Understanding Active Transducers

Transducers are devices that convert one form of energy into another. In the context of measurement systems, they convert a physical quantity (like temperature, pressure, strain, light) into an electrical signal.

Transducers can be broadly classified into two types based on whether they require an external power source to operate:

• Active Transducers: These transducers generate their own electrical signal in response to the physical quantity being measured. They do not need an external power supply. They work on the principle of energy conversion from the physical phenomenon itself. Examples include devices based on piezoelectric effect, thermoelectric effect, or photovoltaic effect.
• Passive Transducers: These transducers require an external power source to operate. They change one of their electrical properties (like resistance, capacitance, inductance) in response to the physical quantity. This change in electrical property is then converted into an electrical signal using an external circuit (like a bridge circuit).

Analyzing the Given Transducers

Let's examine each transducer listed in the question to determine if it is active or passive:

1. LVDT (Linear Variable Differential Transformer): This device measures linear displacement. It requires an AC excitation voltage to produce an output voltage that is proportional to the displacement. Since it needs an external power source (the AC excitation), it is a passive transducer.
2. Piezoelectric Transducer: This transducer generates an electrical charge or voltage when mechanical stress (pressure or force) is applied to certain crystals. It converts mechanical energy directly into electrical energy. It does not require an external power source, making it an active transducer.
3. Thermocouple: This device measures temperature. It works based on the Seebeck effect, where a voltage is produced across the junctions of two different metals when there is a temperature difference between the junctions. It converts thermal energy directly into electrical energy and does not need external power, so it is an active transducer.
4. Photovoltaic cell: This transducer converts light energy directly into electrical energy (voltage or current). It works based on the photovoltaic effect. Solar cells are a common example. It generates its own electrical output from light, making it an active transducer.
5. Strain gauge: This device measures strain (deformation). It consists of a resistive element whose resistance changes when it is stretched or compressed. To measure this change in resistance and convert it into a voltage signal, it needs to be connected to an external circuit, typically a Wheatstone bridge, which requires a voltage source. Therefore, it is a passive transducer.

Identifying the Active Transducers from the List

Based on the analysis above, the active transducers from the given list are:

• Piezoelectric transducer (2)
• Thermocouple (3)
• Photovoltaic cell (4)

The transducers 2, 3, and 4 are active transducers because they generate their own electrical signal without needing an external power supply.