Thermistor: Function and Roles
Key Takeaways
The TCR defines the relationship between temperature and resistance.
Compare thermistors and other temperature-dependent components.
Different uses for PTC and NTC thermistors.
Heat and electricity are heavily intertwined. Many people have experienced this relationship from a young age with an electric stovetop or light bulb, but the effect of temperature on circuit performance is far more involved. As components heat, their material properties change and produce noticeably different functionality. Performance changes are usually detrimental, but material science has discovered or manufactured components where this alteration is a crucial feature of the device. Thermistors are one of several component families with temperature-driven functionality as a defining characteristic.
Relative Comparison of Temperature Sensors
Operating Range | Accuracy | Linear Approx. | Ruggedness | Cost | |
Thermistor | Low | High | Moderate (can be improved with higher tolerance, ADC precision) | High | Low |
Resistance thermometer (RTD) | Moderate | Moderate | High | Low | High |
Thermocouple | High | Low | Low | Moderate | Moderate |
Thermistors and Temperature-Dependent Resistance
The word thermistor is a portmanteau of thermal and resistor, underscoring the relationship between temperature and resistance in these devices. Every resistive element is a thermistor at some level, but the thermistor's resistance sharply rises or falls in response to temperature change. At the heart of the matter is the temperature coefficient of resistance (TCR), an intrinsic property of materials that defines resistance response to temperature variations. Understanding TCR is easiest at its extremes:
A very negative TCR (negative thermal coefficient, NTC) value indicates a rapid resistance decrease with a temperature rise. The more negative the TCR value, the more extreme the reduction in resistance.
A very positive TCR (positive thermal coefficient, PTC) value indicates a rapid resistance increase with a temperature rise. The more positive the TCR value, the more extreme the resistance increases.
A zero TCR indicates a completely temperature-independent resistance. The closer the absolute value of the TCR is to zero, the more gradual the decrease or increase in resistance with changing temperature. For this reason, general-use resistors prefer to have a near-zero TCR value for a broader range of applicability.
Thermistors are not the only devices that operate based on electrothermal principles. Resistance thermometers (also known as resistance temperature detectors or RTDs) and thermocouples also sense temperature. The choice between these sensor modes is usually between the cost, operating temperature range, and accuracy within said range. Thermistors occupy a small temperature range but offer high accuracy therein; thermistors are on the opposite end of the spectrum with an extensive temperature range but relatively poor accuracy. RTDs possess a higher range and accuracy than thermistors at a higher cost and increased fragility.
Temperature sensor selection also relies on the practical needs of the board. Thermistors may be most suitable when board design aligns with the following concerns.
When Are Thermistors Most Suitable?
High-volume production | Thermistors can achieve excellent measurement results even with low tolerances, establishing them as a highly economical option. |
Calibration | All temperature sensors need to calibrate for accuracy relative to the temperature range of interest; these calibrations often utilize the freezing point of water as a reference. Since a thermistor is fundamentally only a resistor, it presents a much simpler waterproofing process. |
Circuit simplification | The thermistor resistance output can directly interface with a microcontroller’s ADC. In comparison, a thermocouple requires amplification of the voltage signal. |
Reliability | Among the temperature sensor types, thermistors exhibit the best reliability and are unaffected by shock or vibration modes. |
Accuracy | Combining tighter tolerance models, calibration, and enhanced ADC bit resolution can improve readings beyond standard levels. |
Differentiating NTC and PTC Thermistors
As mentioned, the primary distinguishing factor between thermistors is the sign of the TRC. Both NTC and PTC thermistors have multiple circuit applications.
NTC
Regarding the current passage, an NTC thermistor is analogous to an inductor experiencing inrush current, although the mechanisms are entirely different. An NTC resistor possesses some resistance at ambient temperatures before experiencing a temperature increase due to power dissipation through the resistor (the Ohmic heating principle). The resistance falls as the temperature increases above ambient, allowing more current to pass. Uses of this technique include:
- Temperature-sensitive processes - Biological and chemical industries reliant on cold chains can monitor a thermistor output to track unsafe temperature levels.
- Current limiters - Thermistors control the rate of current passage to protect sensitive components and sacrificial elements like fuses and circuit breakers. They also reduce power loss as the current is low when resistance is high (and vice versa).
- Predictive maintenance - By monitoring process fluids, a drop in resistance can indicate potential early failure of system components.
- Battery chargers - Output can check for escalating temperatures that could indicate the beginning of thermal runaway conditions.
PTC
Unlike NTC, PTC thermistors are self-regulating: the resistance increases as current passes through the resistor and its temperature climbs above ambient. This restorative action makes PTC thermistors excel at constant temperature maintenance. Assuming the circuit and material conditions do not permit the PTC thermistor to damage itself, the need for temperature control is redundant. The thermistor’s ability to block current spikes makes it valuable as a resettable fuse in series with sensitive circuitry. Other PTC roles include:
- Heating elements - A thermistor's variable resistance is practical in heat-critical processes like diesel engines before fuel injection.
- Current divider - Parallel components with appreciable differences in resistance can lead to current hogging/starvation. PTC thermistors in series will dynamically balance the current between branches.
- Silistor - Silicon thermistors offer improved linearity over standard ceramic material PTCs.
Cadence Solutions for Thermal Components
The thermistor’s temperature-dependent resistance gives circuit designers multiple sensing and circuit-control uses. While this component family is similar to other temperature sensors like RTDs and thermocouples, it occupies a low-cost, high-reliability niche with expandable accuracy, making the thermistor an excellent general solution to temperature sensing. The ability of different materials to scale resistance increase or decrease proportionally to temperature (and vice versa) endears thermistors to a vast range of circuit applications. Taking advantage of thermistor capabilities requires robust electrothermal simulation to model response over operating ranges; Cadence’s suite of PCB Design and Analysis Software grants electronic device development numerous tools to measure performance. When it’s time for prototyping, OrCAD PCB Designer's speed and comprehensive functionality accelerate board layout and reduce time-to-market.
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