Loop powered devices are modules and computer boards which are powered by the system they serve. They derive their power from the input current loop instead of from an external or additional power source. Loop powered devices have two input connections: a positive (+) input where the current loop enters and a negative (-) input where the current loop exits. Typically, loop powered devices such as sensors, transducers, transmitters, isolators, and indicators use a 1-5 mA, 4-20 mA or 10-50 mA current loop, in which a single, twisted-pair cable powers the module and carries the output signal. Outputs are sent to a personal computer (PC), programmable logic controllers (PLC), or digital cross-connect switch (DCS). With 4-20 mA process-monitoring applications, the most common supply voltage is +24 V. Lower supply voltages such as +12 V are used mainly in computer-based systems.
There are many different types of loop powered devices. Examples include sensors, transducers, transmitters, isolators, and indicators. Loop powered sensors produce an electrical signal that is proportional to a physical variable such as pressure, proximity, or linear position. Loop powered transducers convert current or voltage input signals proportionally, to a variable such as output pressure or temperature. Loop powered transmitters are measurement or signal conditioning packages that provide a standard, calibrated output from a sensor or transducer in the form of a current loop output, typically 4-20 mA or 10-50 mA. Signal isolators are used to connect a grounded signal source to a grounded signal follower. Signal indicators provide an audible or visual indication of an industrial process. Other types of loop powered devices are also commonly available.
Loop powered devices differ in terms of certifications and approvals. These include but are not limited to the CE Mark (Europe), FCC (US), VCCI (Japan), DENAN (Japan), TUV-GS (Germany), EZU (Czechoslovakia), RLL (Korea), PCBC (Poland), C-TICK (Australia), PSB (Singapore), IRAM (Argentina), NOM (Mexico), SABS (South Africa), GOST (Russia), T Mark (Taiwan), BSMI (Taiwan), and CSA (Canada). Loop powered devices that meet U.S. military specifications (MIL-SPEC) or adhere to safety standards from Underwriters Laboratories (UL) are also available.