Temperature control

[Amanullah47]

Hi everyone,

I want to control my heater to start/stop by using NI USB 6211. I have the following things

Cartridge heater Voltage: 12V  power 40W

Thermistor:NTC 100K B=3950

I need to know how to wire these things with NI USB 6211 and is there anything i need to buy?

If it is possible just give me hints how to Control the temperature of the heater cartridge

[GerdW]

Hi Amanullah,

I need to know how to wire these things with NI USB 6211 and is there anything i need to buy?

You cannot provide enough power for your heater directly from your USB device so you will need an external power supply and a relais to drive the heater. As the USB6211 cannot measure resistance directly you need to convert your thermistor resistance into a voltage signal: you may start with a simple voltage divider…

If it is possible just give me hints how to Control the temperature of the heater cartridge

Simplest would be to switch the heater on, when it's too cold and switch off, when it's warmer then required…

This all sounds like homework for a electrics/electronics student. Do you have any knowledge in this area of science? Have you done any paper reading before?

As this is a LabVIEW forum you should provide the VIs you already have made and ask for specific problems with these VIs - we will be happy to help you then!

[Amanullah47]

Hi GerdW,

Thanks a lot for your reply. I am using other external power supply for the CARTRIDGE HEATER. But my problem is 

I want to use the heater for up to 200 C and measure that temperature using this thermistor. Finally, I want also control the temperature. That means, there will be a pre-set value (say, 150 C) and my DAQ will measure the temperature and when the temp goes down to 150C HEATER will be activated. Similarly, when temp goes up beyond 150C, HEATER will be de-activated.

For this special issue should I buy anything. In Arduino forum I got this similar problem. And 1 person give the solution like this

"You will need a suitable relay or MOSFET in order to control the heater cartridge.For regulation you can either use on/off regulating or use a PID regulator"

[GerdW]

Hi Amanullah,

You will need a suitable relay or MOSFET in order to control the heater cartridge.

Like I said: you need a relay to switch the heater on/off.

You can use SSRs, there are some capable to switch with just a TTL input like yours from your USB6211…

For regulation you can either use on/off regulating or use a PID regulator

I just suggested the "simplest" method of controlling the temperature. A PID control usually works much better once you found the correct PID gains…

my problem is I want to use the heater for up to 200 C and measure that temperature using this thermistor.

What's the accuracy of the Thermistor at T>100°C? (The resistance deviations are rather small for T>100°C!)

[Amanullah47]

Hi, 

I got this specification with the thermistor

SPECIFICATION
R25	100K±1%
B Value(R25/50)	3950K 1%
Dissipation Factor(mW/)	1.1~1.6 In still Air
Thermal Time Constant(S)	10~17 In Still Air
Operating temperature range	(-40~+250°C)
Dimension	1.8mm Dia X4.1mm
Lead Length	30mm

[GerdW]

Hi Amanullah,

plot the resistance vs. temperature curve for your thermistor. Then calculate the voltage you will read when using a voltage divider with some pull-up resistor. Then decide on your own if the voltage changes are still ok to build up a temperature control for T>100°C…

I made an example for you:

You can use Excel too, but as we are in a LabVIEW forum I used LabVIEW…

From 25°C to 100°C resistance changes from 100kOhm to ~7kOhm, but from 130°C to 200°C the resitance change is quite small with 3kOhm to 0.8kOhm…

General recommendation:

Do some calculations before building your hardware. It will help to verify the hardware specification with the requirements you need to fulfill! (That's basic engineering…)