hello everyone welcome to automation community in this video we are going to learn about the 4 to 20 milliamperes current signals after completing the video you should be able to understand the basics and the history of 4 to 20 milliamperes current signals the learning objectives of this video are industrial history why we use 4 milliamperes to 20 milliamperes as a standard signal advantages and disadvantages in the olden days industries were equipped fully with mechanical operated devices like pneumatic sensors and controllers pneumatic means pressure pneumatic devices use pressure as a medium for the measurement and to control applications in the industries the pneumatic devices generate an output signal in the range of 3 to 15 pounds per square inch the 3 pounds per square inch represents 0 of the measured process variable the 15 pounds per square inch represents 100 of the measured process variable pneumatic devices are bulkier need compressed air for operation higher maintenance costs and takes more time for troubleshooting and repair in the 1950s with the evolution of electronics the analog transmitters started replacing the pneumatic transmitters the analog transmitters generate an output signal in the range of 4 to 20 milliamperes the 4 to 20 milliamperes signals solve many issues provides simple installation and configuration of field instruments during the 1970s the evolution of digital systems started with the introduction of programmable logic controllers plc now let's discuss a pressure transmitter with 4 to 20 milliamperes current output assume the pressure transmitter is configured with a range of 0 to 10 bar 4 milliamperes represents the zero percent process variable and 20 milliamperes represents the 100 process variable in this example zero percent of the process variable is zero bar and one hundred percent of the process variable is ten bar consider a simple plc example in which a pressure transmitter is connected to the plc the pressure transmitter measures the process pressure and generates an output signal in the range of 4 to 20 milliamperes the plc is also programmed similar to pressure transmitter with the same range that is 0 to 10 bar the plc calculates the received milliamperes into the equivalent pressure and displays it to the operator for simple understanding apply the process pressure in terms of zero percent 25 percent 50 75 and 100 to the pressure transmitter if the process pressure is 0 that is 0 bar then the pressure transmitter sends 4 milliamperes to the plc and it displays zero bar pressure to the operator if the process pressure is 25 percent that is 2.5 bar then the pressure transmitter sends 8 milliamperes to the plc and it displays 2.5 bar pressure to the operator if the process pressure is 50 percent that is 5 bar then the pressure transmitter sends 12 milliamperes to the plc and it displays 5 bar pressure to the operator if the process pressure is 75 percent that is 7.5 bar then the pressure transmitter sends 16 milliamperes to the plc and it displays 7.5 bar pressure to the operator if the process pressure is 100 percent that is 10 bar then the pressure transmitter sends 20 milliamperes to the plc and it displays 10 bar pressure to the operator let's discuss why we do not use zero to 20 milliamperes as a standard signal it has a dead zero problem what is dead zero let's discuss this with a simple example take a pressure transmitter with zero to 20 milli amperes output connected to a plc the transmitter is configured with zero to 10 bar pressure transmitters sends zero milliamperes to the plc when process pressure is zero bar and sends 20 milliamperes when the process pressure is 10 bar case 1. assume the process pressure is zero bar so the pressure transmitter sends a zero milli amperes signal to the p-l-c p-l-c displays zero bar pressure to the operator case 2. let's consider a fault in the pressure transmitter loop the fault may be a broken wire faulty transmitter noise high wire resistance or any other problems for our discussion let's assume that the wire is broken between plc and transmitter consider the process pressure is 5 bar because of the broken wire the plc has 0 milliamperes at its input instead of 12 milliamperes the plc thinks that it receives 0 milliamperes and displays zero bar pressure to the operator but in reality it has to be five bar pressure this is the dead zero problem if the transmitter or loop is dead due to any reason the plc shows faulty readings if the current signal range started from 0 milliamperes then it is not possible to differentiate the real process variable and the faulty reading remember we are discussing the olden days plcs which do not have features like diagnostic circuits fault identification etc how to solve this dead zero problem the solution is simple start the current signal range with some value instead of zero milliamperes so we choose 4 milliamperes as the starting value now let's discuss it with a simple example consider the same pressure transmitter example with 4 to 20 milliamperes output current case 1. assume the process pressure is zero bar so the pressure transmitter sends a 4 milliamperes signal to the plc plc displays zero bar pressure to the operator case 2. let's consider wire is broken between plc and transmitter consider the process pressure is 5 bar because of the broken wire the plc receives 0 milliamperes and identifies it as a faulty signal how plc identifies the fault here with 4 to 20 milliamperes current signal the 4 milliamperes is the minimum current signal which represents 0 of the process variable for any faults plc receives zero milliamperes so that faulty signals would be easily identified by the plc this advantage is called live zero we discussed the disadvantage of using zero milliamperes and the advantage of using 4 milliamperes now you must have a question why 4 milliamperes why not 5 milliamperes why not 6 milliamperes why not 8 milliamperes the first reason is in the olden days the power consumption of analog devices was pretty high the analog transmitters require a minimum of 3 milliamperes current for their operation so we have to consider the starting range from above the 3 milliamperes the second reason is 20 percent bias the engineers followed the 20 bias as one of the factors for selecting the 4 milliamperes calculate the 20 bias for the standard 3 to 15 pounds per square inch pneumatic signal 20 percent of 15 pounds per square inch is 3 pounds per square inch so engineers followed the same rule for the current signal 20 percent of 20 milliamperes is 4 milliamperes this 20 bias rule helps the engineers during the transition from pneumatic devices to analog devices and reduces the design complexity of devices like pneumatic to current converters and vice versa now you may have a question why 20 milliamperes why not 25 milliamperes why not 30 milliamperes the reason is in olden days 30 milliamperes is considered as a dangerous threshold limit for the human heart so we have to select a value below the 30 milliamperes a ratio of 1 to 5. this is one of the major reasons for the selection of 4 to 20 milliamperes current as a standard signal range we have a very big advantage with a ratio of one to five the industrial transition from pneumatic devices to analog devices is not simple every industry was using pneumatic devices and were operated with three to 15 pounds per square inch signals we need to design the analog equipment to handle the existing 3 to 15 pounds per square inch signals and eventually replace the pneumatic devices with analog devices wherever possible so design engineers decided to use a ratio of 1 to 5 which helps them to design the equipment with better accuracy and linearity initially for a short period of time they had chosen 10 to 50 milliamperes as the signal range this range is using 10 milliamperes to represent 0 process variable and 50 milliamperes to represent 100 process variable as discussed previously the higher signal range must be below 30 milliamperes so eventually they stopped using the 10 to 50 milliamperes range next they had chosen 4 to 20 milliamperes current as standard signal range it has an advantage of ratio 1 to 5 4 milliamperes higher than the analog transmitter minimum current 20 milliamperes is lower than the dangerous threshold limit of 30 milliamperes this range simplifies the design one of the main advantages of using 4 to 20 milliampere signals is it simplifies the signal conversions from current to voltage a simple resistor is sufficient to convert the 4 to 20 milliamperes signals into the equivalent 1 to 5 volts remember we need a 250 ohms precision resistor for the conversion let's see the example of signal conversion basic formula to calculate voltage equals multiply current with resistance multiply the 4 milliamperes with 250 ohms resistor to get 1 volt multiply the 20 milliamperes with 250 ohms resistor to get 5 volts so it's easy to perform calculations and conversions now you must have a question why do we need to convert the 4 to 20 milliamperes into voltage the 4 to 20 milliamperes signals are connected to analog input modules of plc the analog input module consist of analog to digital converters a-d-c this a-d-c needs voltage as input signal and converts them into the equivalent binary signals ones and zeros then analog input module provides the equivalent binary signals to the plc processor cpu for further operations another advantage is current signals are more immune to electrical noise when compared to voltage signals so the amount of noise or external interference is less one more advantage is current signals can travel a longer distance 4 to 20 milliamperes signals can travel approximately one kilometer when the nominal power is 24 volts dc supply now 4 to 20 milliamperes signals are being used everywhere it's a standard signal choice for industrial users it makes the installation and configuration very easy external disturbances are very less with the help of live zero we can easily detect the faults we do not need any complex equipment for troubleshooting these signals a simple multimeter will do the job we have two disadvantages the first one is the current signals may induce small magnetic fields in the straight wires this magnetic field may affect the current signals traversing in the wires to avoid this problem we use twisted pair cables we need a minimum of two cables means one set of pair this cable pair is twisted as shown in the right side image by using the twisted cables magnetic fields induced in the two cables are cancelled against each other and resolves the magnetic field problems the second one is each pair of cable carries only one process variable we discussed the reasons behind the popularity of 4 to 20 milliamperes signals do you have any questions share with us