NTC Thermistor for Temperature Control

Have you searched the internet? What have you found so far?

yes, i have searched a lot and found a lot of types like this http://www.amwei.com/sort.asp?sort_id=7 but I really don't know which one will i use with plc i need some thing like probe you know it will be connected to plc not to pcb . and i think all i have in my country just epoxy thermistor it's small like ceramic capacitor .

The resistance of a thermistor changes with temperature. It is a passive device.

PLCs typically require a 4-20mA or 0-10VDC input. The thermistor will not achieve this directly. What you need is a transmitter.

You could build one yourself: http://www.thermistor.com/references/420ma.pdf

Or search for one to buy.

Is there a specific reason for using a thermistor?

What PLC are you using?

I don't need a transmitter . it's really resistor but it's connected to voltage supply and the output is taken from the resistor or you can put it in voltage divider and take the output from the other resistor this is not a problem anyway

Thermistors are appealing because they're cheap, but their primary use is in OEM products where the engineering cost can be spread over large quantities. So one finds thermistors in HVAC because thermostats are produced in quantity, but rarely in industrial applications where one finds PLCs.

If you only need to change state at a given temperature, it might be easier to buy a thermostat and take its output into the PLC.

There's probably a PLC brand that has a thermistor card with an associated function block to convert resistance to temperature, but I do not know which brand that is. On the other hand, almost all PLCs have an RTD card that will provide a traceable accuracy equal to a thermistor, without the linearization issues and inverse temperature function of a thermistor.

If you need modulating control, the math in some PLCs allows for look-up table or characterization functions, but you're probably better off using an RTD temperature sensor or a thermocouple and the respective input card.

but thermistors have high accuracy and i need it . i am using fatek plc . and i will use it to control room temperature so if you give help it will be appreciated .

Let us clarify.... Do you intend to use a DC analog input or a digital input on this PLC? A digital input will not work without some external amplifier, but a DC analog input should work with a resistor/thermistor (so long as available DC supply is stable with time). Thermistors usually have a high resistance variation from sample to sample at the same temperature. Their strength is a big proportional change per degree. If you want high ACCURACY, RTDs are superior to thermistors and thermocouples.

i am using analog input , but this thermistor say another talk from it's chart in this site http://www.meas-spec.com/product/t_product.aspx?id=4282

there is small change in resistance with temperature.

i need high sensitivity i think thermistor is the best.

Buy a transmitter and use a 4-20mA analog input on the PLC

http://www.datexelusa.com/DAT4531C_Isolated_converter_Thermistor.html

why should I use a transmitter ? I don't need it, I will connect the thermistor directly to plc.but which one. the range of measurement form 0 to 40 `c

One cannot suggest without data. You know all these things, but we do not read minds!

1. What is DC input range of available analog input? Positive only or +/- ?
2. How many bits Analog/Digital converter does it have?
3. What DC supply is available?
4. Is DC supply free from ripple/noise/drift with temperature-time?

So far as I know, the domestic heating room thermostat has an operate/release difference of about 0.2 Celsius. You need about the same performance.

Do you wish to have a pre-calibrated system or is it good enough to have a "set on site" system? This has direct relation to accuracy and cost of sensor.

If you set on site you will at least need cup of hot water and an accurate thermometer, PLC programmer to read analog input digital value at required temperature. Also the necessary time!

why all these questions to suggest sensor?!.

1.from 0 to 32 +ve Dc . why will i need -ve here?

2.12 bits.

3.23volt.

4.yes it's pure Dc.

no problem with set on site.

so what's in your mind.

The modern way to read temperature accurately with a PIC is to use I2C devices.

Even if the PIC does not support I2C directly (many do), you only need three wires (one being ground, so its called a "two wire" interface), two resistors, two input/output pins and a short bit of code for instance.

Look at the LM75 sensor first, but many companies make similar devices, here is a data sheet:-

http://datasheets.maxim-ic.com/en/ds/LM75.pdf

Some of its properties:-

♦ SO (SOP) and μMAX® (μSOP) Packages

♦ I2C Bus Interface

♦ Separate Open-Drain OS Output Operates as Interrupt or Comparator/Thermostat Input

♦ Register Readback Capability

♦ Power-Up Defaults Permit Stand-Alone Operation as a Thermostat

♦ 3.0V to 5.5V Supply Voltage

♦ Low Operating Supply Current 250μA (typ), 1mA (max)

♦ 4μA (typ) Shutdown Mode Minimizes Power Consumption

♦ Up to Eight LM75s Can Be Connected to a Single Bus

♦ Pin- and/or Register-Compatible with Improved Performance Maxim Sensors Including MAX7500,

MAX6625, MAX6626, DS75LV, and DS7505

I hope this helps.

thank you ,but i will use plc so i don't like any ICs

Curiosity got the better of me, so I looked up "fatek plc" (post #6).

Guess what? They offer plug-in temperature measurement modules for their PLCs -- including one for NTC thermistors!

Spec sheet:http://fatek.com/

So, once again, it simply comes down to "RTFM". (Maybe it's time to update that acronym to RTFPDF?)

Is it necessary to use temperature module ? i think i can measure temperature without that module.

Just a suggestion...

Farnell part No 1299929 EPCOS, 5 kohm @ 25 Celsius, tolerance +/- 0.5 degree, self heating ~1 degree at 2.5V bias, 4% change/degree, £1-85 each.

http://uk.farnell.com/epcos/b57863s502f40/sensor-miniature-ntc-5k/dp/1299929

Farnell have hundreds of thermistors and a good search facility to find the one you want.

Is it necessary to use temperature module ? i think i can measure temperature without that module.

Well, you need either one of those modules, or some other box of electronics between the sensor and the PLC. You cannot simply connect a thermistor, RTD, or thermocouple directly to the PLC and expect anything useful.

Please, please, please, spend some time reading the information on Omega's website. It's very useful for learning how sensors function, how to choose the proper sensor for each application, etc.

http://www.omega.com/temperature/Z/zsection.asp

if i couldn't connect thermistor directly to plc ,what is this http://www.tri-plc.com/appnotes/F-series/ThermistorSensorFPLC.zip

if i couldn't connect thermistor directly to plc ,what is this http://www.tri-plc.com/appnotes/F-series/ThermistorSensorFPLC.zip

The summary for "this" says:

"This application note explains how to physically connect a 10K Precon thermistor to the PLC using the PLCs on board ADC as well as how to use a look up table and interpolation method to calculate accurate temperature readings from the non-linear thermistor inputs."

The zip file contains an example of the lookup table and instructions on building a voltage-referenced resistor divider network for connecting a thermistor to an ADC (analog digital converter) input, which their PLC models have on-board.

Does the PLC you are using have an available ADC input?

So, one last time, you need either a temperature measurement module, or some other box of electronics (the resistor divider network) between the temperature sensor and the PLC.

Are we done yet?

I told you many times I have analog input cardfor the plc . and i shared trhis link to see the thermistor connected directly to plc ,isn't it?

So, I suggested cheap, 0.5 degree accuracy thermistor in post #21 [and where to search for suitable types].

The Triangle Research Application note in post #23 (by Mado himself) tells everything about how to use it.

Is there still a problem?

67model

Actually, if you "know everything" about how to do this, why are you posting here in the first place? Just go an do it.....

OH!! You want help from us!! And we are supposed to remember EVERYTHING you post!!! I simply forgot that........How forgetful of me.......

Goodness gracious me........

Try being a tad more friendly please to great CR4 people who are ONLY trying to assist you further!!!!

Please note, for next post, Mado. If at the very beginning you had written, for example......

1. I have an 0 - +10 V range A/D with 12 bits.
2. I have +23 V supply available to energise a sensor.
3. Accuracy +/- 1 Celsius and resolution 0.1 Celsius is required for room temperature sensor.
4. Can anyone suggest a suitable thermistor sensor?
5. Has anyone used a similar method and how well did it work?
   

You would not have had to write several posts to tell us that information.

Nor would your information have been lost in 25 posts, as we try to find out how much you already know, and what you actually want.

67model

ok, thank you every body for your help.