There can be only one transmitter in a current loop. Many 4-wire devices can be powered from either a dc or ac supply. Both loop connections float off ground, which minimizes ground loop problems in systems with multiple loops or distant receivers. In most installations, one side of the power supply is tied to ground or earth. To avoid adding additional devices to the system, the 3-wire transmitter can be replaced with a 4-wire unit, which has isolation built-in.Ī 4-wire transmitter has 2 connections to power the unit separately from the loop. This device accepts a 4-20 mA signal, functions as a repeater or re-transmitter, and delivers a reconstituted 4-20 mA current loop signal that is fully isolated. Another method to provide separation is to add a loop isolator. If an application using a 3-wire transmitter requires isolation in the loop, a separate dc power supply can be installed for each transmitter. The shared connection in a 3-wire transmitter requires careful consideration of grounding issues to prevent potential ground loops. In many installations, this terminal is wired to a system ground or earth. The low side of the supply is connected to the common terminal, which is also part of the current loop. However, these units need additional wiring compared to 2-wire devices, so the configuration is more complex and the installation cost is typically higher.Ī 3-wire transmitter has one additional connection for an external dc supply. The availability of extra power means these units can offer extended capabilities and additional features, such as higher sensor excitation, LED display, relay outputs and digital communications. This eliminates the need for a separate loop supply. At some point below 4 mA loop current, the device will stop operating.īoth 3- and 4-wire devices are powered from an external supply. If a loop-powered device has a display, it will typically be an LCD. It may lack sufficient power to drive certain types of sensors. Conversely, limited power places constraints on the transmitter’s capabilities. This is helpful when operating in hazardous locations where intrinsic safety is needed. However, the amount of power available is very limited. This loop-powered instrument is an ideal solution when external power is not accessible. A 2-wire transmitter obtains its operating power from the current loop. Transmitters are classified as 2-wire, 3-wire, or 4-wire. It converts a physical or electrical parameter into a 4-20 mA signal, where 4 mA represents the zero value and 20 mA is full scale. The transmitter is the device that controls the current circulating in the loop. Three devices are needed to make a 4-20 mA current loop – a transmitter, receiver and loop power supply. Home » News & Blog » 4-20 mA Loop Instrumentation
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