Resistor Tolerance
No resistor is manufactured with a perfectly exact resistance value. Resistor tolerance specifies the maximum allowable variation between the resistor's marked value and its actual measured resistance. For example, a resistor marked as 100 Ω ±5% may measure anywhere between 95 Ω and 105 Ω and still be within its specified tolerance. Understanding tolerance is important when designing reliable electronic circuits, especially those requiring high accuracy.
What Is Resistor Tolerance?
Tolerance is the percentage by which the actual resistance value may differ from the nominal (marked) value.
It is expressed as a percentage and is usually indicated by the last colour band on through-hole resistors or in the component's datasheet.
Common Tolerance Values
| Tolerance | Typical Applications |
|---|---|
| ±20% | Older carbon composition resistors. |
| ±10% | Low-cost general-purpose circuits. |
| ±5% | Most consumer electronics. |
| ±2% | Improved accuracy circuits. |
| ±1% | Precision electronics. |
| ±0.5% | Instrumentation. |
| ±0.25% | Laboratory equipment. |
| ±0.1% | Calibration equipment. |
| ±0.05% | Ultra-precision circuits. |
Tolerance Colour Bands
| Colour | Tolerance |
|---|---|
| Brown | ±1% |
| Red | ±2% |
| Green | ±0.5% |
| Blue | ±0.25% |
| Violet | ±0.1% |
| Grey | ±0.05% |
| Gold | ±5% |
| Silver | ±10% |
| No Band | ±20% |
Calculating Tolerance
To determine the allowable resistance range, multiply the resistor value by its tolerance percentage.
Example 1
100 Ω ±5% 5% of 100 Ω = 5 Ω Minimum = 95 Ω Maximum = 105 Ω
Example 2
10 kΩ ±1% 1% of 10,000 Ω = 100 Ω Minimum = 9,900 Ω Maximum = 10,100 Ω
Why Tolerance Matters
| Application | Recommended Tolerance |
|---|---|
| LED current limiting | ±5% |
| Power supplies | ±5% or ±1% |
| Audio amplifiers | ±1% |
| Voltage dividers | ±1% |
| Sensor circuits | ±1% or better |
| Laboratory instruments | ±0.1% to ±0.5% |
| Calibration equipment | ±0.05% to ±0.1% |
Factors Affecting Accuracy
- Manufacturing process.
- Temperature changes.
- Ageing of the resistor.
- Humidity.
- Power dissipation.
- Mechanical stress.
Tolerance vs Temperature Coefficient
| Tolerance | Temperature Coefficient (TCR) |
|---|---|
| Initial manufacturing accuracy. | Change in resistance with temperature. |
| Expressed as a percentage. | Expressed in ppm/°C. |
| Fixed at manufacture. | Changes as temperature changes. |
How to Measure Tolerance
- Read the resistor's nominal value.
- Identify its tolerance rating.
- Measure the resistance using a calibrated digital multimeter.
- Compare the measured value with the allowable range.
- Replace the resistor if it falls outside its specified tolerance.
Common Mistakes
| Mistake | Explanation |
|---|---|
| Assuming every resistor has the exact printed value. | Every resistor has a manufacturing tolerance. |
| Ignoring tolerance in precision circuits. | Can significantly affect circuit performance. |
| Confusing tolerance with power rating. | These are completely different specifications. |
| Confusing tolerance with TCR. | TCR describes temperature-related resistance changes. |
Choosing the Right Tolerance
- ±5% is suitable for most hobby and consumer electronics.
- ±1% is recommended for audio, analogue and measurement circuits.
- ±0.1% or better is used in laboratory and calibration equipment.
- Lower tolerance generally means higher cost.
Key Points
- Tolerance defines the allowable variation from a resistor's nominal value.
- Gold indicates ±5% and brown indicates ±1% tolerance.
- Precision circuits require tighter tolerances.
- Tolerance and temperature coefficient are different specifications.
- Always choose a tolerance suitable for the circuit's required accuracy.