Replacing Resistors
Replacing a faulty resistor involves more than simply matching its resistance value. To ensure safe and reliable operation, the replacement resistor should also match or exceed the original component's power rating, voltage rating, tolerance and construction type. Choosing the wrong replacement may lead to poor circuit performance or cause the new resistor to fail.
Step 1 – Identify the Resistor Value
Determine the resistor's nominal resistance before replacing it.
- Read the colour bands.
- Check the printed marking (SMD resistors).
- Measure the resistor if it is still readable.
- Consult the circuit diagram or service manual.
- Refer to the PCB silkscreen if available.
Step 2 – Select the Correct Power Rating
The replacement resistor should have a power rating equal to or greater than the original.
| Original | Suitable Replacement |
|---|---|
| ¼ W | ¼ W, ½ W or 1 W |
| ½ W | ½ W or 1 W |
| 1 W | 1 W or higher |
| 5 W | 5 W or higher |
Never replace a resistor with one having a lower power rating.
Step 3 – Match the Tolerance
Whenever possible, use the same tolerance as the original resistor.
| Original | Recommended Replacement |
|---|---|
| ±5% | ±5% or better. |
| ±1% | ±1%. |
| ±0.1% | ±0.1%. |
Using a tighter tolerance is usually acceptable, but replacing a precision resistor with a lower-accuracy part may affect circuit performance.
Step 4 – Check the Voltage Rating
High-voltage circuits require resistors with suitable maximum working voltages.
Always check the manufacturer's datasheet when replacing resistors in switch- mode power supplies, valve amplifiers or other high-voltage equipment.
Step 5 – Choose the Correct Type
| Original Type | Replacement |
|---|---|
| Carbon Film | Carbon Film or Metal Film |
| Metal Film | Metal Film |
| Metal Oxide | Metal Oxide |
| Wirewound | Wirewound |
| SMD | Equivalent SMD package and value |
Replacing Surface-Mount (SMD) Resistors
- Identify the package size (e.g. 0603, 0805, 1206).
- Match the resistance value.
- Match or exceed the power rating.
- Use proper SMD soldering techniques.
- Avoid overheating the PCB pads.
Soldering Tips
- Disconnect all power.
- Remove the faulty resistor carefully.
- Clean the solder pads.
- Fit the replacement resistor.
- Solder using clean, bright joints.
- Trim excess lead length.
- Inspect for solder bridges or dry joints.
After Replacement
- Inspect the surrounding components.
- Measure the new resistor if necessary.
- Check for short circuits.
- Power the circuit using a current-limited supply if possible.
- Monitor the resistor for excessive heating.
- Verify correct circuit operation.
Common Mistakes
| Mistake | Result |
|---|---|
| Replacing only the resistor. | The new resistor may fail if the underlying fault is not repaired. |
| Using the wrong resistance value. | Incorrect circuit operation. |
| Using a lower wattage resistor. | Overheating and premature failure. |
| Ignoring voltage rating. | Possible insulation breakdown. |
| Poor soldering. | Intermittent or unreliable operation. |
Real-World Examples
| Equipment | Replacement Considerations |
|---|---|
| Audio Amplifier | Use flameproof emitter resistors of the correct wattage. |
| SMPS (UC3842 / IR2153) | Match resistance, power rating and voltage rating exactly. |
| Battery Charger | Replace current-sense resistors with low-value, high-power types. |
| LED Driver | Use flameproof current-limiting resistors where specified. |
| Bench Power Supply | Use precision resistors for voltage reference and current sensing. |
When Should You Upgrade the Resistor?
In some cases, using a resistor with a higher power rating or better tolerance can improve reliability without affecting circuit operation, provided there is sufficient space on the circuit board.
However, changing the resistor type or value without understanding the circuit may lead to unexpected behaviour.
Key Points
- Match the resistance value exactly unless the circuit design requires otherwise.
- Never use a resistor with a lower power rating than the original.
- Match the tolerance and voltage rating where appropriate.
- Choose the correct resistor construction for the application.
- Always investigate the cause of the original resistor failure before fitting a replacement.