> From:   Peter Homann[SMTP:peterh@ADACEL.COM.AU]

> I'm investigating using a cheap toaster oven as a SMD reflow oven
> for prototype boards.
>
> The idea is to use a PIC in provide an accurate temperature
> controller, possibly providing a programmable temperature profile
> capability.

I just designed a new toaster oven with a PIC at the heart of the
controller.  The requirement was for cheap, not for accurate, so I
cut a lot of corners on accuracy.    All the PIC is really doing is
analog comparing the thermistor voltage to a potentiometer voltage.

However, using a simple thermistor for feedback gives about +/- 5 C
temperature control, with a very simple non-PID control algorithm.
That's probably as good as you'll need.

I used a 16C620 and two of the analog comparators.  One analog
comparator is hooked to a FENWALL GEC series glass body thermistor,
400Kohms cold.  This thermistor is in parallel with a  68K resistor
which gioves a more linear response.  The FENWALL GEC series is good
to about 300 C, plenty for soldering.   I read the comparator once a
second and let it decide whether to turn on the elements.

You'll want more sophistication if you're doing ramping and PID.

You don't have to read the actual cavity temperature either.
Position the thermistor about 1/2 cm away from one of the interior
metal walls of the toaster oven.  It will read a lower temperature
than the interior, but will track very consistently and respond very
quickly.  This trick works for thermistors of lower temperature
ratings.

Don't bother trying to use a thermocouple unless you need +/- 4C
accuracy and are willing to spend $30 US to get it.

To implement real PID you'd need to go up to one of the parts that
have a real analog input

Let me know how you progress!

-- Lawrence Lile

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http://members.sockets.net/~llile/index.htm