In the context of SMD resistors, ppm stands for parts per million. It is primarily used as a unit of measurement for the Temperature Coefficient of Resistance (TCR).
The TCR of a resistor indicates how much its resistance value changes in relation to changes in temperature. As the reference states, TCR is usually expressed in ppm/°C (parts per million per degree Centigrade) units.
Understanding ppm/°C for Resistors
When a resistor has a specified TCR in ppm/°C, it means that its resistance value will change by that many parts per million for every one-degree Celsius change in its temperature.
- ppm is a relative unit, similar to percentage, but on a much smaller scale. 1% is equal to 10,000 ppm.
- ppm/°C therefore quantifies the rate of resistance change per degree of temperature shift.
For example:
- A resistor with a TCR of +100 ppm/°C will see its resistance increase by 100 parts per million of its nominal value for every 1°C rise in temperature.
- A resistor with a TCR of -50 ppm/°C will see its resistance decrease by 50 parts per million of its nominal value for every 1°C rise in temperature.
- A TCR close to 0 ppm/°C indicates very stable resistance across temperature changes.
Why is TCR (in ppm/°C) Important for SMD Resistors?
SMD (Surface Mount Device) resistors are used in a wide range of electronic circuits, often in applications where temperature stability is crucial.
- Precision Circuits: In circuits like amplifiers, sensors, or measurement equipment, even small changes in resistance due to temperature can affect accuracy. Resistors with low TCR values (e.g., 10 ppm/°C or 25 ppm/°C) are preferred for these applications.
- Environmental Variations: Electronic devices often operate in environments where temperature fluctuates. Understanding and selecting resistors with appropriate TCRs helps ensure the circuit performs reliably under these conditions.
- Component Self-Heating: Even without external temperature changes, current flowing through a resistor generates heat (Joule heating). This self-heating can change the resistor's value based on its TCR.
Common TCR values for SMD resistors can vary significantly depending on the resistor technology and intended application:
Resistor Type/Application | Typical TCR (ppm/°C) |
---|---|
General Purpose | ±100 to ±250 |
Semi-Precision | ±50 to ±100 |
Precision | ±5 to ±50 |
In summary, when you see "ppm" associated with an SMD resistor's specification, it is almost always referring to parts per million as the unit used to express its Temperature Coefficient of Resistance (TCR), indicating how sensitive the resistor's value is to temperature changes.