Current Transducers

Transducers convert electrical energy into other forms of energy, as in the case of audio loudspeakers, or from other forms of energy into electrical signals, such as with pressure transducers. Current transducer is the device used to convert alternating current (AC) or direct current (DC) electrical current signals into analogue instrumentation signals used in industrial control systems. Here they would 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 analogue process value, or a 3-15 psi pneumatic signal for pneumatic control systems.

Relevant to the controls industry, the term is also used to define specific conversion devices. These devices change 4-20 mA DC analogue process signals into 3-15 psi analogue pneumatic signals to actuate proportional pneumatic control valves, or convert pressure signals to proportional 4-20 mA DC analogue 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.

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 to current transducers.

There were also hundreds of pneumatically actuated control valves in these plants which had to receive their output instructions from the control computer, which sent these signals as 4-20 mA DC analogue 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.

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 transducers 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.

Current transducer can also describe a type of sensor that measures the magnetic flux of a power conductor to sense drive motor currents for machinery and process equipment and transmits an analogue milliamp or voltage signal to control systems. Solid core current transducers have closed loop ring transformers that must be slipped over the temporarily open end of a power conductor. Split core current transducers 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 analogue control systems.