4-20 mA to 1-5 V conversion
Most of transmitters use 4-20 mA as ay signal standard for transmitting process parameters .Two wire method is common nowadays . supply and signal uses the same pair of wires . And in 4-wire type transmitters we can see seperate wires for power supply and signal. In both case current transmission method is used instead of voltage .This is because in voltage transmission , drop in voltage occur as the length of the signal cable increases .But in current transmission we know the current transmitted will be same throughout the loop . The transmitter acts as a current source and generate a current signal which can be interpreted as the process variable.This article describes how a current signal can be converted to a standard the voltage form (4-20 mA to 1-5 V)
The above diagram shows a transmitter connected to a 24 V power supply .The transmitter uses this power and generate a current in the loop which is equivalent to the process variable.
In process automation systems the process variables are analysed by proper measuring circuits or devices .The below figure is an example .A transmitter connected to the analogue input card of a control system .The input card is a 4-20 mA input type . current is always measured in series .So inorder to measure these 4-20 mA signal we should connect the card series in the loop . see below diagram .
In some measuring instruments or control panels I/o points , they accept the signals in to their terminal as voltage form They accept signals as standard voltage form and not as current signal . In such case we need to convert the current signal in to corresponding voltage signal before fed in to its terminals.
Current signal to voltage signal conversion
To convert a varying current signal in to a voltage signal is easy .The principle is current produces a voltage drop which is equal to the product of current and resistance…..
V =IR
Yes it is the Ohms law which is going to help us.
For example , suppose we passed a 4-20mA signal through a 100 ohm resistor, then voltage drop across it can be calculated by Ohm’s law .
when current = 4 mA,
voltage drop V = IR
= 4 ×100
= 400 mV
=0.4 V
When current =20 mA,
Voltage drop =20×100
=2000 mV
=2 V
So on passing 4-20 mA through a 100 ohm resistor ,we will get 0.4 -2 V signal across the resistor .
250 ohm resistor
In violtage transmission technique, 1-5 V is the standard form .which means 4-20 mA current form is equivalent to 1-5 V voltage signal .
So a 250 ohm resistor is used for this conversion .
See figure below the voltage across the 250 ohm resistor can be found as below.
When current is 4mA
V =IR
=4×250
=1V
When current is 20mA
V = IR
=20×250
=5V
So 4-20mA converted in to 1-5 V using a 250 ohm resistor .This 1-5 V is used as a signal standard in voltage form..Some control system cards or some measuring devices only accept this type of voltage standard in to its connecting terminals .
So inorder to convert the 4-20ma signal to 1-5 V we only need to insert a 250 ohm resistor in the loop and connect the measuring device or measuring circuit or I/o device parallel with this 250 ohm resistor .
Connection diagram with I/o module.
The below diagram shows a transmitter connected to the 1st channel of a analogue input card .It is a 1-5 V card .The transmitter send a 4-20 mA in the loop .A 250 ohm resistor connected in this loop will convert this signal in to corresponding 1-5 V signal.The measuring device or card can be connected parallel with this resistor for voltage measurement .see diagram .In this case two wires are connected to the card input terminal. The input card is a 1-5 V input type card.
Another configuration like only one wire connected to the input terminal are also popular. See diagram 2
