Analog Fluctuation, Bad card?

Davis_1377

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Join Date
Apr 2020
Location
Tennessee
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I am measuring 12v from 0-10v analog input with a voltage divider, 2 resistors that are 20m Ohm (also tried it with 2k Ohm resistors). I get a accurate reading but when the analog input gets a certain voltage there is a odd fluctuation in the reading thats higher and lower than the correct value. I have tried different inputs on the card, different resistors even a potentiometer to adjust voltage from a 9v battery. Ive eliminated all possible nearby interference. Nothing I try seems to affect the signal. I dont believe in a programming issue as other voltages are correct. Filtering does not help everyway Ive tried. The fluctuation pattern is too perfect but not always and is higher and lower than the correct value. My PLC is DL205 with 260 CPU and F2-04AD-2 Analog Input. Attached is the analog signal and steady voltage input. Any help is greatly appreciated.

2020_04_19_18_24_IMG_2002.jpg 2020_04_19_18_24_IMG_2004.jpg
 
How much current can your 12V source supply?
You tried 2 x 20 ohms as a voltage divider, current = Voltage / Resistance, so at 6V that would be 6/40 = 0.15Amps or 150mA and 0.9watts, Power = current^2 x Resistance.
Two 2K resistors would still pull 0.0015Amps or 1.5mA.
 
How many bits are in the amplitude of that fluctuation?


What is the time scale?


Answers to those queries might rule some things out and others in.
 
the pdf may suggest that unit can be run from 12V. Can you power it with a car battery temporarily to eliminate 50/60Hz as a noise source?
 
another straw at which to grasp ...

where is the power for this signal loop coming from? ... is the output voltage of that supply "grounded" in any way? ...

sometimes just providing a "bleed off" path to ground - even if just through a 1 meg-ohm resistor - can "clean-up" a signal ...
 
the pdf may suggest that unit can be run from 12V. Can you power it with a car battery temporarily to eliminate 50/60Hz as a noise source?

i read that too but from what I can research its only from the newer single board cards. I have tried it with a seperate 24v battery
 
another straw at which to grasp ...

where is the power for this signal loop coming from? ... is the output voltage of that supply "grounded" in any way? ...

sometimes just providing a "bleed off" path to ground - even if just through a 1 meg-ohm resistor - can "clean-up" a signal ...

ive tried it grounded and not grounded. event resistance to ground on a separate clean battery voltage
 
How much current can your 12V source supply?
You tried 2 x 20 ohms as a voltage divider, current = Voltage / Resistance, so at 6V that would be 6/40 = 0.15Amps or 150mA and 0.9watts, Power = current^2 x Resistance.
Two 2K resistors would still pull 0.0015Amps or 1.5mA.

plenty, its agm the size of a car battery.
 
Can you confirm that you are measuring the voltage across the "bottom resistor"; across the Ch 1 (-) & Ch 1 (+)
as shown in the sketch below?

0-12-Vdc-divider-for-0-10-V-input.jpg
 
new info. for some odd reason when the input voltage is at the certain point and the fluctuation starts it is making the other 3 channels jump around between 0 and 32769. the other 3 channels are shorted together. When I do get a good reading the other 3 channels set on 0.

Capture.JPG
 
Last edited:

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