Thoery - 10 : -Temperature Dependent Resistors (NTC& PTC)

Temperature Dependent Resistors (Thermistors)

A thermistor (a portmanteau of "thermal resistor") is a type of resistor whose resistance is intentionally designed to vary significantly and predictably with temperature. This property makes them ideal for use as temperature sensors and in circuits that need to react to temperature change.

Thermistors are broadly classified into two main types based on their response to temperature:

1. NTC (Negative Temperature Coefficient)

2. PTC (Positive Temperature Coefficient)

---

1. NTC (Negative Temperature Coefficient) Thermistors

1.1 Definition and Core Principle

An NTC thermistor exhibits a decrease in electrical resistance as its temperature increases. This relationship is inverse and highly non-linear.

• At low temperatures: The thermistor has high resistance.

• At high temperatures: The thermistor has low resistance.

1.2 Construction and Working Principle

NTC thermistors are typically made from a sintered mixture of semiconductor materials, such as metallic oxides of manganese (Mn), nickel (Ni), cobalt (Co), copper (Cu), and iron (Fe).

In these semiconductor materials, the charge carriers (electrons and holes) are responsible for conducting electricity. At low temperatures, most charge carriers are bound within the atomic structure, resulting in fewer free carriers and thus high resistance. As the temperature rises, thermal energy excites the electrons, causing them to break free from their valence bands and become available for conduction. This increase in the number of free charge carriers leads to a significant drop in resistance.

1.3 Characteristic Curve and Equation

The relationship between resistance (R) and temperature (T) for an NTC thermistor is exponential.

A common and useful approximation for this curve is the Beta ($beta$) Parameter Equation:

$$R(T)=R_0\cdot e^{\beta \left(\frac{1}{T}-\frac{1}{T_0}\right)}$$

Where:

• R(T) is the resistance at the temperature T (in Kelvin, K).

• R_0 is the nominal resistance at a reference temperature T_0 (e.g., 10textkOmega at $25^\\circ\\text{C}$ or 298.15textK).

• T_0 is the reference temperature in Kelvin (K).

• beta (Beta) is the material constant (in Kelvin, K), typically ranging from 3000 K to 5000 K. It defines the "steepness" of the resistance curve.

For higher accuracy, the Steinhart-Hart Equation is used:

$$\frac{1}{T}=A+B\ln (R)+C(\ln (R){)}^3$$

Where A, B, and C are coefficients derived from experimental measurements.

1.4 Circuit Symbol

The standard symbol for a thermistor indicates its temperature dependency with a -T°.

1.5 Applications

The high sensitivity of NTC thermistors makes them ideal for:

1. Precise Temperature Sensing: Used in digital thermometers, thermostats (in air conditioners, refrigerators), and medical equipment.

2. Inrush Current Limiting: When a circuit is first turned on, the NTC is cold (high resistance), which limits the initial surge of current. As it warms up from the current flow, its resistance drops to a very low level, allowing normal circuit operation with minimal power loss.

3. Temperature Compensation: Used to counteract the effect of temperature on other electronic components in a circuit.

---

2. PTC (Positive Temperature Coefficient) Thermistors

2.1 Definition and Core Principle

A PTC thermistor exhibits an increase in electrical resistance as its temperature increases. This effect is typically non-linear and can be extremely dramatic in "switching" type PTCs.

• At low temperatures: The thermistor has low resistance.

• At high temperatures: The thermistor has high resistance.

2.2 Construction and Working Principle

There are two main types of PTC thermistors:

1. Silistors: Made from doped polycrystalline silicon. They show a relatively gradual, linear increase in resistance with temperature. They are less common than the switching type.

2. Switching Type (Polymer PTC): These are more common and are composite materials made from a polymer matrix loaded with conductive particles (e.g., carbon black).

   • Normal Operation (Low Temp): The polymer is in a crystalline state. The carbon particles are packed closely together, forming numerous conductive paths. The overall resistance is very low (e.g., $\< 1 \\Omega$).

   • Tripping (High Temp): When the temperature rises to a specific point, called the Curie Temperature ($T\_C$), the polymer matrix undergoes a phase transition and expands rapidly. This expansion separates the conductive carbon particles, breaking the conductive paths. As a result, the resistance increases abruptly by several orders of magnitude (e.g., to 10textkOmega), effectively becoming an open circuit.

2.3 Characteristic Curve

The resistance-temperature curve for a switching PTC is highly non-linear. It is characterized by a very sharp increase in resistance around its specified Curie Temperature (T_C).

2.4 Circuit Symbol

The symbol is similar to the NTC but with a +T° to indicate a positive coefficient.

2.5 Applications

The unique switching characteristic of PTCs makes them suitable for:

1. Overcurrent Protection (Resettable Fuses): This is their primary application. A PTC is placed in series with the load. During normal operation, its resistance is negligible. If an overcurrent fault occurs, the PTC heats up rapidly due to I2R heating. When it reaches its T_C, its resistance skyrockets, limiting the current to a safe, low level. When the fault is cleared and the device cools down, the PTC returns to its low-resistance state, "resetting" the circuit.

2. Self-Regulating Heaters: A PTC can act as its own thermostat. As it heats up, its resistance increases, which in turn reduces the current flowing through it and limits its own power dissipation ($P=V^2/R$). It will stabilize at a specific temperature.

3. Time Delay Circuits: The time it takes for a PTC to heat up and "trip" can be used to create simple time delays, for example, in the degaussing coil of older CRT monitors.

---

3. Summary and Comparison

Feature	NTC Thermistor	PTC Thermistor
Full Name	Negative Temperature Coefficient	Positive Temperature Coefficient
Core Behavior	Resistance decreases as temperature increases.	Resistance increases as temperature increases.
Material	Semiconductor (Metal Oxides)	Doped Polysilicon or Polymer Composite (Carbon in Polymer)
R vs. T Curve	Exponential decay (smooth, continuous)	Sharp, sudden increase at a specific point (the Curie Temperature)
Key Parameter	Beta (beta) value	Curie Temperature (T_C)
Primary Use	Temperature Sensing, Inrush Current Limiting	Overcurrent Protection (Resettable Fuse), Self-Regulating Heaters

4. General Advantages & Disadvantages of Thermistors

Advantages:

• High Sensitivity: A small change in temperature results in a large change in resistance, making them very sensitive.

• Low Cost: They are generally inexpensive to manufacture.

• Fast Response Time: Due to their small size, they can react quickly to temperature changes.

• Small Size: Can be integrated into compact electronic assemblies.

Disadvantages:

• Non-Linearity: Their resistance-temperature relationship is not linear, often requiring complex calculations or lookup tables for precise measurements.

• Limited Temperature Range: They typically operate within a narrower temperature range (e.g., -50°C to 200°C) compared to other sensors like thermocouples.

• Self-Heating: The current flowing through the thermistor to measure its resistance will dissipate power ($P=I^{2}R$), causing it to heat up. This self-heating can introduce errors in the temperature reading if not properly accounted for.

• Fragility: Ceramic-based thermistors can be more fragile than metal-based sensors like RTDs or thermocouples.