Current Transducers are the devices that converts input energy into output energy, the latter usually differing in kind but bearing a known relation to input. Originally, the term preferred to a device that converted mechanical stimuli into electrical output, but it has been broadened to include devices that sense all forms of stimuli such as radiation, heat, sound, strain, vibration, pressure, acceleration, and so on and that can produce output signals other than electrical such as hydraulic or pneumatic. Many measuring and sensing devices, as well as thermocouples, loudspeakers, microphones, and phonograph pickups, may be termed transducers.

There are hundreds of types of transducers, many of which are designated by the energy change they accomplish. For Instance, piezoelectric transducers contain a piezoelectric element that produces motion when subjected to an electrical voltage or produces electrical signals when subjected to strain. The latter effect may be applied in an accelerometer and a piezoelectric vibration pickup, or a strain gauge. An electro acoustic transducers may convert electrical signals to acoustic signals or vice versa. An example is the hydrophone, which responds to waterborne sound waves and is useful in underwater sound detection or presence. A photoelectric transducer reacts to visible light to produce energy in electrical form. Electromagnetic transducers forms a large group, the major categories of which are differential transformers, Hall-effect magnetic transducers, inductance transducers, induction transducers, and saturable reactors. These works on electromagnetic principles.


Current Transducers may be classified as passive or active. The active transducers create electric current or voltage directly in response to stimulation. An example is the thermocouple; here, the fact that a current will flow in a continuously circuit of two metals, if the two junctions are at different temperatures, is used to create electricity. The passive transducer produces a change in some passive electrical quantity, such as resistance, capacitance or inductance, as a result of stimulation. Passive transducers usually requires additional electrical energy. A simple basic example of a passive transducer is a device containing a length of wire and a moving contact touching the wire. The position of the contact discovers the effective length of the wire and, thus, the resistance offered to electric current flowing through it. This is the basic version of what is called a linear-displacement transducer, or linear potentiometer. For practical use, such transducers employ thin-film, wire-wound or printed circuits to allow for a long resistor within a relatively small device. The longer the resistor, the greater the drop in voltage passing through the equipment; thus, changes in position are converted to electrical signals.


Transducers also may produce hydraulic or pneumatic output. Pneumatic systems communicate with the help of compressed air. An example is a equipment in which motion is applied via a system of pivots to a baffle that can be moved closer to or farther away from a nozzle that emits a stream of air. The amount of resistance created by the amaze affects the amount of back pressure behind the nozzle, creating a pneumatic signal. Hydraulic systems lean to be designed similarly to pneumatic systems, except that hydraulic systems use hydraulic (liquid) pressure rather than air pressure. Fluidic principles, which apply to the communication between two fluid streams, have also been used to create transducers.


Types of Transducers

Most microphones use either an electrostatic or an electromagnetic technique to convert sound waves into electrical signals. The dynamic microphone is made with a small magnet that oscillates inside a coil attached to the diaphragm. When a sound wave affects the diaphragm of the microphone to vibrate, the relative motion of the magnet and coil creates an electrical signal by magnetic induction. Either a moving-coil or a moving-magnet system may be employed, based on which element is connected to the moving diaphragm; the moving coil is used more often. The dynamic microphone is ragged and has reasonably good linearity, so that high-quality models are very useful in recording. Because a moving-coil microphone and a moving-coil loudspeaker are very similar or related, intercoms are often made with the same element serving both functions.


The condenser or electrostatic microphone is constructed with the diaphragm as one plate of a parallel-plate capacitor. The most popular form of this kind of microphone is the electret condenser microphone, in which the plates are given a permanent electrical charge. When a sound wave aims the charged diaphragm plate to vibrate, the voltage across the plates changes, creating a signal that can be amplified and transmitted to the recording device. An amplifier is generally mounted in the microphone, so this type of microphone requires the use of a battery to power the amplifier. Because the diaphragm of a condenser microphone can be very light in weight, compared with the more massive dynamic microphone, it is able to respond faster and at higher frequencies. Therefore, condenser microphones generally have better linearity and a greater frequency range than dynamic microphones.


The crystal microphone uses a piezoelectric crystal as its transducer and piezoelectric crystals are durable and cheap, and they have relatively large electrical output; for this reason, they are generally used in telephones and portable sound systems. They do not have very good linearity and so are sketchy for quality sound recording.


The ribbon microphone is different in that it responds to the air velocity of the sound wave, not to the pressure variation. Because ribbon microphones are very sensitive and they cannot be used where they will suffer mechanical shocks. Ribbon microphones are bidirectional and can be used to pick up sounds coming from both sides of the microphone equally in very well manner.

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