On Sun, 07 May 2006 17:59:45 +0200 Bill Sloman <bill.sloman@xxxxxxxxx> wrote: > Your original idea strikes me as extremely sound. Isolating the Pt100 > sensor, on the other hand, strikes me as a really bad idea. Well, I was thinking about that this solution won't work either. After all, I don't want to control the temperature of the PT100, but the PCB. > What you actually want to control is the temperature of the board you > are reflowing. That's true. That's way I put the PT100 on a PCB. I guess the problem with it is that there is no copper on that pcb. So it's not a good model of a PCB reflowed. > Putting the sensor onto a significant lump of printed circuit board - > something that is at least ten times longer and wider than it is > thick - seems to be to be pretty much essential if you want your > sensor to be sensing a temperature that is anywhere near the > temperature of the board to be reflowed. Again, I wanted to model the PCB reflowed. > Such a sensor will necessarily react more slowly to changes in > infra-red power input than a bare sensor, so you are going to need a > proportional/integral/derivative controller to get a stable and quick I guess the PID is the magic word for me. Believe or not, my room mate has a degree in automatics, and he was not able to help me. I'm a telecom engineer. So far, what I did is incrementing my output PWM when the temperature was low, and set the power to 0, when it was too high. > control loop. You might also need to linearise the voltage output of > the control amplifier to get a roughly linear relationship between > error signal at the input and corrective temperature rise at the > sensor, bearing in mind that power dissipation in the infra-red > source is proportional to the square of the applied voltage, its > temperature rise (if dominated by convection) will thus be roughly > proportional to the applied voltage, while its radiated output > increases as the fourth power of its absolute temperature. Indeed. That's basic electronics. > Messy physics ... No, it's very interesting for me. Thank you very much for the overview! Next week, I'll be busy implementing the PID control. Levente -- http://web.interware.hu/lekovacs
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