PLC Thermistor Input

Here is a dual Thermistor converter. They are around $225. Datexel DAT4532C. you can find them on www.datexelusa.com.

PLC has power supply and main brain and several attached PLC input and output cards. Most output cards are digital on/off cards. Some are analog output cards which require calibration with the potentiometer on the card and some PLC programming that sets output registers to zero and span. PLC input cards can be digitial input to monitor on/off conditions of solenoids or switches and PLC input cards can be analog inputs. These cards are not calibrated rarely unless special card.

Inputs of analog cards range in precision from 8 bits, 16 bits, 32 bits, 64 bits, etc.

The higher the precision the greater the cost.

To get an analog input into a PLC card, you can input current, voltage or other depending on type of card and precision of card.

Once range and type is selected, you need PLC programming knowledge to program PLC analog input.

PLC analog input card either accepts variable range 4-20mA current input or 1-5 voltage input or 1-10 voltage input. PLC program register uses analog register to store information. Transfer analog register value into storage register. Transfer storage register value to comparator register or math register to perform calculation.

PLC programming is required.

For Example in modbus Ladder Logic ProWorX uses 3X analog register to store value. Additionally value can be defined in 8 bits, 16 bits, or 32 or 64 or 128 bits depending on how expensive a PLC you are using. Generally cheapest PLC's are used.

8 bits was common, then 16 bits. You will find many 16 and 32 bits out there.

Once 3X register stores value, perform block transfer to 4X register.

Once this is done, you just take that stored value and insert it into some math register to add it to something or perform some mathematical comparison with another value.

From there to transfer the result to Scada. Place final result in another 4X register and specify that 4X register in the Scada machine database. You will need to construct a register map in your PLC. With this map you can specify which registers you want to transfer information from your PLC to your Scada database. Generally the database includes fields and tags. Once you do this, in your Scada software you can link the fields and tags via a GUI (graphic user interface) to an operator screen that the operator can push buttons or slides with pictures of your process.

Different Vendors have different strategies and schemes to get analog information into their PLC's using their programming. GE has their own scheme, Schneider Electric has one, Allen Bradley, Crouzet, and many others each have their own PLC languages.

Concept is a popular one, ProWorX is also used. Generally in real life only engineers are allowed to do this. Technicians are allowed to do some stuff but more and more ITS takes over so technicians are left with installation and field calibration. Calling them potentiometer tweakers. Electricians sometimes do this work as well depending.

http://www.kontrolkalemi.com/forum/elektrik-motorlari/52842-ptc-termistor-role.html & PLC

An economical way to get several thermistor inputs is to use a multiplexed input module, with a common interface such as Modbus RTU. An example is http://www.icpdas-usa.com/products.php?PID=494 . It has six channels wich comes to $70/channel at the price of $419.

Thank you for posting this. It is good information!