Can someone briefly describe how a 4-20mA circuit works? 1

[bdn2004]

We have a meter that is providing a 4-20mA output to a PLC and an external meter. The PLC has a 4-20mA input card and is also looped to a digital meter. It works fine. But why? What does the internal circuit diagram look like for what's generating the 4-20mA current? What is the driving voltage as the literature doesn't say on the meter? At what point does that generator get overloaded, is this just a never ending source of 4-20mA? I just which I understood this in full circuit diagram format.

 

[benta]

This should help:

http://www.ni.com/white-paper/6940/en

Driving voltage i usually 12...24 V, but may be higher if the wires are long.

Benta.

 

[xnuke]

There are multiple types of 4-20 mA transmitters, but you didn't specify what type you have, so I'll discuss the two most common.

One type of transmitter, known as two-wire (since only two wires are required) or loop-powered or passive, requires an external DC voltage source (24 V is typical). This source can be either from a PLC power supply or from a separate DC power supply. The transmitter (or "meter" as you called it) acts like a variable resistor that takes a value proportional to the measured span of the transmitter. For example, if you have a temperature transmitter that is 20-120 F corresponding to 4-20 mA, when the temperature sensed is 20 F, the transmitter will provide whatever resistance is required in the loop to make the loop current equal to 4 mA.

A second type, known as four-wire or externally-powered or active, is a transmitter that requires two wires for power and an additional two wires for signal transmission. This type of transmitter acts like a current source. It uses the power provided to it from the external source to generate current in the signal wires, with one wire sourcing current and the other wire acting as the current return.

In the U.S., a 4-20 mA transmitter must be able to provide current into a maximum of 600 ohms without a problem. For more information, see ANSI/ISA-50.00.01-1975 (R2012) Compatibility of Analog Signals for Electronic Industrial Process Instruments.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[engineeringcalc]

The reason of using current instead of voltage is that with current you avoid errors due to voltage drop in the cable. If the transmitter sends for instance 10mA, the controller will receive 10mA, independently of the impedance of the wires.

You have a short explanation in the following link:

http://www.herramientasingenieria.com/onlinecalc/4_20mA.html

And a online calculator in the following one:

http://www.herramientasingenieria.com/onlinecalc/4_20mA.php

I hope it helps.

Regards,

 

[waross]

Start by arbitrarily selecting a 0% point and a 100% point for the measured variable.
Some examples are:
0 PSI = 0%, 50 PSI = 100%
100 deg. F = 0%, 250 deg. F = 100%
4 ma corresponds to 0% and 20 ma corresponds to 100%. The system is flexible, dependable and accurate.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[OperaHouse]

Actually I think it is pretty amazing that the development of 4-20ma happened prior to the availability of micro power electronics. Given that you have at least 3ma and 6V to play with, there are endless devices that you can power remotely with just two wires and get a signal back.

 

[mikekilroy]

4-20ma came about to

a) show if a wire is broke in the loop (0-3.99ma = fault)
b) feed same command to LOTS of devices
c) run it over long lengths

any of the above.

consider if you send 0-10v command to something; if wire breaks, it will be 0vdc command - VALID.... so by moving low point up to 4ma min, if a wire brakes and command goes to 0ma, it is known to be a fault instantly. satisfies choice a) above. consider a nuclear power plant - gotta know that stuff ASAP

consider power plants: we sometimes supply 3-30pc electric actuators to open/close/modulate air entry, exit, etc., to the boilers, scrubbers, etc. all must do same thing. 4-20ma is great way to accomplish b) above.

0-10v command into typical 10-20kohm inputs all in parallel with drop the voltage over long lengths to unknown value.... 4-20ma is constant and does not get effected by 1-2-3-4 mile lengths. satisfies c) above.

Lastly, do the match: 4-20ma.... IS THE SAME THING AS 2-10VDC COMMAND!!!!!!!! the device that accepts 4-20ma simply is a 0-10vdc input command device with a 500 ohm resistor across the input..... huh? V=I*R so 4ma*.5k=2v input. 20ma*.5kohm= 10vdc input.... so each device will convert the 4-20ma into simple 2-10vdc command anyway! but no need to worry about Vdrop over the long wire run maybe miles thru the plant.

HOw may devices can you command with same command? No limit - simply have command source start with enough volts to drop 10v across each in the series run - so sometimes it may require 0-100-200 volts for 10 or 20 units!

 

[waross]

Historically plant instrumentation was controlled with air pressure signals. 3-15 PSI or 6-30 PSI. With low level air signals there was a problem with dropping pressure signals near 0 PSI. The elevated zero gave quicker response on a dropping signal as it approached the elevated zero (3 PSI) compared to approaching 0 PSI. The time constant of a low pressure dropping to zero PSI through a small orifice could become onerous. The elevated zero also allowed fault alarms in the event of loss of signal.
The first 4-20 ma devices were not all loop powered. The present day paradigm where virtually all 4-20 ma devices are loop powered came about piece-meal, over quite a few years.

Bill
--------------------
"Why not the best?"
Jimmy Carter