What is meant by feedback in electronics?

1. **Material Choice**:

- RTDs are usually made from pure platinum, though nickel and copper are also used. Platinum is preferred due to its linear resistance-temperature relationship, chemical stability, and repeatability over a wide range of temperatures.

2. **Temperature-Resistance Relationship**:

- The electrical resistance of the RTD material increases with temperature. For platinum, the resistance increases almost linearly with temperature.

3. **Construction**:

- The RTD element can be a thin film or a wire wound around a core.

- **Thin Film RTDs**: A very thin layer of platinum is deposited on a ceramic substrate. Thin film RTDs are compact and have a fast response time.

- **Wire-Wound RTDs**: A fine platinum wire is wound around a ceramic or glass core. Wire-wound RTDs can be more accurate and stable over a wider temperature range.

4. **Operation**:

- The RTD is placed in the environment where the temperature is to be measured.

- A constant current is passed through the RTD, creating a voltage drop across it. This voltage drop is measured.

- The resistance \( R \) of the RTD is calculated using Ohm's Law: \( R = \frac{V}{I} \), where \( V \) is the voltage across the RTD and \( I \) is the current flowing through it.

5. **Temperature Calculation**:

- The resistance value obtained is then converted to temperature using a calibration curve or an equation specific to the material of the RTD. For platinum RTDs, the Callendar-Van Dusen equation or a polynomial approximation is often used to relate resistance to temperature.

- The Callendar-Van Dusen equation for platinum is:

\[

R_t = R_0 (1 + A \cdot t + B \cdot t^2 + C \cdot (t - 100) \cdot t^3) \quad \text{for} \quad t < 0^\circ \text{C}

\]

\[

R_t = R_0 (1 + A \cdot t + B \cdot t^2) \quad \text{for} \quad t \geq 0^\circ \text{C}

\]

where \( R_t \) is the resistance at temperature \( t \), \( R_0 \) is the resistance at 0Â°C (typically 100 Î© for a Pt100 RTD), and \( A \), \( B \), and \( C \) are constants specific to the platinum material.

6. **Advantages**:

- **Accuracy**: RTDs are highly accurate over a wide range of temperatures.

- **Stability**: They provide stable readings over long periods.

- **Linearity**: The resistance change with temperature is almost linear, simplifying calibration and interpretation.

7. **Disadvantages**:

- **Cost**: RTDs, especially platinum types, are more expensive than other types of temperature sensors like thermistors and thermocouples.

- **Size**: The physical size can be larger, particularly for wire-wound RTDs.

- **Fragility**: Thin film RTDs can be fragile and require careful handling.

By carefully measuring the resistance and using known calibration data, RTDs provide precise temperature measurements, making them invaluable in scientific, industrial, and laboratory applications.