Wagner Lipnharski wrote:
> The idea of the regulator is the automated SMD
> soldering profile.  Press [start] and wait for the beep to remove the
> boards.  The microcontroller also consider 3 minutes delay after complete
> power shutdown, to reduce boards temperature naturally without stressing
> components by a possible fast temp break down (fast door open).  The total
> operation takes 10 minutes; 4min=preheat, 2min=heat, 1min=melt,
> 3min=cooldown.

When I worked in the instrument fitters we maintained
a lot of heating ovens for baking steel product.

There is more to consider than just the electronic
control, the most important part is the thermal
characteristics of your cabinet and load (target PCB).

For instance, if you put your accurate sensor closely
coupled to the heating element, it will keep the element
at the right temp, BUT it may take considerable time
for the load temp to rise to match, and worse on
cool-down. It your sensor is too close to the load,
there is a large thermal lag at the element and thermal
ripple will be very high and you get overshoot problems
similar to what you show in your charts. A fan helps a lot,
as does 4 elements around the cavity (like my small oven)
and smaller cavity vs element power helps too.

With correctly considered thermal properties of the oven
you can get great temp regulation even with a crappy
mechanical thermostat. But with bad thermal properties
even a perfect controller might leave you with overshoot,
slow response, or hot/cold spots on your PCB.


> I will not bother to build a stainless steel oven, double wall, with a vent
> duct window system to control the cool down rate.  I believe that a well
> insulated oven can hold and distribute the temperature pretty well without
> much re-heat, mostly for a max of 4 minutes time.  This home-build oven
> doesn't need to be bigger than 15x22x8cm (6x8x3") inside (useful area).
> Heater elements could be anything well distributed, even plain
> Nickel-Chromium wire trying to cover most of the walls

Why not just buy a decent toaster oven? These are the
right size and a good one has the elements correctly
spaced and enough elements to give goo thermals.

The cool down can be done from the controller, I think
the oven will cool quick enough (thats why you don't want
extra insulation) that your controller can "ride" the
down curve adding a tiny bit of power and keep it exact. :o)


> How a force ventilation works inside an oven?  I was never lucky to see one
> yet. It is a small metal fan blades with a long shaft and external motor?

I have, mainly in microwave convection ovens. The fan
is a blower (tube) type, motor is on a shaft a few
inches away. Most appliance repairers will have a
heap of these junked around the place, a polite phone
call should get you one to play with. :o)
-Roman

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu