UNDERSTANDING 4-20mA

Acronym Legend:

  • mA- Milliamp
  • PID- Proportional Integral Derivative
  • SCADA- Supervisory Control and Data Acquisition
  • PLC- Programmable Logic Controller
  • DC- Direct Current
  • AC- Alternating Current
  • HMI- Human-Machine Interface
  • GPH- Gallons per hour
  • pH- power of Hydrogen
  • ORP- Oxidation-Reduction Potential

 

 

4-20 mA is a DC loop that is the standard electric process control signal in many industrial applications where the loops are used to control and carry the signal from field instruments to PID controllers, SCADA systems and PLC cabinets, respectively. It is the ideal method of transferring process information because current does not change as it travels from transmitter to receiver, much like water in your home; the flow is constant. Over the years there have been other loops used for applications such as 4-10mA because it corresponded to 1-5V analog voltage across a 250 Ohm resistor, making it easy to adapt the 4-20mA current loop to a 1-5VDC analog input voltage as well. Older times say 10-50mA because technology at the time used magnetic amplifiers which required a minimum of 10mA to operate. Current is only restricted by resistance as water is only restricted by pipe bends and size reducing, but the flow is still constant, the drop is in pressure for water, and voltage, for electricity, which is why current loop is recommended over voltage analog. This is why using current as a means of conveying process information is so reliable, it is kept low and sufficient so there is rarely a significant drop.

Prior to the popularity of electric controls, businesses were using pneumatic control systems with compressors driving 3psi to 15psi of pneumatic signal throughout the plants where 3 psi represented a “live-zero” and 15 psi represented 100%. The 4-20mA signal standard is emulated from the pneumatic 3-15psi standard and both are used to this day.

The reason for the 4mA is because it should be enough to drive 2 wire devices and since semiconductor devices require 3mA to operate, the standard had to be above 3mA to supply power before actually reading or regulating. Besides transmitters needing minimal to do the job, if 0 was used instead of 4mA, we could not differentiate the actual 0 value of the sensor or the problem of the transmission of the signal, such as if a cable was cut or complete power loss from provider.

The reason 20mA was used as the maximum is stated to be because the human heart can withstand up to 30mA of current, so as a safety point of view, 20mA was chosen.

With a span of 3-30 and wanting to stay linear with base value, the only options present were 4-20 or 5-25. Since calculations are easier in multiples of 2, 4-20mA had more votes, thus 4-20mA is what we get.

Those numbers represent the real value, in percentages, of the process once you have a minimum and maximum established. Here is a breakdown of what those numbers mean in real value;

 

4-20mA;

4mA=0%, 8mA=25%, 12mA=50%, 16mA=75%, 20mA=100%

Likewise for the pneumatic 3-15psi;

3psi=0%, 6psi=25%, 9psi=50%, 12psi=75%, 15psi=100%

 

The steps and requirements necessary to make a 4-20mA loop are;

Power Source

The most common DC power source for 4-20mA is 24V but it has been used with 12V, 15V and even 36V since some older systems used higher voltage. The reason DC is used over AC is the magnitude of the current, with DC being constant and AC continuously changing, making it difficult for the signal level to transmit. With that in mind, the power supply must be greater than the sum of the minimum voltage required to operate the transmitter, plus the IR drop of the Receiver, and in the event of long transmission runs, the IR drop in the wire. When calculating that drop, consider the maximum level of the current that can flow through the 4-20mA loop, not just your 20mA value, but the over-scale or alarm limit of the transmitter.

 

Sensor

The device used for measuring the process variable; commonly being temperature, humidity, valve position, motor speed, flow, PH, ORP, Ammonia, level, and/or pressure

 

Transmitter

This is the key for transmitting the 4-20mA signal from the sensor to the controller, conveying the real world signal of flow, speed, pressure, etc. into a control signal necessary to regulate the flow of current in the loop. It takes that signal the sensor gives it and converts it to the 4-20mA source that the controller can understand; 4mA being the 0 to 20mA being the 100% of what the scaling/span is.

 

Receiver/Controller

This device is at the other end of the transmission line, receiving that transmitted signal. This unit itself can be any number of different devices, such as; panel meter, PLC, motor speed control, or some other digital control system like SCADA or BACTALK that you span to the process; 0-X feet, 0-X degrees, 0-X gallons per hour, etc. “X” being whatever you program as the maximum capacity that your instrument will read. It reads the output that the transmitter gives it and either displays it itself in a simple form so operators can say or it stores the info in a database for trends, for example; if you have a tank that is 30 feet and you set a span of 0-30, the sensor sends a signal which the transmitter converts to 12mA, the control system will reflect 15 feet since the 12mA is 50% of the set-point. It will either show this level on a display of some sort or add it to a software trend-log for future reference.

To go over those 4 steps in an easy loop process you got;

Power Supply à Controller à Sensor à Transmitter à Controller

That’s it!

 

Here are some examples of uses;

  • You have a powered local controller spanned 0-300 GPH for a flowmeter, the flow-meter is transmitting 12mA back to the controller giving it a reading of 150 GPH, if that controller has an output source to an HMI, that will reflect the same value.
  • You have a tank that is 40 feet deep so you span your controller for 0-40’ and place your transducer on it, it reads 10’, your controller is receiving an 8mA signal transmitted from the sensor.

 

4-20mA isn’t limited in just reading, it regulates too;

  • If you have a motor and want to regulate the speed which is an RPM of 1750, and you wanted to bring it down to 875, you would set your current output to 50%, which would reflect 12mA going the opposite way.
  • If you have an HVAC system linked with a span of 0-100 degrees and your thermostat is set at 71 degrees, it would show 15.36 mA, if you adjust it to 78 degrees, it would then send a 16.48 mA signal.

 

 

 

 

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