At 12:23 7-5-2006, you wrote:
On Sat, 6 May 2006 16:25:24 -0500
"Phil Taylor" <[EMAIL PROTECTED]> wrote:
> Do you know if there is heating occurring during cool-down? If
there isn't,
> can you remove heat another way?
Thanks for the reply. The original idea was to have a piece of PCB,
and measure the temperature of the PCB, because the target will be a
PCB too. So I attached the pt100 to the pcb with some heat
conducting paste. I think this was a bad idea. I think I'll mill out
the area of the PCB where the pt100 mounted, so the only heat couple
will be is the pins of the pt100. I'll see if it helps or not...
Your original idea strikes me as extremely sound. Isolating the Pt100
sensor, on the other hand, strikes me as a really bad idea.
Your control loop consists of an infra-red heater irradiating your
Pt100 sensor, and a feedback loop that regulates the voltage across
your infra-red heater to maintain an infra-red flux that sustains the
desired temperature at your Pt100 sensor.
What you actually want to control is the temperature of the board you
are reflowing.
For a given infra-red flux, the equilibrium temperatures of your
sensor and of the board being reflowed are unlikely to be the same.
The heat flowing into the sensor depends not only on the infra-red
flux, but also on its absorbtion coefficient (1 - reflectivity) at
the wavelengths involved. This doesn't have to be the same as the
absorbtion coefficient of the board you want to reflow.
Heat flows out of the sensor and the board being reflowed by
conduction, convection and radiation. Conduction won't be an issue
for the relatively large board being reflowed, and probably won't
matter much for the sensor. Radiation isn't too important at the
temperatures in question - the board and sensor will be re-radiating
at much longer wavelengths than they are absorbing, so that while the
re-radiation coefficient would be the same as the absorbtion
coefficient at the same wavelengths, they don't have to be the same
in practice, not that the relatively small proportion of the heat
being lost by radiation will allow this to be a real problem.
Convection is the real problem.
The sensor is going to be much smaller than the board being reflowed,
which makes a difference. In practice, the convention currents around
the board being reflowed and the sensor are going to be determined by
the board, which is going to be much bigger than the sensor, so that
the sensor is likely to be more effectively cooled by forced
convention acting on the sides as well as the top and bottom of the
sensor than is the board, which hasn't got any significant side area.
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.
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
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.
Messy physics ...
--
Bill Sloman, Nijmegen