Type numbers with suffix letters are used as transmitter identifiers. Type number is the number of wires necessary to provide transmitter power. Shields and IO wires are excluded. Suffix letters identify the load resistance capability. Figure 4 illustrates three basic transmitter types.
Type 2 is a 2-wire transmitter energized by the loop current where the loop source voltage compliance is included in the receiver. The transmitter floats and signal ground is in the receiver. Type 3 is a 3-wire transmitter energized by a supply voltage at the transmitter.
The transmitter sources the loop current. Transmitter common is connected to receiver common. Type 4 is a 4-wire transmitter energized by a supply voltage at the transmitter. The transmitter sources the loop current to a floating receiver load.
If a transmitter has field inputs, which provide signals referenced to field grounds potential ground loops exist. This potentially will cause signal gradation. Table 1 shows transmitter class designations. It is necessary to understand that all mA transmitters may not necessarily be identical in their ability to provide current into different loads. For example, a typical mA transmitter module could not drive a k-ohm load.
The class standard ensures that modules of identical classes are interchangeable with respect to their drive capabilities. It is noteworthy to mention here that one should always completely examine all module specifications before replacing units.
Practical mA Circuit Field inputs are usually referenced to field grounds or in some cases actually connected to a field ground for example, the grounded thermocouple. Receiver grounds are rarely identical to field grounds; therefore, isolation is required to eliminate potential ground loop problems. Very long cables have resistance, that causes a voltage drop.
The drop in voltage affects the reading. The advantage of using a mA signal is that Current signals do not have this problem. The current can induce a magnetic field and cross talk to parallel lines. One should use twisted wire and keep it distant from other communication lines. Skip to content Any process parameter such as Temperature, Pressure, Flow, Density is sensed, measured by a sensor and is converted to an electrical signal in the form of voltage.
Most of the applications use mA signal instead of mA or V, or V. Calculate percentage of signal corresponding to mA signal Measured Value -.
The disadvantages of using mA for an active sensor input loop are that current loops can transmit only one process signal, which requires multiple loops when there are numerous process variables that need to be transmitted. Furthermore, using multiple loops can lead to ground loops problems if the individual loops are not isolated.
One other advantage of mA current loops is safety. Also, the mA current loop is intrinsically safe for hazardous areas that may include dangerous levels of dust or vapor because the low power consumption does not cause combustion if normal operating or fault conditions are in play. Transmitter: The transmitter amplifies and conditions the sensor output, then converts the voltage to a direct current level in a range of mA that circulates in series through a closed loop.
The output on the transmitter is current and is proportional to the physical variable. The loop power supply: usually provides any energy requirement to both the transmitter and the receiver. A source of 24 Volt is widely used in mA monitoring applications. As well There are 12 volt power supply for computers. The mA transmitter is a current loop which simplified with an ideal Norton current source composed of Signal, and R signal models the mA transmitter.
The line resistance is shown as Rline, and Vnoise represents randomly induced loop noise. In this example, a ohm controller and a ohm digital display are connected in series with the signal current.
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