Question

What type of resistor is used as a thermistor?

Answer 1

A **thermistor** is a type of resistor whose resistance changes significantly with temperature. The specific type of resistor used as a thermistor is made of a **semiconductor material**, typically ceramic or polymer composites, that exhibits a predictable change in resistance with temperature. Thermistors come in two main types:

---

### **1. Negative Temperature Coefficient (NTC) Thermistors**
- **Behavior**: In NTC thermistors, resistance **decreases** as temperature **increases**.  
  - For example, as the thermistor gets hotter, more charge carriers (like electrons) are available, causing the resistance to drop.
- **Materials**: They are typically made of metal oxides such as manganese, nickel, cobalt, or a combination of these.
- **Applications**:
  - Temperature sensing in appliances like air conditioners and refrigerators.
  - Inrush current protection in circuits, where they limit the initial surge of current when the device is powered on.

---

### **2. Positive Temperature Coefficient (PTC) Thermistors**
- **Behavior**: In PTC thermistors, resistance **increases** as temperature **increases**.  
  - This behavior occurs due to the material undergoing a phase change, reducing the ability of current to flow.
- **Materials**: They are usually made from polycrystalline ceramic materials based on barium titanate.
- **Applications**:
  - Overcurrent protection (acting like resettable fuses).
  - Self-regulating heating elements in devices such as hair dryers or de-icing systems.

---

### Key Differences Between NTC and PTC Thermistors
| **Property**        | **NTC Thermistor**        | **PTC Thermistor**        |
|---------------------|--------------------------|--------------------------|
| Resistance-Temperature Relation | Resistance decreases with temperature | Resistance increases with temperature |
| Typical Use Cases   | Temperature sensing and control | Overcurrent protection, self-regulation |
| Material            | Metal oxides             | Ceramic (barium titanate) |

---

### Summary
A thermistor is a **semiconductor-based resistor** that comes in NTC or PTC types, depending on whether resistance decreases or increases with temperature. The choice of thermistor depends on the specific application, such as sensing, protection, or heating.