I would like to create a system with feedback that:
- monitors the temperature of the textile fabric material (it has 80+-40 ohm resistance)
- heats the material and maintaining its average temperature around a certain range (e.g. 40 degrees). It was necessary to supply about 4V (800mW) to bring it to 43°C

---------------------------------------------------------------------------------------------------------------------------------------------------------------------
1° idea:

With a microcontroller I supply Vi to a source current which, by adjusting Vi and R, determines the current that will flow in the conductive textile material.
The conductive textile material has 80+-40 ohm, so by adjusting the current flowing through it I can set the temperature of the material due to the Joule effect.
How do I measure/monitor its temperature? With a temperature sensor (NTC or RTD .. I don't know yet) that sends the measured values to the micro I used to supply the Vi. This way, I close the feedback loop
---------------------------------------------------------------------------------------------------------------------------------------------------------------------

2° idea: (as @Sensacell suggested)
Use a MOSFET and PWM.
@ericgibbs we could continue the conversation started here

 

Is this homework?

When you say Labview- does this mean the control system must be Labview? or just the command setpoint?
A linear Opamp is a overkill way to control a resistive heater load, why not just use a MOSFET and PWM?

 

Is this homework?

When you say Labview- does this mean the control system must be Labview? or just the command setpoint?
A linear Opamp is a overkill way to control a resistive heater load, why not just use a MOSFET and PWM?

It's a project I want to do on my own, I don't know if I can call it "homework".

On LABview I must surely display current, resistance, temperature variation in real-time etc. and, probably, the commands 'start', 'stop'.

You're right! I try to think of a solution with mosfets and PWM.

 

hi k89,
Do you know the resistance of the heating coil and its thermal resistance coefficient.?
How many Watts do you consider it needs to keep the wristband at the desired temperature versus the ambient temperature.
E

 

hi k89,
Do you know the resistance of the heating coil and its thermal resistance coefficient.?
How many Watts do you consider it needs to keep the wristband at the desired temperature versus the ambient temperature.
E

The only thing I know is that it has a resistance of 80+-30 ohms... I did some tests and to get to a temperature of 43°C... the corresponding power required was 775mW at 3.5V output from the power supply (and about 3.16V measured at the ends of the patch)

 

hi k89,
Do you know the resistance of the heating coil and its thermal resistance coefficient.?
How many Watts do you consider it needs to keep the wristband at the desired temperature versus the ambient temperature.
E

I think the easiest solution is this:

looking at the Vds-Id characteristic of any mosfet (if I keep it in saturation), depending on the Vgs value I supply it with ... the Id flowing on the drain (where the fabric material to be heated is) will vary ... even though I know that the maximum current will be about Vcc/80ohm (I hope it is enough to heat it up to 40°C).
By controlling the voltage on the gate with a signal from an Arduino, I will use a logic mosfet.

 

By controlling the voltage on the gate with a signal from an Arduino, I will use a logic mosfet.

Switching PWM control is much more efficient, than trying to control the MOSFET gate in its linear mode (which can waste up to 50% of the power).

 

Switching PWM control is much more efficient, than trying to control the MOSFET gate in its linear mode (which can waste up to 50% of the power).

I have an IRLZ44N and a 24V power supply (I have about 240mA which is sufficient to heat the fabric material). I simulated that the power dissipation (see the image below) is about 1.5W ... I don't think it is necessary to put a heatsink and I read in the datasheet that the maximum power dissipation of this device is 110W.

The problem is this:
I would like to keep the temperature of the material in a certain range (for example between 45°C and 50°C) .. so as soon as I read (from an NTC attached to it) that the temperature has exceeded 50°C I turn off the mosfet .. as soon as it drops below 40°C I turn it back on .. then when it drops over 50°C I turn it back off again etc.
If this happens repeatedly those power peaks I have in simulation during the switching (when it has not yet reached saturation) become frequent!
How frequent? I don't know, I'm still not familiar with the behaviour of this heating material.