Wherever a process is being monitored or controlled, you find 4 to 20 mA DC as the standard analog signal of choice. There are others - 0 to 5 V DC, for example - and the many digital signal standards. As process variables, 4 to 20 mA signals emerge from signal converters and transducers (or transmitters) representing variables such as temperature, pressure and power. They are routed to controllers, indicators, programmable logic controllers (PLCs) and distributed control systems (DCS) to be used in display, control and alarm monitoring of the process.
Transducers handling nonlinear inputs - for example, thermocouples or differential pressure flow meters - often hand the job of linearizing the signal over to the receiving device. Some models, called "smart," have digital processing circuitry that can linearize the process signal. Even smarter models are configurable, meaning that you can select and range any one of several types of process signals as the input. It is a short step from here to incorporate a digitally coded output, typically for entry into a DCS.
Compared with the low millivolt signals from thermocouples or RTDs, the 4 to 20 mA signal is robust enough to be free from electrical interference in plant-wide wiring. The signal wire is copper, not the more expensive dedicated thermocouple extension wire. Why is this signal is known as a current source? It means that the receiving circuit and wiring can have any resistance between typically 0 and 600 A without affecting the accuracy of the milliamp signal. Similarly, a voltage source remains stable for the whole range of different load resistances within its specification.
Transmitting the Signal EffectivelyLoop-Powered Transmitters. With this setup, the external DC power supply, transmitter output and receiving devices are strung in a series loop. Some 12 V of the DC supply is dropped across the transmitter as power for the internal electronic circuit. If your receiving device resistance is too high for the power supply, you can use a higher voltage DC supply, up to about 90 V.
Self-Powered Transmitters. These devices have a second pair of terminals that take the normal 115 VAC supply, from which the internal DC supply is derived. You can achieve electrical isolation among all of the transmitters on the same AC power supply as well as between inputs, outputs and ground. This setup avoids the gross, unpredictable measurement errors that connection to grounded receiving devices can cause.
Input/output isolation is important, especially when the thermocouple tip is exposed and vulnerable to touching high heater or process voltages (such as those in furnaces or duct heaters). Isolation keeps hazardous voltages off the plant signal wiring and avoids common-mode measurement errors.
As a control signal, the 4 to 20 mA signal would come from, say, the output of a temperature controller, and feed into a final control device such as an electropneumatic or motorized control valve, motor drive or silicon-controlled rectifier (SCR). Usually, a high control signal calls for high output of your final control device (direct-acting mode). Reverse acting is the other option - high milliamps will be used to close your valve rather than open it. Pause and decide which action you want. If your controller or wiring should break with loss of milliamps, choose the mode that you reckon is fail safe.
Force-Balance Principle. The 4 to 20mA signal is about the right size for use in the force-balance system found in pressure transmitters. Here, the magnetic force of the milliamps in a coil performs a null-balance against the diaphragm pressure, resulting in an accurate transduction of pressure to milliamps. The same principle is used in I/P (current-to-pressure) converters (often incorporated into electropneumatic control valves) and in valve positioners where, by servo action, the valve is forced to a position proportional to the milliamp signal.
Signal Converter. A simple example of a signal converter is the signal isolator with 4 to 20 mA in and 4 to 20 mA out. It is used to isolate an incoming, grounded signal from a receiving device that cannot tolerate a second ground reference. The input/output/ground isolation usually will withstand for about 1,500 V.
Many combinations of signal ranges are available for both inputs and outputs. Other options include multiple channels, mathematical functions and user-defined linearization.