What is transducer? How it is used in electronic circuits?

Transducer is a device which converts one form of energy into another form. It is also defined as a device that converts a non electrical quantity into proportional electrical quantity.

For example –

An electric generator converts mechanical energy into equivalent electrical energy. A solar cell used in calculators converts light energy into equivalent electrical energy. A pencil cell converts chemical energy into equivalent electrical energy.

Active transducer: it is a device, which does not require external electrical power. It is a self generating transducer. For example à Solar cell, piezo electric crystal etc.

Passive transducer: it is a device which requires external electrical power like battery. It cannot generate its own voltage or current. It only changes its resistance or capacitance etc. during conversion. For example à LDR, thermistor etc.

Classifications of transducers – broadly the transducers are classified into two main types: active transducers and passive transducers. The active transducer generates its own electrical voltage during conversion. Thus it does not require any battery supply for conversion (e.g. solar cell, thermocouple etc.). In passive transducer, it requires external battery supply. It only changes its parameter during conversion like change in resistance or capacitance etc. (e.g. LDR, thermistor etc.)

Active transducers –

Definition – an active transducer is defined as a transducer which generates its own electrical voltage during conversion. It does not require any external battery supply for its working.

Examples –

1. Solar cell – when it is exposed to strong sunlight or any other light, it converts light energy into proportional DC voltage.
2. Piezo electric crystal – when it is subjected to changing pressure it produces proportional AC voltage.

Passive transducers –

Definition – passive transducer is defined as the transducer which requires external battery voltage to operate. Also it only changes its parameter like change in resistance or capacitance during conversion.

Examples –

1. LDR (Light Dependent Resistor) – when LDR is exposed to light, its resistance decreases (less than 10W) proportionally & when it is dark its resistance is very high (several MW).
2. Thermistor – when thermistor is exposed to heat its resistance decreases and when it is cooled its resistance increases.

According to working principle of transducers they are classified into four main types –

1. Mechanical transducers – for example strain gauge, LVDT etc.
2. Thermal transducers – for example thermistor, thermocouple etc.
3. Magnetic transducers – for example search coil etc.
4. Radiation transducers – for example solar cell, photo diode etc.

Following factors must be considered while selecting transducer for a particular work or system –

1. Physical quantity to be measured must be considered for –
2. The type of physical quantity whether it is electrical quantity (AC or DC) or non-electrical quantity (pressure, intensity, displacement, speed, heat etc.)
3. Range of quantity like pressure (0–10N), intensity (0–250L), temperature (–10°C to 200° C) etc.
4. The principle of transducer must be considered for –
5. The system and transducer must be compatible i.e. the output characteristics of transducer and input characteristics of the system and must match.
6. This means that principle of maximum power transfer theorem must be satisfied.
7. The measurement accuracy of the transducer must be considered which depends on –
8. Type and range of quantity under measurement.
9. Physical conditions like mechanical and electrical connections, mounting style of transducer.
10. Surrounding conditions like nonlinearity effect and frequency response etc.
11. Environmental conditions like temperature effects, shocks or vibrations etc.
12. Compatibility of some associated equipment’s like zero balancing provision, sensitivity tolerance, impedance matching etc.

Instrumentation system – this is special type of measurement system which uses highly accurate measurement techniques. Such system has three major parts –

1. Input circuit – it receives the quantity under measurement. It then produces proportional electrical signals to the processing unit.
2. Processing circuit – it receives proportional electrical signals from input circuit. It amplify, filter or modify the signals in such a form that it is compatible with output circuit.
3. Output circuit – it receives the processed signals from processing circuit and connects it to a simple indicating meter, CRO, magnetic tape recorder or even computer.
4. Necessity of transducer – transducers are used in instrumentation system to provide a medium to interface with electronic circuits with physical (i.e. non electrical quantities).

Types of transducers –

• Temperature transducers – this transducer converts heat energy into its equivalent electrical energy. They are of two types –
• Active temperature transducers – thermo-couple which converts heat energy into equivalent electrical voltage.
• Passive temperature transducers – thermistor or resistance thermometer is a passive transducer. It only changes its resistance due to change in temperature.
• Pressure transducers – these are of two types: the stress and strain types. When either stress or strain is applied, they produce a proportional electrical voltage.
• Active pressure transducers – piezo electric crystal is a good example of active pressure transducer. It produces proportional electrical voltage when pressure is applied on it.
• Passive pressure transducers – strain gauge, capacitive transducer. When either stress or strain is applied, their passive parameter like resistance or capacitance proportionally changes.
• Light transducers – it converts light energy into equivalent electrical energy. There are two types of light transducers –
• Active light transducers – in this photo–voltaic cell, photo multiplier tubes (made up within vacuum tubes) and solar cells (made up of semiconductor material) are used. They convert light into electrical energy.
• Passive light transducer – this contains LDR – light dependent resistor. Its resistance changes as light on it changes.
• Sound transducers – it converts sound energy into equivalent electrical energy and vice versa.
• Active sound transducer – carbon microphone is good examples of active transducer. It converts sound into proportional AC voltage. This happens because carbon granules in it vibrate and produce proportional voltage across two dissimilar metal plates.
• Passive sound transducers – capacitive microphone is passive transducer. Its capacity (C) changes proportionally due to change in sound intensity.