It went totally dead after a while so my son-in-law assumed it needed a battery and replaced it to no avail. a while back I posted about the key getting stuck in the ignition and that if I disconnected the battery it would take about 20 minutes after reconnecting it before it would restart. The sensor measures a process variable, the transmitter translates that measurement into a current signal, the signal travels through a wire loop to a receiver, and the receiver displays or performs an action with that signal.More woes with the 2007 Equinox. ![]() These components are all it takes to complete a 4-20 mA current loop. Digital displays, controllers, actuators, and valves are common devices to incorporate into the loop. This device also needs to either display the information received (for monitoring purposes) or automatically do something with that information. ![]() ![]() This current signal must be translated into units that can be easily understood by operators, such as the feet of liquid in a tank or the degrees Celsius of a liquid. Receiverįinally, at someplace in the loop there will be a device which can receive and interpret the current signal. However, over long distances (greater than 1,000 feet) it can add up to a significant amount, depending on the thickness (gauge) of the wire. While the wire itself is a source of resistance that causes a voltage drop on the system, it is normally not a concern, as the voltage drop of a section of wire is minuscule. This component is typically overlooked in a current loop setup because wire is so intrinsic to any modern electronic system, but should be considered in our exploration of the fundamentals. The current signal on the loop is regulated by the transmitter according to the sensor's measurement. In addition to an adequate VDC supply, there also needs to be a loop, which refers to the actual wire connecting the sensor to the device receiving the 4-20 mA signal and then back to the transmitter. The use of improper power supplies can lead to equipment failure. When deciding on what voltage of power supply to use for your particular setup, be sure to consider that the power supply voltage must be at least 10% greater than the total voltage drop of the attached components (the transmitter, receiver and even wire). There are many common voltages that are used with 4-20 mA current loops (9, 12, 24, etc.) depending on the particular setup. Remember that the power supply must output a DC current (meaning that the current is only flowing in one direction). In order for a signal to be produced, there needs to be a source of power, just as in the water system analogy there needed to be a source of water pressure. If the tank were half full the transmitter would signal at the halfway point, or twelve milliamps. Conversely, it would translate fifty feet as the tank being full and would then transmit a twenty milliamp signal. If, for instance, a sensor was measuring the height of a fifty foot tank, the transmitter would need to translate zero feet as the tank being empty and then transmit a four milliamp signal. This is where a transmitter will come into play. Second, whatever the sensor is monitoring, there needs to be a way to convert its measurement into a current signal, between four and twenty milliamps. The technology that goes into the sensor will vary drastically depending on what exactly it is intended to measure, but this is not relevant for this discussion. A sensor typically measures temperature, humidity, flow, level or pressure. Sensorįirst, there needs to be some sort of sensor which measures a process variable. Related: Why a 0 mA Signal is Not Practicalįigure 4. Having a grasp on the history, workings, pros and cons of the 4-20 mA loop will help you to understand why it is the dominant standard for the industry and allow you to make informed decisions about your process control. ![]() Not knowing the basics could potentially cost you money when it comes time to make decisions about process display and control. Of all possible analog signals that can be used to transmit process information, the 4-20 mA loop is, by far, the dominant standard in the industry.Īs major as the 4-20 mA loop standard has become in the process control industry, many do not understand the fundamentals of its setup and use. Analog signals, where information about the process is transmitted via varying amounts of voltage or current, are the predominant type of input in industries requiring process control today. Thermocouples and RTDs provide direct temperature reading while digital signals such as Modbus ® provide exacting control over process variables and display. In the world of process control, there are a myriad of different types of process inputs.
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