s7-1200 and NTC

[rQx]

Hi!

I'm not to familiar with temperature sensors but I have this cooling machine and the guy who will deliver it wants us to control the defrost cycles. He said he will provide NTC sensors that I can use. I don't recall there beeing any NTC or PTC options avaliable for the s7-1200 or am I mistaken?

/Tim

 

[nonuke]

The S7-1200 CPU's only accept 0-10V analog input.
Just ask the guy to supply a PT100 temperature probe. With a PT100/4-20mA converter, you can convert the signal to 4-20mA. Just add a 500 ohm resistor across the 4-20mA terminals to convert the signal to 2-10V that you can connect to the Siemens CPU.

 

[rQx]

The S7-1200 CPU's only accept 0-10V analog input.
Just ask the guy to supply a PT100 temperature probe. With a PT100/4-20mA converter, you can convert the signal to 4-20mA. Just add a 500 ohm resistor across the 4-20mA terminals to convert the signal to 2-10V that you can connect to the Siemens CPU.

I can buy signal modules for other temperature sensors, I have used PT100 before and then I used a module to input it directly to my PLC, no need for resistors

 

[nonuke]

You only need a resistor if the sensor is 4-20mA and the PLC analog input is 0-10V. This is the case of the analog inputs of the S7-1200 CPU. The S7-1200 also has 4-20mA cards, but if you want to use the analog inputs in the cpu, you need the resistor (don't need to buy a card).

 

[rQx]

I will have four of these sensors and the cabinet will be brand new so I buy signal modules that's required for my application. Thanks for the tip though

 

[BryanG]

NTC and PTC does't tell you what type of sensor the guy will be supplying, it only tells you how it responds to change in temperature. PTC - positive temperature coefficient, NTC - negative temperature coefficient. You need to know what technology the sensor will be. In terms of how the PLC will use the signal you might just need to invert the scaling. As you know you can get Thermocouple and PT100 signal modules for the S7-1200. It is late on Friday and my head is a bit fuzzy but if the guy offers Thermistors then be careful, they aren't linear output so you would have to do extra maths.

 

[kallileo]

This a job for a HVAC controller to which NTC probes can connected directly like the new Schneider M171 series.
Our company mostly uses Carel and Dixell (Emerson) HVAC controllers but you need to buy the programming software as well.

This guy was able to use thermistor with a S7-1200 RTD input module.
http://www.plcengineers.com/sm1231rtd.html

 

[rQx]

Ok, so PTC and NTC is just a name for many sensors in terms of how they act?
A Pt100 can be a PTC for example?
I can simply tell him to use Pt100, this is just what he suggested and what they regulary use, but since I have the authority to tell him what to use (since I have the PLC ) I can tell him to use a Pt100 for example. This would be mush simpler.

 

[Lare]

Ok, so PTC and NTC is just a name for many sensors in terms of how they act?

Yeah, it only says how resistance works

PTC = Positive Temperature Coefficient (resistance increases when temparature raises)
NTC = Negative Temperature Coefficient (resistance decreases when temparature raises)

A Pt100 can be a PTC for example?

PT100 is allways PTC resistor, it's resistance will increase with temparature, name comes from point: 0'Celsius = 100 ohms.


http://en.wikipedia.org/wiki/Resistance_thermometer

PT100 and 4-20mA loops are common at industry, it depends of hardware if you can connect different types than these sensors. Usually plc have only 0-10V or 0/4-20mA analog inputs and for other type sensors you need signal converters or different type of analog cards, if it even exist.

 

[rQx]

Ok, so I start to get a grip of this. I need to know more about the sensors he talks about. Simply knowing NTC wont be enough, I'll try to see if I can get him to use Pt100 (to use with special s7-1200 RTD module), or some sort of 4-20mA. Thanks all for the support!

 

[kallileo]

In HVAC most of the time NTC 10k@25 C sensors are used because they are very cheap, accurate, have fast temperature response and since their internal resistance is high cable length doesn't have an impact on the actual reading as with RTD sensors like pt100 or pt1000.
Unfortunately for most of the known PLC I know do not support NTC sensor while their are so widely used in HVAC.