Making a self governing heat pad

is there a way to make a self regulating heat pad that will not exceed a certain set point and that will stay at a specific temperature while being powered wihtout using any type of microcontroller.

Well you can just use a bimetallic strip sensor, but they have noticable
hysteresis. How accurately do you want the temperature control, and
what are the thermal time constants of the system roughly?

hi Im not sure if it was you a talked with earlier. I lost the thread

The pad I mentioned was actually controlled by embedded thermistors and hardware in the pad.

The control circuits for this can be quite simple and you should find examples on the web.

There is also an arduino shield that can use thermistors for measurement and pwm control. using that and the pid library could get you a close tolerance controller .

Alternatively ebay on PID controller. mine cost less than 10 and had a PWM output. They can be specced for either platinum resistance or most thermocouple and thermistor types.

example

http://www.ebay.co.uk/itm/85V-to-265V-PID-DIGITAL-TEMPERATURE-CONTROLLER-KILN-FURNACE-OVEN-COFFE-MACHINE-/371246237841?pt=UK_Home_Garden_Hearing_Cooling_Air&hash=item5670000891

Pad I saw actually had many of these distributed throughout the mat apparently in zones. lab gear though, cost an arm and leg.

Yes that was me that you talked to earlier however the device was far too bulky im looking for a thin flexible pad type heater that would be about the size of a golf ball. I'm just wondering now if i would be able to accomplish what i want without even using a micro controller. I will have to look into the control circuit and PID you were talking about thanks for the link too. I thought i may be able to accomplish this with an OP-AMP so im looking into that.

Sure, lots of temperature controllers have been built using simply an Op Amp or two.

If the heater has a lot of mass, the sensor is tightly coupled so as to limit overshoot, and you can tolerate a few degrees of hysteresis, a bang-bang controller will work fine.

PID can be built into hardware, too. Weird, huh? :slight_smile:

A bit more complex, an adjustable PID controller:

Brought to you by the late, great Robert Pease.

http://electronicdesign.com/analog/whats-all-p-i-d-stuff-anyhow

Hi yes thats pretty much what i was thinking about.

From the op post though i think the control he wanted was pretty tight on hysterisis.

Im not sure if a simple circuit could maat that criteria.

Use a PTC in series with the heater and couple it to the heated mass. It will self-regulate the power delivered.

// Per.

A PID controller, designed correctly, should be able to hold temperature pretty close. You have to specify how closely you need it held.