Engineering Reference
Resistor Tolerance, Power and TCR Reference
Lookup reference for resistor tolerance, power rating, derating, working voltage, pulse limits, temperature coefficient in ppm/K, drift, and design-margin boundaries.
Rating and Stability Terms
| Term | Unit | Meaning |
|---|---|---|
| Tolerance | ±% | Initial resistance deviation at stated conditions; not total lifetime accuracy |
| Rated power | W | Permitted steady dissipation under stated ambient, mounting, and temperature conditions |
| Derating | % or W/°C | Reduction of allowable power as ambient or body temperature rises |
| Maximum working voltage | V | Continuous voltage boundary that can limit use before P=V²/R |
| Overload / pulse rating | curve | Time- and waveform-dependent short-duration capability |
| TCR | ppm/K | First-order fractional resistance change per kelvin |
| Voltage coefficient | ppm/V | Resistance change with applied voltage for technologies where specified |
| Thermal resistance | K/W | Body or film temperature rise per dissipated watt under stated mounting |
| Long-term drift | % | Change caused by load life, aging, humidity, and thermal cycling |
| Noise | µV/V or dB | Technology- and operating-condition-dependent excess noise |
Common Tolerance Classes
| Tolerance | Typical context |
|---|---|
| ±20% | E6-era/general low-precision use |
| ±10% | E12 general-purpose |
| ±5% | E24 general-purpose |
| ±2% | Tighter general-purpose |
| ±1% | Precision/common E96 |
| ±0.5% | Precision |
| ±0.1% | High precision |
| ≤±0.05% | Specialized precision; verify stability and TCR separately |
TCR Conversion
| TCR | Fraction per K | Change over 60 K |
|---|---|---|
| 25 ppm/K | 0.000025/K | 0.15% |
| 50 ppm/K | 0.000050/K | 0.30% |
| 100 ppm/K | 0.000100/K | 0.60% |
| 250 ppm/K | 0.000250/K | 1.50% |
Canonical Examples
| Check | Inputs | Result |
|---|---|---|
| Ohm's-law power | 5 V × 20 mA | 0.100 W |
| Divider loading | 5 V, 10 kΩ / 10 kΩ | 2.500 V |
| LED resistor | 5 V supply, 2 V LED, 20 mA | 150.000 Ω; 0.250 W rating |
| TCR drift | 100 ppm/K over 60 K | 6000 ppm = 0.6% first-order change |
Common Reference Mistakes
- •Treating tolerance as total accuracy.
- •Ignoring TCR across operating temperature.
- •Running continuously at nameplate power without derating.
- •Ignoring maximum working voltage.
- •Applying steady-state power to pulses.
- •Confusing ppm/K with percent/K.
- •Averaging positive and negative tolerances incorrectly.
- •Ignoring self-heating in precision networks.
- •Assuming equal package size means equal rating.
- •Ignoring PCB copper and airflow in thermal behavior.
- •Using nominal resistance after excessive rounding.
- •Skipping the exact part datasheet and reliability conditions.
Support reference
FAQ
What does resistor tolerance mean?
Tolerance is the allowed initial deviation from nominal resistance under stated conditions. It does not include all temperature, voltage, aging, and load-life effects.
Is a 1% resistor accurate to 1% at all temperatures?
No. TCR and other drift terms add to initial tolerance across temperature and life.
What is resistor power rating?
It is an allowed dissipation under specified ambient, mounting, and temperature conditions, not a universal safe value.
Why must a resistor be derated?
Higher ambient and body temperature reduce thermal margin and can increase drift or shorten life.
Can voltage exceed a resistor limit even below rated power?
Yes. Maximum working voltage can be reached before the calculated power rating, especially for high resistance values.
What is resistor TCR?
Temperature coefficient of resistance is first-order fractional resistance change per degree, commonly stated in ppm/K.
Are ppm/K and ppm/°C equivalent?
Yes for temperature differences because one kelvin and one degree Celsius have the same interval size.
How do I convert ppm to percent?
Divide ppm by 10,000. For example, 5000 ppm equals 0.5%.
Does a pulse below average power always remain safe?
No. Pulse energy, duration, repetition, peak voltage, element temperature, and technology-specific curves still apply.
Should I select the exact calculated power rating?
Usually no. Apply margin and the manufacturer's derating curve for ambient, mounting, enclosure, and reliability goals.
Does E-series define resistor tolerance?
Preferred-number series and tolerance are related historically but separate specifications. Verify the actual component tolerance.
Does this page replace a resistor datasheet?
No. Use the exact part's power, voltage, pulse, TCR, drift, noise, package, and environmental specifications.
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