# Reason for 4-20mA current as a standard signal for transmitting data to and from PLCs



## Tsmil (Jul 17, 2011)

I recall being told in instrumentation class that some sensors use the 4-20 loop also supplies the excitation required for the sensor to work. The current required to do this is less than 4ma so they they needed a value as a positive 0 point.

This is what we were taught but I think that this is only one factor.


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## triden (Jun 13, 2012)

Biggest reason for current is that you can counter losses due to voltage drop for long runs. 

I would think the reason that it is limited to 20ma is because it would not require too large of an input voltage to induce 20ma across a high resistance load/line. If the resistance of the cable was 1000ohms and you needed 100ma, you would need a driver that could put out 100volts. 20ma keeps the driver voltage at a reasonable level for most applications with enough signal strength to counter the noise floor.


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## bill39 (Sep 4, 2009)

In addition to what the others are saying, 4-20mA signals have very good immunity to noise.


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## wildleg (Apr 12, 2009)

I believe that the 4-20ma just evolved from the power levels that were being used at the time for ttl logic ckts.


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## JRaef (Mar 23, 2009)

triden said:


> Biggest reason for current is that you can counter losses due to voltage drop for long runs.
> 
> I would think the reason that it is limited to 20ma is because it would not require too large of an input voltage to induce 20ma across a high resistance load/line. If the resistance of the cable was 1000ohms and you needed 100ma, you would need a driver that could put out 100volts. 20ma keeps the driver voltage at a reasonable level for most applications with enough signal strength to counter the noise floor.


Hit the nail on the head! I was taught that it was also related to early attempts to keep the signal energy at non-incendiary levels so that they could be transmitted around in explosive gas areas without having to be in conduit and have sealing fittings. Later they had to add intrinsic safety barriers anyway, but that was a part of the original intent. At something like 100VDC in the above example, not producing a spark at a wire break is pretty much impossible.


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## erics37 (May 7, 2009)

Didn't they used to use to use 0-10 psi or something for signalling? A process control guy around here was telling me about that; a lot of devices would create an analog electrical signal and then a transducer would convert it to psi and that would actuate whatever device was measuring it.


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## ScooterMcGavin (Jan 24, 2011)

erics37 said:


> Didn't they used to use to use 0-10 psi or something for signalling? A process control guy around here was telling me about that; a lot of devices would create an analog electrical signal and then a transducer would convert it to psi and that would actuate whatever device was measuring it.


 
Still Do! Many Pneumatic control valves are still 3-15PSI. The only difference is that there is that the positioner will have a 4-20ma to 3-15PSI transducer built-in to it which converts the signal.

We have plants which are still entirely pneumatic. All the level pressure and temperature transmitters output a 3-15PSI air signal over instrument tubing to the Pnuematic single loop controller which can do an entire PID control scheme pneumatically. It's quite impressive Lots of springs, levers and bellows inside. We have been upgrading to electronic loop controllers because of the inherent delay in pneumatic systems and its tougher to get parts for them. Plus us young guys don't know how to work on them, but the ones we have still work fine.

On the 4-20ma. One of the reasons I was told that they used this as the standard is because when you get nice clean 1-5 volts across a 250 ohm resistor which is how PLC's actually read the signal. It was explained to me that voltage measuring circuits are much cheaper to make and they don't produce as much heat as current reading circuitry would.


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## sparky970 (Mar 19, 2008)

erics37 said:


> Didn't they used to use to use 0-10 psi or something for signalling? A process control guy around here was telling me about that; a lot of devices would create an analog electrical signal and then a transducer would convert it to psi and that would actuate whatever device was measuring it.



3-15 psi is most common but some valves are 6-30 psi. Find an old Foxboro 13A and rebuild it. That will give you a good understanding of pneumatics


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## oliquir (Jan 13, 2011)

and now a lot of transmitter can be powered by the 4ma signal so no need of separate supply


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## sparky970 (Mar 19, 2008)

oliquir said:


> and now a lot of transmitter can be powered by the 4ma signal so no need of separate supply



:blink::blink:


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## oliquir (Jan 13, 2011)

sparky970 said:


> :blink::blink:


never heard of 2 wire transmitter??
here an example 
http://www.omega.ca/shop/pptsc.asp?ref=PX764&flag=1


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## Lone Crapshooter (Nov 8, 2008)

Old electronic instrumentation was 10 to 50 but that was years and years ago. 
Instrumentation signal transmission is a 1 to 5 ratio. 4 to 20, 3 to15 are 1 to 5 ratios. The reason for 4 mA as a zero (live zero) is that if you look at a process controller and it reads less than 0 (down scale) you know have a instrument problem. If you used 0 as 0 and you lost your control loop you would mot have any indication.
LC


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## sparky970 (Mar 19, 2008)

Lone Crapshooter said:


> Old electronic instrumentation was 10 to 50 but that was years and years ago.
> Instrumentation signal transmission is a 1 to 5 ratio. 4 to 20, 3 to15 are 1 to 5 ratios. The reason for 4 mA as a zero (live zero) is that if you look at a process controller and it reads less than 0 (down scale) you know have a instrument problem. If you used 0 as 0 and you lost your control loop you would mot have any indication.
> LC



10-50 was millivolts


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## sparky970 (Mar 19, 2008)

oliquir said:


> never heard of 2 wire transmitter??
> here an example
> http://www.omega.ca/shop/pptsc.asp?ref=PX764&flag=1


Right, you need a supply voltage


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## triden (Jun 13, 2012)

scameron81 said:


> On the 4-20ma. One of the reasons I was told that they used this as the standard is because when you get nice clean 1-5 volts across a 250 ohm resistor which is how PLC's actually read the signal. It was explained to me that voltage measuring circuits are much cheaper to make and they don't produce as much heat as current reading circuitry would.


This is true, but I don't think the resistor value was the main concern for the choice. After all, a 100 ohm resistor would make 1-5volts for a 10-50ma current source and so on. There are many "nice" resistor values that work with a plethora of current values.


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## EJPHI (May 7, 2008)

Lots of good info here. The noise immunity point is important for many industrial processes. The current source in 4-20 mA systems has very high impedance compared to the load of a hundred Ohms or so. An interfering voltage coupled to the wiring in 4-20 mA will mostly show up across the current source so the load won't see much of this interference. They start at 4mA because setting a process parameter at 0 mA would be have greater noise content as a percentage of the desired signal.

4-20 mA ROCKS!! :thumbup:

EJPHI


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