Xander_M

50+ Thermistors on PLC

7 posts in this topic

I am trying to build something (not for discussion) which involves using 50+ NTC resistors using a PLC to measure and do some other stuff.

My question is, does anybody have an idea of how to deal with the high amount of individual NTC resistors? Of course, they need to be amplified and measured by the PLC directly or a device in between which converts it to digital. I have absolutely no idea in what way to connect the resistors to the PLC, does anybody have ideas?

Let me know!

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well... get appropriate number of inputs that can either connect directly to thermistors or to thermistor transducers. at 50, i would prefer direct connection. another option is multiplexing if you don't need continuous reading (in general, temperature is a slow changing process).

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16 hours ago, panic mode said:

well... get appropriate number of inputs that can either connect directly to thermistors or to thermistor transducers. at 50, i would prefer direct connection. another option is multiplexing if you don't need continuous reading (in general, temperature is a slow changing process).

Multiplexing is the best I guess, using a microcontroller with a good ADC and adding a thermistor amplifier.

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Presumably you want temperature units.

There are not a lot of resistance-to-4-20ma transmitters that linearize to a thermistor curve.  The one I know of is Acromag's TT230, a single channel thermistor transmitter that has "Customizable thermistor linearization table with preset curves for popular resistances".  

A PAC I use has an 8 channel RTD input card that has several straight resistance input ranges, 0-200, 0-500, 0-1000, 0-2000, and 0-4000 ohms with programmable look-up table functionality for conversion to temperature units.

I suspect that most RTD input cards have some configurable resistance input option, but you'd have to investigate.

Considerations when using a multiplexor
Synchronizing an external multiplexor to the input scan of a PLC could be a challenge if the Mux is not driven by an external pulse.   A mux's solid state switch (as opposed to dry contact switching) might not be designed for switching resistance and affect the measurement.

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you might look at pyromation, they have transmitter pucks that are software configured(connect resistive device to puck), i only know of single channel, we had hundreds of them at one place i worked, they were great easy to configure and you end up with configuration files saved for each device, so if they need replaced just download original file, done.

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Procon Electronic's(https://www.proconel.com/product/pt8...dule-isolated/)We use this, I did not set it up so I don't know what all you can do but we pull over ethernet 8 TC values to our scada system. Also Check out (https://www.mccdaq.com/temperature-measurement.aspx)CBCOM CB-7018 we use to use it, it is RS-485, they have units with more than 8 channels, and other devices that do similar function, again I did not setup so I don't know what all you can do. Also pyromation has devices, we called puck transmitters for temp sensor thermocouples and RTD's etc. 

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