Help ... set point scaling

[Shohadawy]

I want to enter a set point by a potentiometer to a PIC16F877. This set point should order the pic to order a heater to heat to the temperaure corresponding to the potentiometer voltage ... how can I scale the potentiometer output so that each voltage value that the potentiometer gives become corresponding to a temperature that the thermocouple measures ... Note that the thermocouple gives 41 microvolt per degree centigrade

 

[ericgibbs]

I want to enter a set point by a potentiometer to a PIC16F877. This set point should order the pic to order a heater to heat to the temperaure corresponding to the potentiometer voltage ... how can I scale the potentiometer output so that each voltage value that the potentiometer gives become corresponding to a temperature that the thermocouple measures ... Note that the thermocouple gives 41 microvolt per degree centigrade

hi,
The way I read your post is:
You want to set a 'temperature' value using a pot connected to a PIC's analog input?
When the thermocouple output is less than the set temp, the heater is switched ON.
When the thermocouple output is equal to or greater than the set temp, the heater is switched OFF.?

You will have to amplifiy the thermocouple output, so that the maximum temp range of interest gives say +5V. It will also be required to 'offset' the output of the amplified signal.

The amplified/offset thermocouple signal could be fed to a second analog input of the PIC. The PIC program would be written so that you get the temp range of the setpot and the thermocouple to track over the range of interest.

By comparing the two analog inputs it is possible control a relay/power transistor which in turn is connected to the heater.

Does this help.?

 

[Shohadawy]

You've understood me ... but how can I calibrate the voltage of the potentiometer sothat the PIC inderstands that it corresponds to a temperature say 50

 

[ericgibbs]

You've understood me ... but how can I calibrate the voltage of the potentiometer sothat the PIC inderstands that it corresponds to a temperature say 50

hi,
Its done within the PIC program.
Say the set pot is on analog channel 0.
Assume that you use the PIC's internal +Vref [+5v] and that you use both the analog high and low registers, this will give a 10bit resolution ie; a decimal value range of 0 thru 1023, for 0 thru +5V from the set pot.

If you require the setpot to set a tempr of say, 0 thru 100degC, a setting of 50degC would correspond to a voltage of +2.5V on the setpot. [adc value of512]

If you have amplified/offset the the thermocouple signal to span from 0 thru 100degC, then the output of the thermo amp would be +2.5V at 50degC.
Say this signal is connected to analog channel 1.

The PIC program would test for less/greater signals between chan 0 and chan 1 and take the required action with the heater.

Does this help?

 

[Shohadawy]

Yes, that's enough thanks alot