intermittent operation.
Key Points
- A digital multimeter is the preferred tool for testing resistors.
- Disconnect power before measuring resistance.
- Lift one lead for the most accurate measurements.
- Compare measured values with the resistor's tolerance.
- Visual inspection is often the quickest way to identify damaged resistors.