# PLC Analog Input Card Jumper



## lynes.nathan (Oct 24, 2012)

Hey everyone:


I saw this at work today. Does anybody know why they put a jumper between Ch0- and Ch0_iRtn? I noticed they also did this for a few more of the inputs. I also noticed that some of the inputs did not have a jumper.


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## bill39 (Sep 4, 2009)

what brand and model # is the module? Check the manual, it will tell you.

Allen Bradley has a couple of analog modules that require that jumper to the voltage terminal even though it's a 4-20mA signal.


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## lynes.nathan (Oct 24, 2012)

bill39 said:


> what brand and model # is the module? Check the manual, it will tell you.
> 
> Allen Bradley has a couple of analog modules that require that jumper to the voltage terminal even though it's a 4-20mA signal.



It is a ALLEN BRADLEY 1769-IF4I Analog INPUT MODULE


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## xpertpc (Oct 11, 2012)

That jumper is used when inputting a current source as 4-20 ma, when inputting a voltage source as 0-10 vdc then no jumper is needed.


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## DriveGuru (Jul 29, 2012)

Just a guess


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## xpertpc (Oct 11, 2012)

I hate to be the one who states 4-20 ma dropped across a 250 ohm resister equals 0-10 volts. Personally I have never seen a 500 ohm one unless its a 5 volt bastard. j/k


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## DriveGuru (Jul 29, 2012)

xpertpc said:


> I hate to be the one who states 4-20 ma dropped across a 250 ohm resister equals 0-10 volts. Personally I have never seen a 500 ohm one unless its a 5 volt bastard. j/k



Lol, just figured I'd throw that out there, you'd be surprised how many plant electricians/technicians I've run across that really don't even understand ohms law


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## xpertpc (Oct 11, 2012)

I said that all wrong but that's the danger of early retirement - hitting the Jim Beam a bit early,

But your guess is right on track and will go one step further and explain the need for the jumper in a current loop and how it adds the resistor that you drew in the picture.

First we use current over voltage on long runs because of wire resistance and voltage drop, current is also less susceptible to electrical noises.

Using ohms law E = I x R, when you drop current across a resistor you get a voltage drop which is then read by the input.

0 ma x 250 ohm = 0 volt
4 ma x 250 ohm = 1 volt
20 ma x 250 ohm = 5 volt

0 ma x 500 ohm = 0 volt
4 ma x 500 ohm = 2 volt
20 ma x 500 ohm = 10 volt

The general reason we use 4-20 ma over 0-20 ma is for a fail-safe mode, so in the case with your modulating valve if using 0-20 ma and the sensor went bad or the wire broke it would just assume the valve is closed even though it could be in any position.

Using 4 ma as a low end would always guarantee a signal to be present, if it drops less than 4 then a fault would occur and signal the system to alarm.


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## DriveGuru (Jul 29, 2012)

xpertpc said:


> I hate to be the one who states 4-20 ma dropped across a 250 ohm resister equals 0-10 volts. Personally I have never seen a 500 ohm one unless its a 5 volt bastard. j/k



I thought you knew you stated it wrong and were being facetious, thought the j/k was just kidding ...and technically I think it might be better stated that when current is pulled through a resistor voltage is dropped...just sayin


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## triden (Jun 13, 2012)

Even Schneider PLC's require the third input on their 4-20ma modules. Pretty common occurrence.


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## DriveGuru (Jul 29, 2012)

triden said:


> Even Schneider PLC's require the third input on their 4-20ma modules. Pretty common occurrence.



Many drives are the same way as well, except they will normally use a dip switch or jumper.


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## lynes.nathan (Oct 24, 2012)

xpertpc said:


> I said that all wrong but that's the danger of early retirement - hitting the Jim Beam a bit early,
> 
> But your guess is right on track and will go one step further and explain the need for the jumper in a current loop and how it adds the resistor that you drew in the picture.
> 
> ...



Makes sense! Thank you.


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