# 4-20 milli amp for Dummies



## MechanicalDVR (Dec 29, 2007)

petek57 said:


> I kind of --sort of know how this works and how it is supposed to work. But if anyone has a good pdf or website about 4-20 milli amp controls I would appreciate it!


 
Just a different type signal but works to scale up and down just like 0-10vdc or 5-10vdc. What do you need to know?


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## drsparky (Nov 13, 2008)

4-20 works well because you never worry about voltage drop.


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## petek57 (Mar 3, 2009)

Here's how I surmise 4-20 ma system works. A power supply sends voltage to an instrument. This instrument measures _whatever _and translates that to a scale somewhere between 4-20 ma and sends that signal back to either a plc or a meter. 

Is that about right?


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## drsparky (Nov 13, 2008)

petek57 said:


> Here's how I surmise 4-20 ma system works. A power supply sends voltage to an instrument. This instrument measures _whatever _and translates that to a scale somewhere between 4-20 ma and sends that signal back to either a plc or a meter.
> 
> Is that about right?


Depending on the setup you do not need a separate power supply; the PLC can run the instrument through a current loop. Here is a example of a basic analog card setup. Scale the software at the PLC to match the instrument. If you have a temperature probe that reads 0 to 100 degrees, that would be 6.25 degrees per milliamp over the base line of 4 milliamps or .16 milliamps per degree. Thus 4 = 0, 12 = 50, 20 =100. Room temp of 70 degrees should read about 15.2 milliamps. 
Also remember that noise can affect the readings so use proper shielded cable and only ground the shield on one end of the cable. :thumbsup:


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## petek57 (Mar 3, 2009)

Good stuff!


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## 76nemo (Aug 13, 2008)

Why did the post about the LM317 get thrown to Diy forums?


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## B4T (Feb 10, 2009)

76nemo said:


> Why did the post about the LM317 get thrown to Diy forums?


I just checked.. he used "hobby" for the question of how he was related to the electrical field. I don't see a dire danger there to close the thread


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## PLCMentor.com (Apr 15, 2009)

Pete,
Also keep in mind that since it is current additional devices should be added in series. so if you want a PLC and a chart recorder to both monitor the reading then you would connect them in series with the source.


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## Mach (Dec 17, 2007)

http://www.mtroubleshooting.com/mm5...&Product_Code=MTM-9001&Category_Code=INSTRMNT

This one doesn't change like the Uglys.

Ohms law is very useful for loops.


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## John (Jan 22, 2007)

The reason for the use of 4 ma as the low value is because some instruments will see a signal or loop fault if the reading of the circuit loop is less then 4 ma. Also some instruments are powered by using the 0-4 ma range supplied by another instrument.:thumbsup:


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## petek57 (Mar 3, 2009)

*Do I have it right?*



PLCMentor.com said:


> Pete,
> Also keep in mind that since it is current additional devices should be added in series. so if you want a PLC and a chart recorder to both monitor the reading then you would connect them in series with the source.


Like many things my first experience with 4-20 was when the owner of the company told me to go install about 50 devices. Sink or swim basically. Thankfully there were some onsite techs to babysit me.

Here's the way I remember it. A rectifier provides a DC common that then goes to a measuring device (chlorine is one I remember). This device measures the chemical content and reports via a 4-20ma signal to the plc.

The rectifier's + gets tied into the instrument's -. The + from the instrument is then sent to the plc. Which then interprets the strength of the signal into an analog reading. 

The process is then adjusted depending on that reading and how the plc is programmed.

As you said, other meters etc. can be placed in series with the instrument.

Am I on track?


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## petek57 (Mar 3, 2009)

*That looks excellent*



Mach said:


> http://www.mtroubleshooting.com/mm5...&Product_Code=MTM-9001&Category_Code=INSTRMNT
> 
> This one doesn't change like the Uglys.
> 
> Ohms law is very useful for loops.



Thanks!


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## petek57 (Mar 3, 2009)

*More*



Mach said:


> http://www.mtroubleshooting.com/mm5...&Product_Code=MTM-9001&Category_Code=INSTRMNT
> 
> This one doesn't change like the Uglys.
> 
> Ohms law is very useful for loops.



Looking thru their website they have many other references that are going to be helpful.

Again Thanks!


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