Active transducers are self generating transducers taht do not require external power source to work. They are of four types.
Thermocouple & thermopile
Principle: An emf is generated across the junction of two dissimilar materials ( metals or semiconductors) when that junction is heated. Application : Temperature,heat flow, radiation
Moving coil generator
Principle: Coil moving in a magnetic field generates voltage Application : Velocity, vibration
Piezoelectric pickup
Principle: External force acting on certain crystals called piezoelectric crystals (Eg. Quartz) can generate a emf . Application: Pressure change, Sound, vibration, acceleration
Photovoltaic
Principle: Incident light stimulates semiconducting materials, constituting a cell, to generate voltage across their junctions. Application : Solar cell, light meter.
Disadvantages of active transducers
Rather than being disadvantage, these are the differences in active and passive transducers in terms of output, that can be exploited for a completely different set of applications.
- Low Resolution,
- Low output amplitude.
Mostly used in devices that use very low voltages, read microelectronics, for working.
Comaprison
| Active Transducers | Passive transducers |
|---|---|
| Do not require external power supply | Requires an external power supply |
| Compact systems | Bulkier to accommodate the power supply |
| Outputs are current or voltage generated proportional to the input | Physical inputs are mostly converted to resistance, capacitance or inductance. |
| The signal conversion is simple as output is directly measurable | Complex signal conversion as additional conversions are needed for measurable voltage or current |
| Simple design | Complex design |
| Resolution is less, A large input is to be measured for very less change in output | High resolution, slight change in input is translated to the output |
| Feeble output would need amplification | Amplification is not always necessary |