A current transducer is a device that coverts alternating or direct electrical current signals into analog instrumentation signals so that they can be used and readily interpreted by certain industrial control systems. Most of the time they don’t look like much from the outside; they’re usually square or rectangular in shape, and might resemble a portable speaker with jacks for input and output cables. Their function is really important to a range of different mechanical operations, though. Electricity is essential for running many types of machinery and high-powered processes in manufacturing and other industrial settings. Simply plugging machines into outlets isn’t always the best solution, though, and this is where transducers come in. The device takes the raw energy from the source and converts it into exactly the ratings required by the machine on the input side. The engineering behind how this process happens can be quite complex, but the simple fact of its operation is the most important aspect in most cases.
In general, the term “transducer” is used for any device or mechanism that converts electrical energy into other forms of energy. Transducers are commonly a part of audio loudspeakers, for instance, which convert electrical energy into audio energy; in pressure transducers, they convert into electrical signaling. The combined term “current transducer,” however, is usually used exclusively in settings that require the conversion of alternating current (AC) or direct current (DC) electrical current signals into an analog signal base. They convert the measured AC or DC current value, perhaps the power supply to a motor or pump, into either a 4-20 milliamp (mA) DC analog process value, or a 3-15 psi pneumatic signal for pneumatic control systems. These devices usually appear to be running on normal electricity, but the transducer is regulating the energy and the output.
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Current transducers came into widespread use during the 1970s and 80s as large process plants, such as petroleum refineries, were being computerized. Most had been controlled by pneumatic control systems to that point, due to the intrinsic safety of pneumatics in flammable and explosive environments. In order for the computers to use the real time information coming from the many pneumatic process transmitters measuring temperatures, pressures, flows, levels, and other variables, their pneumatic outputs had to be converted to electronic signals using pressure.
There were also hundreds of pneumatically actuated control valves in these plants which had to receive their output instructions from the control computer. In most cases those computers sent signals as 4-20 mA DC analog current signals. This required a current to pneumatic conversion. The industry soon termed these I to P or I/P transducers, with “I” denoting the current input and “P” signifying the pneumatic output.
Importance of Magnetic Fields
Most I/P transducers convert their electronic signals to pneumatic by passing the current through a wound coil in a magnetic field, which modulates a backpressure nozzle in a pneumatic pilot circuit that provides the 3-15 psi pneumatic output. Pressure to current devices employ pressure diaphragms that are mechanically linked to excited strain gages, piezo sensors, or capacitance sensors that drive an output amplifier section. These then transmit the required 4-20 mA DC current output to control systems.
Relevant to the controls industry, the term is also used to define specific conversion devices. These devices change 4-20 mA DC analog process signals into 3-15 psi analog pneumatic signals to actuate proportional pneumatic control valves or to convert pressure signals to proportional 4-20 mA DC analog process signals. These measurements are typically used to describe a process operating normally. By converting the actual signals from an instrument to these standardized ranges, measurements falling outside these ranges can be noted and used in diagnostics.
Certain types of sensor, usually those that measures the magnetic flux of a power conductor to sense drive motor currents for machinery and process equipment, can also carry the current transducer name. These are usually concerned with transmitting an analog milliamp or voltage signal to control systems.
Solid core transducers have closed loop ring transformers that must be slipped over the temporarily open end of a power conductor. Split core transducers, on the other hand, have a hinged side of the transformer ring that can be temporarily opened to allow the ring to be slipped around a conductor that cannot be disconnected. They usually incorporate rectifier and adjustable output conditioning circuitry to allow specific calibrations for analog control systems.