Senales 4-20 ma

A 4-20mA Signal Goes The Distance

A 4-20 mA Signal Goes The Distance
by George Tsakir

A 4-20 mA control signal is one of the signals of choice in the Building Automation industry as well as inthe Industrial Process arena. Most of the various transmitters and output devices available from Kele utilize a 4-20 mA input or output signal. In large commercial buildings, expansive factories,multi-building university campuses and other large projects, it is not unusual to find an application where a transmitter is located a long distance from the nearest building automation controller. Weare frequently asked, "How far can 4-20 mA signal wires be run?". The answer is quite amazing. A 4-20 mA current loop consists of a number of devices wired in a series loop. These devices include apower supply, a sensor/transmitter and one or more loads such as a building automation input, a chart recorder, an alarm module or a digital display. Depending on the type of transmitter used, the powersupply may either power the transmitter external to the loop or the power supply may be one of the devices in the series loop as shown in Figure 1.

The transmitter is a 4-20 mA current source thatrequires a DC voltage supply to operate. The transmitter in Figure 1 is a model PTX1 pressure transmitter. Reviewing the PTX1's specifications in the Kele catalog shows that it can operate with a powersupply voltage from 10 to 30 VDC. However, the actual power supply voltage used will directly affect the output capability of the transmitter. To provide its full output, the transmitter must becapable of supporting the total resistance of all loads that are wired in the loop. Again looking to the specifications of the PTX1, there is a graph (Figure 2) that indicates the output load capabilitiesof the PTX1 for supply voltages from 10 to 30 VDC.

http://www.kele.com/tech/Electric/TRefCLoop2.html (1 of 3) [11/03/2008 05:36:57 p.m.]

A 4-20mA Signal Goes The Distance

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