PTC Thermistor for motor temperature

<will we use any signal convertor that will change ohm value to digital or thermistor will give us a digital signal it self>

Clearer English will help.

Otherwise a complete Circuit Design Drg + PLC's input signal conditioning Instruction set.

Good Idea. However very expensive one as ADC is a special thing in PLC.

What is needed is a trip unit that converts PTC thermistor resistance to a switching contact that the PLC can receive. Check out the Lee Dickens range of 'alphabet block' signal conditioners (usual disclaimer) as something suitable will be found there.

Just checked Lee Dickens and there doesn't seem to be anything immediately applicable, though they can custom-manufacture just about anything.

The next possibility is to get back to the motor manufacturer and see what they recommend.

You need to know the followings for the PTC thermistor. Its power rating, temperature switching point and minimum and maximum resistance change near the temperature switching point.

Assuming that the thermistor will switch between 240 ohms to 100k Ohms or higher resistance value.

24V power connected to thermistor in series with relay of similar resistance will pass 50mA current through coil and will have 12V drop across thermistor. Hence, take a 12V 50mA coil current relay SPDT type and connect the coil in series with thermistor to 24V standard industrial DC power supply.

Relay will remain ON most of the time and will cause some amount of heating in thermistor (0.6W) which will be dissipated in motor coil. If motor heats up then thermistor resistance will go up and relay will become OFF. Potential free contacts of relay can drive your PLC input.

If you wish to connect thermistor to PLC relay input then use slightly greater voltage for the thermistor in the range of 48V for 24V PLC relay input sense coils.

If you have opto isolators in your PLC for sensing then be very careful not to exceed the input voltage and current of the opto isolator else they will get permanently damaged due to over current. Read the PLC catalog before you start connecting anything to it.

Ask for more details if you still have problem in understanding.

Very good and practical solution.

I believe an example of a diagram would help

This is a simple interface to PLC through PTC thermistor. You have to use 24V or 36V or 48V excitation voltage depending on the relay coil rating. 100mA relay sense coil will require 48V power.

For Opto Isolator input use only 6V, 12V or 24V as permitted by the opto isolator input scheme of your PLC and also watch out for input polarity. Relay does not require polarity but opto isolator have polarity to input LED.

This scheme is also useful for PNP type input stage of the PLC but not for NPN type input stage. You can take DC power output of your PLC to interface.

For NPN type input stage simple connect the thermistor to input and ground and no external power required.

It is also possible to make PTC output as 4-20mA if your PLC is located at far point. I need the response curve of the PTC to give you such proper design for direct use.

If you require a digital input signal to your PLC use a thermistor relay such as supplied by www.dold.co.uk part Nos MK9163, BA9038 or MK9039.

These units also eliminate the resistance of the connecting cables

As previously stated... more background information is needed.

So basically, by needing to convert the thermistor output to a digital out... you just need to turn on an input if the motor reaches a specific temperature? I'd say Shyam has the right idea.

If you want to measure temperature (analog input)... i'd say go with a thermocouple.

There are many ways to do this.

1. simple resistor pair:

V---R---(sense)--Therm ----- gnd.

at node sense, depending upon Thermistor characteristics, you will get voltage swing with minimum resistance coupled to V (supply). This is the 5 cent solution.

You may need more:

use transistor PNP with thermistor pulling up to supply and suitable resistor pulling down to gnd. Again we assure no excessive current through thermistor.

You may need more:

Use comparator for higher input impedance.

In all cases use filter caps since this is near a motor and noise may be a problem.

Write for more info if you need it.

You can also use resistance to frequency converter if your PLC has a frequency counter. I prefer such measurement with uC based embedded systems.

I want to measure temperature of room with ntc thermistor and control it with plc.

want some help in choosing the thermistor accurate type.

thanks.

Hi mado,

The following links may help

http://www.omega.com/kwbld/ntcthermistorsensor.html

http://www.omega.com/prodinfo/thermistor.html

Shyam

Hi Everyone

Sorry to bump this 10 year old thread...

Has anyone wired PTC thermistor to a PLC analog voltage input using a voltage divider to provide a known voltage trip level.

This is how PTCs are wired on VSDs. I.e. see Rockwell Powerflex 525 VSD wiring.

Cheers, Michael.