Analog input voltage drop.

[Bazel27]

Hello , 

Need some help.

I have NTC sensor 10k Ohm at 25 C , connected through the voltage divider to Jazz 10-11-ua24 analog 0-10V input.  When divider is not connected to plc , and i measure voltage between R1 and ground it shows 5 V , and then i connect it to the plc , voltage drops to 4V. 

What is the reason ?

i have made a drawing of connection diagram

Thanks in advance 

 

Bagel NTC calibration.spl7.pdfAleksei 

[Joe Tauser]

You're adding a resistor in parallel to R1 when you connect the analog input; the spec lists 20K ohms as the input impedance.  The effective resistance (Re) below the sensor will be 6.67K.  

 

Applying the voltage divider formula Vr = Vs*Re/ (Re + Rs) = 10 * (6.67) / (6.67 + 10) = 4.00 

 

 

You're going to have to re-size R1 to compensate for the input impedance of the module.  I'll leave that as an exercise for you.  Or you could just adjust your scaling logic.

 

Joe T.

[Bazel27]

You're adding a resistor in parallel to R1 when you connect the analog input; the spec lists 20K ohms as the input impedance.  The effective resistance (Re) below the sensor will be 6.67K.  

 

Applying the voltage divider formula Vr = Vs*Re/ (Re + Rs) = 10 * (6.67) / (6.67 + 10) = 4.00 

 

 

You're going to have to re-size R1 to compensate for the input impedance of the module.  I'll leave that as an exercise for you.  Or you could just adjust your scaling logic.

 

Joe T.

Thanks , i have made an adjust of scalling logic already.

Thanks for the fast response.

Cheers 

Aleksei Bagel