Sensing temp without transmitter to PLC input? 1

[dieselvette]

Anyone know of a temperature probe with mA or V output, that doesn't require an $80 transmitter? The purpose here is to read a temperature 60-150 degF and send it to an analog PLC input.

Must be a way to do this without spending big bucks.

After all, I have a temp sensor in my house with digital readout that costed $10!

Nic Van Engen
Electrical Technician

 

[nbucska]

If you can build and calibrate it, you shoud be able to build one for a few $-s

Plesae read FAQ240-1032
My WEB: <

http://geocities.com/nbucska/>

 

[JLSeagull]

Many PLC and DCS companies have analog input modules that accept thermocouple and RTD elements. These are especially desirable for projects with lots of temperature inputs. I am a dinosaur and prefer to buy the transmitters.

 

[dieselvette]

more specifically:
accuracy of 2% per 100 degF and precision of 1 degF increments is sufficient.

Repeatability & consistency is more important. The sensor will never be calibrated or tested for accuracy (after intitial testing & production) so it needs to stay within the 2% accuracy as long as the equipment is in service (many many years).

I've found after shopping around for temp sensors that these kind of specs are easy to satisfy, it's input-ing the info to the PLC that is the challenge (as far as cost is concerned).

Nic Van Engen
Electrical Technician

 

[dieselvette]

JLSeagull:
The trouble is that a Pt-100 (thermocouple/RTD ready) analog module isn't any cheaper than a transmitter (as a general rule), and is capable of ONLY temperature inputs. Main advantage at that point would be asthetics & easier setup.

I agree that if I were in need of 2 or more temperature inputs, (or more PLC inputs altogether) that would be a good way to gain some and therefore MUCH more cost effective than a transmitter.

Nic Van Engen
Electrical Technician

 

[ScottyUK]

National's LM35 integrated temperature sensor is a good place to start. A bit of simple analogue circuitry and you have a fairly accurate sensor with an output you can interface to. Some application ideas at the back of the datasheet:

http://www.national.com/ds/LM/LM35.pdf

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Sometimes I only open my mouth to swap feet...

 

[Skogsgurra]

If you are looking for high output voltage without any electronics, low cost, ruggedness and if you do not need very high precision - an NTC thermistor is just right.

I have been using them in white-goods like bath-room fans, in industrial equipment lige spot-welders and a few other products that I have designed over the years.

They are non-linear, yes. But linearization to better than +/-2 degrees celsius is possible (it will cost you a simple resistor) over your temperature range. The output voltage can usually be taken to several volts without introducing self-generated heat errors.

This gives you some ideas:

http://www.users.bigpond.net.au/tmchong/ECS_webpage/index.html

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Skogsgurra]

If you need good sensitivity, low cost, moderate accuracy and robust temperature sensors, then NTC thermistors are hard to beat. I have used them in white-goods like bath-room fans, in spot welders and battery chargers that I have designed over the years.

Yes. They are non-linear. But it doesn't cost you more than a simple resistor to have the +/-2 degree celsius you need over that temperature range. It will, in fact, be better than that.

The output voltage can easily be in the "several volts" range without introducing self-heating errors. The output signal can be filtered with an easy capacitor across your PLC input (no resistor needed, the NTC+R is the resistor).

They exist in many shapes. There are even some in teflon and stainless steel housings. And still at a low low cost.

http://www.users.bigpond.net.au/tmchong/ECS_webpage/index.html

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Skogsgurra]

Huh? I thought that first post was lost forever. So I wrote a new one. Have your pick!

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...