Load Cell output resistance of 350ohms into Analog Input

[darkesha]

Hi, I have analog load cell 0-10V feeding into 1771 Analog Input card.
I scaled it simply
0-20,000 <=> 0-4096 counts <=> 0-10V
for multiplier of 4.88

Now I have some doubts since the spec sheet mentions input/output resistance of load cell being 350ohms and I am not really sure if that has anything to do with voltage drop on the PLC card (250ohms) and if my scaling might be out of whack ?

 

[DamianInRochester]

If it is truly just a "load cell" then you would normally require a Load cell amplifier to bring the output voltage up to the 10V range. You should have a mV/V spec on the cell. Provide the specs and Part number.

 

[darkesha]

C2P1 by BHL

2mV/V

I have no experience with load cells prior looking at this one.
Just peaked into JB - there is something in there (amplifier most likely).

1771 IFE/C is the analog card. Unfortunately machine is running but not required for this product, so there is no voltage output changes, and of course, there are no drawings to follow.

 

[JesperMP]

Post a photo of the "junction box". Of the inside, and of the type label.
It may be summing box, and then you need an external loadcell converter.
Or it may actually be a loadcell converter.

 

[DamianInRochester]

C2P1 by BHL

2mV/V

I have no experience with load cells prior looking at this one.
Just peaked into JB - there is something in there (amplifier most likely).

1771 IFE/C is the analog card. Unfortunately machine is running but not required for this product, so there is no voltage output changes, and of course, there are no drawings to follow.

That is just a straight up load cell

http://www.bsicontrols.net/specifications/BLH/Literature/LoadCells/C2P1/c2p1.pdf

You will require something to bring the signal into a range your PLC card can use -10/+10V, 4-20mA, etc.

You will need to find something that matches the 2mV/V and your PLC's input requirements at full scale in order to get the full resolution out of it.

 

[darkesha]

This load cell had been wired for a long time, I was tasked to check if everything was correctly set scaled and calibrated.
I will post some photos tomorrow - need cable to transfer them off camera.

So again, the machine most likely works ok, I just need to make sure and in order to do this I need to learn the system inside and out.

Any good sources to inform myself about load cells ?

 

[DamianInRochester]

http://www.transducertechniques.com/wheatstone-bridge.aspx

Pertaining to your original question, the bridge resistance of your cell (350ohm) will have no direct effect on the PLC analog input. If you go through the link above you will see that the cell itself is just a resistor bridge.

If you are getting incorrect readings, there are a number of things that could be wrong.

loose wire anywhere between the cell and the input card.
Scaling
Excitation voltage to the cell.
Damaged cell.
Something damaged to the mounting of the cell such that the load is not being transmitted properly.
etc.

Do you have a methods and a means to hook up some external device to check your values?

 

[darkesha]

Analog input and scaling work really good, there is no problems with that. I want to make sure that 4.88 multiplier is correct, so for every count the analog signal goes up, i get 4.88 ft/lbs.
I thought it will be super simple expecting load cell to have linear output between min and max which would correspond 0 and ten volts.
Thanks very much for the link, I worked night shift and will read up on it tonight when I come back.