Analogue electrical signals are used to represent process va

Analogue electrical signals are used to represent process variables such as pressure, speed, temperature, flow rate and so on. The instruments produce electrical outputs that are processed into a standard form so that standard equipment can be connected to it such as analogue to digital converters and meters. One standard form is 4-20 mA.

(1)Give the other common standard forms used in PLC for I/O interconnection?

(2)Why is the minimum in the above standard 4mA instead of 0 mA?

Solution

1.) Other standard forms used in PLC for I/O interconnection are:

4-20 mA

10-50 mA

0-10 V

1-5 V

2.) The reason for choosing 4 mA is that, when the transmitter was first introduced it was working on pneumatic pressure signal for transmission. Later on electronic chips were introduced which required minimum of 3 mA of current to function. Hence, a margin of 4 mA is taken as reference

Apart from this, if 0 is used instead of 4 mA,we cannot differentiate the actual 0 value is of the sensor or the problem in the transmission of signal ( cable is cut or damaged ). For the upper limit, 20 mA is used as maximum because the human heart can withstand upto 30 mA of current only. So,as a safety point 20 mA is chosen

mA reading will be accurate from 3 mA. But getting exact ratio between 3 to 20 mA is bit difficult. So, 4-20 mA is commonly used so that 0, 25, 50, 75, 100% can be converted in to 4- 20. i.e.

0-4 mA

25-8 mA

50-12 mA

75-16 mA

100-20 mA.