Temperature control using a PID contoller.

[D.M.]

The picture attached is my schematic.

I use the op-amp as a comparator to sense temperature and as the NTC thermistor's temperature increases its resistance decreases thus making the voltage at V(-) larger. When V(-) reaches a high enough temperature the op-amp will toggle "off" and turn the heater off.

I plan on changing the temperature level in which the comparator toggles "off" by changing the voltage at Vref via a digital to analog converter. I plan on using the outputs from the PID to create a boolean number into the D/A to make a voltage at Vref. As Vref changes the "toggle" point for the comparator will change.

**broken link removed**
Questions:

Will the PID control the D/A converter in the manner i have chosen?
Is that a wise approach in changing the desired temperature?

Do i need 21 different outputs from the PID to operate 3 seven segment displays?

Any help/feedback is appreciated. This is one of my first practical applications. I decided to try to use my engineering knowledge for at home purposes.

Thanks in advance.

 

[mneary]

Putting a DAC and a comparator together like that is the same as making a crude ADC. Most modern microcontrollers already have ADCs built in, which are easier to use and usually more accurate than a DAC you can build.

 

[MikeMl]

I agree with mneary. Just set the OpAmp gain/offset to span the range of the controller's ADC. Most uControllers have an ADC with sufficient resolution that you can do the entire PID implementation in software.

 

[D.M.]

ahh been busy with school and havent had time to be on here much.

ok, so i already looked into the whole DAC from my cntroller. The thing is when i connect it to my scope the signal is PWM and didnt think that would work well for the Vref on the comparator.. I was thinkin i need an actual Analog Voltage to be at Vref, otherwise the comapartor would toggle on off repeatedly because it would go High for a split second and off for another periodically depending on the value i wish to output.

Am I to believe that the DAC output will actually sustain a votlage at Vref in which would allow my compartor to operate as planned?

Was this clear enough to understand my concern?

Thanks again for any help.

 

[mneary]

Yes, it's true that a DAC can sustain a voltage. But what we're suggesting is to get rid of the DAC and comparator and use a micro that already contains an ADC that will measure the input directly.