Answer / arindam

why we require a 4-20 mamp for transmission? the main reason
behind it because of electro magnetic interference the
voltage can get interfered very easily and we can't get our
desired o/p value.when we are using 4-20 mamp this problem
is solved but since, PLC is a digital device it can
understand mainly High=5V or low so we design the sink to
have a resistance as high to get the desired voltage, the 4
mamp is also helpful to power the electronics at the
transmitter end. to have a 4-20 mv instead can firstly :
cause em interference and secondly we need to have a
continuous level monitoring for different voltages...which
can increase the size of PLC . Also because of change of
impedance at the transmission line the voltage can drop
below 4 volt ocassionaly then loop powering of field
instrument will not be possible.

Answer / biswarup das

There is different module in a PLC . Out of which one is T/C
module if you connect the o/p of T/C directly to this module
of PLC , then the PLC will recognise it.

T/C module is not inbuilt we have to select it seperately.
There are many modules like this such as RTD module, 0-10
volt module etc.

Answer / altamash

In plc, there is a separate I/O MODULE FOR T/C which already have inbuit converter so we dont have to worry on this,First we have to know complete module of plc.thanx

Answer / guest

we will convert mV in to the ma....by connecting resistance
in parallel

Answer / kishore

There will be a Convector for mv to ma what ever the
temperature Sens by the t/c that will convert to the ma and
signal goes to DCS.

Answer / sattar

Voltage to current converter using op-amp

Answer / sreekanth

standard thermocouple output is 0 to 10 mV. There will be 4-
20MA converter either in DCS in the field(Transmitter)as
the modules will accept only 4-20 MA output.

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