Rockwell Automation 2080-LC20-20QBBR Micro820 20-point Programmable Controllers User Manual - Manual (Page 43)

Rockwell Automation Publication 2080-UM005A-EN-E - December 2013 35
Wire Your Controller Chapter 4
Calculate for Thermistor Resistance
While connecting Analog input to thermistor as shown in previous diagram,
calculate input voltage using the following equation:
Where:
Vi = Voltage input (±5% without calibration; ±2% with calibration)
Ri = Resistance input (14.14 K
Ω ±2%)
Rt = Thermistor resistance (10 K
Ω Thermistor is recommended)
Vref = 10V ±0.5V
To calculate for thermistor resistance, use the following equation.
Calibrate Thermistor
1. Connect a resistor (10 K
Ω is recommended) across Vref and Analog
Input 00 of your Micro820 controller following the diagram, Analog input
to thermistors on page 34 . The resistor is measured as Ri using a precision
multimeter.
2. Calculate the ideal counts (C1) for resistor (Ri) following this equation:
C1 = 14.14 K
Ω / (14.14 KΩ + Ri) * 4095
3. Read the actual counts (C2) of Analog Input 00 from Connected
Components Workbench.
4. Calculate for calibration Gain.
Gain = C1/C2
For example:
If Ri is measured as 10.00 K
Ω, then
C1 = 14.14 / (14.14 + 10.00) * 4095 = 2399 counts;
C2 is read from Connected Components Workbench as 2440; so
Gain = 2399/2440 = 98% .
IMPORTANT
Micro820 controllers support 10 KΩ type thermistors.
In order to get the best results, the system must be calibrated.
Vi =
Ri + Rt
Ri
* Vref
Rt=
Vi
Vi Vref - Vi Ri