Principle of 4-20ma


Tony Willard

4-20mA signals. If you have per say a measuring device that is changed by resistance and you have a consistent voltage, then you will have a change in amperage. If your change in resistance in comparison to the scale or range that it uses, you can measure this change in mA. Therefore you can monitor the change from 4-20mA and use this information to give you feedback for i.e.
position, temperature, fluid levels, etc..

Johan Bengtsson

Well, the 4-20mA does not really have anything to do with the principle of the control. 4-20mA is commonly used for signal transmitting to and from the controller (what the controller do is a completely other subject)

4mA represents the lowest reading on the process value or the output and 20mA represent the highest. Everything between is a linear function like this:

4mA - 0%
8mA - 25%
12mA - 50%
16mA - 75%
20mA - 100%

This is but one (but one of the most common) way of transmitting a signal from the sensor to the controller and then to transmit the output from the controller to the actuator. Other ways include:

Voltage signals:

Air pressure signals:
3-15Psi, almost equal to 20-100kPa

Others do exist, some of them are really uncommon however.

What the controller do is another matter, like PI, PID, on/off, fuzzy, model based, whatever...

/Johan Bengtsson

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<P>4-20 mA control has been chosen for several reasons.<BR>
It is used to transmit analog signal (ex.: 0_100 C temp./4_20 mA)<BR>
It needs only 2 wires.<BR>
It can cover long distance (several Km of wire)<BR>
It has a better noise immunity than voltage signals<BR>
Many devices can be connected in the same current loop (ex.: Temp transmitter---loop indicator---PLC---chart recorder)<BR>
A faulty transmitter can be detected (0 mA current output)</P>

<P>Here is an example:</P>

<P>2 wires temp.<BR>
(0_100 C)<BR>
] 24 VDC
-]------->>--[+loop indicator-]------- -
<P>If temp. transmitter reads 0 C, signal on loop will be 4 mA<BR>
If temp. is 50 C, signal output is 12 mA<BR>
If temp. is 100 C, signal is 20 mA<BR>
and so on.</P>

<P>The loop indicator reads loop current and display an engineering value (let's say 0 to 100 C)</P>

<P>Hope it helps a bit.</P>