Instrumentation control

Theye are 4-20 mA so that a loss in signal can be detected. If they were 0-20 mA, the controller would just think the process was a the LRV and send the corresponding output to the valve.

Dean Kindrai

Because you couldn't distinguish if 0mA was a zero signal (possibly a good situation) or if it was a broken wire (an open circuit, possibly a very bad situation).

Dean Kindrai
Neff Engineering of WI
This is called "Live Zero". If the standard was 0 to 20, how would you know the difference between a dead transmitter and one which is transmitting its minimum output?
Also, the 4mA is consumed in powering the device.


Paul Broughton

The 4mA serves two purposes,
1. sensor failure / cable break
2. many transmitters/sensors are "2 wire, loop powered", rather than output a signal to a controller they regulate the current suplied from the controller (the amount of current allowed to pass being proportional to the measurement taken) the 4mA is the minimum to power the electronics in the sensor.
Just a guess... 0 can be interpreted as an OPEN circuit while 4 mA would be minimun "process voltage" so that the two could be distinguished.... Just a thought, I don't know for sure. :)

Zan Von Flue

Not all Instrumentation signals are 4 to 20mA. Either with a switch or with software, it mostlikely can also handle 0 to 20mA and maybe even 0 to 10V. A 4 to 20mA signal (input) means at 0 mA the Instrumentation knows the controller is not powered up, not connected or there is a wire break. This 4mA check can't happen with a 0mA signal. This also applies for the output signal. So the Controller can test if the Instrumentation is properly connected it can check the 4mA signal.

For this question there are many reasons:

1. For every electric signal minimum current required is 3.75mA.So min. level is 4mA.

2. To differtiate between zero & null value.

3. If you see industry standard as 0,25,50,75 &
100%...............4 to 20mA fits exactly as
4, 8, 12,16,20mA.

4. If you see actual graph, after 20mA there is saturation level.

I don't know if it is "THE" answer, or plays a role in all current sources, but those using a transistor to produce the current would have nonlinear effects until the base-emitter voltage reached its .6 to .8 value. Hope it helps.

Our controls are supplied with either 4-20 or 1 to 20 or 1 to 10 as you describe it. It can actually even be 1 to 5.
The 1 to something is measured in VDC or usually called a resistive output. You would want the resistive output in, say, computer control as the computer can output 1 to 5 VDC. So if you are controlling a liquid level in a tank and 2.5 volts is received back by the computer means the tank is half full. It is cheaper in local indication. A simple battery or power supply can input 10 VDC with a proportional output. A local dial can be installed as can a PC board to activate relays etc.

4-20 is required in many applications when long wire lead length is an issue. It is more common today and is an excellent standard. The signal is converted to two wires for easy reading of many instruments. You still need a power supply usually 24 VDC.

We convert a resistive load to 4-20 by using a surface mount module that easily hooks up to most resistive 3 wire devices. You can see the unit by going to then to controls – then 4-20.

Bob Hogg

Johan Bengtsson

Because since 0mA is not a valid signal you can easily detect when the wire is broken or there is some other fault giving 0mA. if 0mA would have been a valid signal you could not make any such detection since you would not know if the signal really should be 0 or if something is broken.

/Johan Bengtsson

P&L, Innovation in training
Box 252, S-281 23 H{ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: [email protected]

Anthony Kerstens

I don't have a solid answer since I'm not an instrumentation expert.

The only thing I can be sure of would be that some analog devices provide broken wire detection. A signal of 0mA is needed to do this.

Other reasons may involve noise at low signal levels, non-linear ranges of electronic components, and calibration of circuits.

The one instrumentation book I have does not go into great detail on electronics.

Anthony Kerstens P.Eng.
The most obvious answer is... Using the 0-20 range... does 0ma represent "0" or a break in the cable. While with the 4-20 range there is no chance of this ambiguity.

Best Regards... Rick Kelly

Chief Technician N/C
Cheese Operations
Kraft Canada Inc.
Ingleside, Ont.

V (613) 537-8069
F (613) 537-8044
[email protected]
Because, 2-wire transmitters (no external power supply) use the 4mA signal to energize the transmitter.

In Europe, people prefer 0-20mA, because it is possible (with new technology) to "energize" the transmitter with "almost zero" power.

In the US, 4-20mA is a standard signal.

Jim Pinto
email : [email protected]
San Diego, CA., USA

C. Ward Yelverton

Also loop powered devices and indicators won't work with 0-mA. 4-mA establishes a minimum power availability to all devices on the loop.

Michael R. Batchelor

The minimum power availbility is just a freebie. (An important one to be sure.) The 4-20 ma is reminicient of the older 3-15 psi signals. The live zero gives an indication that the loop is broken by indicating 25% below zero if there is no signal.
Yes, but also the 4 to 20 mA allows for a low cost diagnostic for broken wire or lack of power supply. 0 mA is not the beginning of the scale, but a signal of malfunction.

the devices for two wires we can't start from 0 ma because 0 ma means the charge is very high (open circuit). for 4 wires it's better start from 4 ma to know the situation for your devices or wire. this is what I understand during my small experience.


Jeffrey Eggenberger

It also allows for 0 (Zero) ma to be a faulted condition. A nice way to check for an open circuit.

There is no such thing as a natural-born pilot. Chuck Yeager

Jeff Eggenberger


Forget about the 4-20 mA stuff anymore,the future is going to be the Fieldbus( all Digital, bi-directional communication) devices/systems.