Transductores 4-20Ma Alimentados Por Lazo

Whitepaper: INTRODUCTION TO THE TWO-WIRE TRANSMITTER AND THE 4-20MA CURRENT LOOP

Background First appearing in the 1950’s with the advent of electrical and electronic controls, the 4-20mA signal standard reigns as one of the most popular mediums for signal transmission and electronic control in industrial environments nearly 60 years later. Prior to the widespread adoption of electrical andelectronic controls, buildings often used pneumatic control systems. Large and powerful compressors drove 3psi to 15psi pneumatic signals throughout a plant and these pneumatic lines connected to pneumatically controlled valves and pneumatically controlling valves in order to drive proportional controls and actuators throughout the building, all powered from compressed air. Air pressure at 3psiserved as the “live-zero” and 15psi represented 100%. In this way, the more modern 4-20mA signal standard emulated the earlier 3-15psi pneumatic controls. Any pressure below 3psi was considered “dead zero” and an alarm condition. Some installations still use pneumatic control today. Modern I/P converters (current-to-pressure transducers) are available to convert the 4-20mA control loops to commonpneumatic ranges, such as 3-15psi, 118psi, 3-27psi, and 6-30psi. In two-wire 4-20mA control loops, we use 2-wire transmitters to convert various process signals representing flow, speed, position, level, temperature, pressure, strain, pH, etc., to 4-20mA DC for the purpose of transmitting the signal over some distance with little or no loss of signal. This paper reviews the operation of thistransmission standard and its advantages, in particular as it relates to two-wire transmitters and the associated 4-20mA current loop. The Basics of 4-20mA Current Loops To help understand new concepts, I like to simplify them with easy to remember relationships. For the two-wire 420mA current loop, I use a simple triangle as follows:

+ T I

R
Figure 1: Symbolic 2-Wire Current Loop
From thetriangle, I can quickly identify the three common components of a current loop, how they are wired together, and even the direction of current flow. For my mental model, each side of the triangle represents a component of the current loop. The vertices of the triangle represent a wired connection between these components. For reference, I also place a positive/plus sign on the “peak” of the triangle. Iwill also use the first three letters of “TRIangle” to identify the principle components. Accepting the convention that current flow will move from the supply positive to and return to the supply negative, I see that current moves counter-clockwise in my symbolic current loop.

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Whitepaper:INTRODUCTION TO THE TWO-WIRE TRANSMITTER AND THE 4-20MA CURRENT LOOP __________________________________________________________________________________________

+ I T + R
Figure 2A: 2-Wire Current Loop With Component Substitutions

+ V + I

+ -

Figure 2B: Traditional 2-Wire Current Loop w/ Earth Ground

The left side is labeled “T” and refers to “Transmitter”. The base of the triangle is labeled“R” and refers to “Receiver”. The right side of the triangle is labeled “I” and refers to the source of current or the power supply. The vertices refer to the wired connections between these elements. Each element shares two connections to the loop. The “+” label at the peak shows that the positive side of the power supply connects to the positive side of the transmitter. Using the convention ofcurrent flow from positive to negative, the direction of current flow from the power supply is shown counter-clockwise. You should also note that the Transmitter is not the source of current, but simply regulates the flow and magnitude of the current through it. The current is sourced by the power supply, flows in controlled fashion through the transmitter, then into the receiver, and returns to...