Analog input negative numbers

bcarbo · Jan 25, 2017

I am using a 1769-IF8 analog input with a 0-10volt load cell input. When I configure the input to 0-10v, it always reads as 0. When I configure the input to -10v to +10v, it reads negative numbers. Even when the input is 0 volts, it still reads a negative number. I checked the input with a meter and it is reading and positive voltage.

gclshortt · Jan 25, 2017

You are reading noise on the line. The signal to noise ratio at 0 volts is infinite.
When the setting is at 0 to 10 volts the PLC will clip the noise in half because it cannot go below 0.

Regards,

bcarbo · Jan 25, 2017

I would have thought that the numbers would jump if there was noise. They are pretty steady.

gclshortt · Jan 25, 2017

They will be jumping around if you are viewing on a oscilloscope or similar device. You could minimize the amount of noise on the analog line by installing a capacitor from the analog to ground.

James Mcquade · Jan 25, 2017

gclshortt,

Wouldn't installing a capacitor from the line to ground defeat the purpose of an analog input?
when the capacitor charges to 5 volts lets say, it stays at 5 volts until it discharges?

regards,
james

gclshortt · Jan 25, 2017

https://www.solo-labs.com/bypass-capacitors-why-and-how-to-use-them/

Here is a good article on use of the capacitor to filter out noise.

Regards,

geniusintraining · Jan 25, 2017

bcarbo said:
I am using a 1769-IF8 analog input with a 0-10volt load cell input. When I configure the input to 0-10v, it always reads as 0. When I configure the input to -10v to +10v, it reads negative numbers. Even when the input is 0 volts, it still reads a negative number. I checked the input with a meter and it is reading and positive voltage.

Whats the reason for configuring -10 to +10 if the load cell is 0-10 then you should only use 0-10 and not -10/+10?

bcarbo · Jan 25, 2017

I put it at -10 to +10 just to see what the values were.

bernie_carlton · Jan 25, 2017

Let's look at simpler reasons ...

If a greater weight causes the signal to move in the negative direction then either the excitation or signal leads to the load cell are backwards.

If the signal is negative but moves in a positive direction (less negative) with more weight then the offset is incorrect.

geniusintraining · Jan 25, 2017

bcarbo said:
I put it at -10 to +10 just to see what the values were.

If you dont use what the manufacture set it up for then you will get funky values... Just like if you used 0-5 you would think it would just go to 5v and stop but it wont most of the time... if you dont want it to go below 0 then put a MOV in your logic and write a =/< (equal to or less than) MOV 0 to the register and it will never go negative

Toine · Jan 25, 2017

I find 0..10V quite high for load cells. The typical load cells that I come across get a 5V or 10V excitation voltage. The signal voltage that comes back on inputs is typically between 5mV and 30mV for max load on the load cells.

bernie_carlton · Jan 25, 2017

I missed that the OP didn't mention a load cell conditioner. Yes, attaching the output leads of a loadcell to an ordinary analog input card probably won't work very well.

Could you perhaps provide a model number for the load cell and a diagram showing how you have it wired to the card?

shooter · Jan 26, 2017

did you also connect the gnd?
and as bernie says more info would help.

Analog input negative numbers

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