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Advantages and Disadvantages of 4 t0 20 mA analog signals
ADVANTAGE
This range simplifies the design. One of the main advantages of using 4to 20 milli amperes signals is it simplifies the signal conversions from current to voltage. A simple resistor is sufficient to convert the 4to 20 milli amperes signals into the equivalent one to five volts. Remember, we need a 250 ohms precision resistor for the conversion. Let's see the example of signal conversion. The basic formula to calculate voltage equals multiply current with resistance. Multiply the formula amperes with 250 ohms resistor to get one volt and multiply the 20 milliamperes with 250 ohms resistor to get five volts. So it's easy to perform calculations and conversions. Now, you must have a question. Why do we need to convert the 4to 20 milliamperes into voltage before 20 milliamperes signals are connected to analog input modules of PLC? The analog input module consists of analog to digital converters ADC, this ADC needs voltage as input signal and converts them into the equivalent binary signals 1 and 0. 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 advantages current signals can travel a longer distance of 20 milliamperes signals can travel approximately one kilometer when the nominal power is 24 volts DC supply. Now for 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 line 0 we can easily detect the faults. We do not need any complex equipment for troubleshooting these signals are simple multimeter will do the job.
DISADVANTAGE
We have two disadvantages. The first one is the time signals may induce small magnetic fields in the strength wires. This magnetic field may affect the current signals diversity 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 and do to them the two tables are cancelled against each other and resolves the magnetic field problems. The second one is each pair of cable carries only one process variable.
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