Skip.

Just because the thermocouple is producing a voltage, doesn't mean it
isn't damaged in some way. Do you still get the error message if you
short out the thermocouple??

I'd look initially at a simple relay control with adjustable duty
cycle and an external thermometer of some type (eg a known
thermocouple).
You can then work out the on/off ratio to maintain a given
temperature. The cycle time could be a minute or two so you wouldn't
need anything too elaborate - a 555 timer should be able to be
configured to do the job.

This should give you a bit more time to come up with a more elegant
solution, and allow you to work out the characteristics of the unknown
thermocouple.


Richard P


2008/9/26 Dr Skip <drskip@gmail.com>:
> I'm having a creative thought blockage to what should be an easy EE problem....
>
> I have an kitchen oven (with range top) that is electronically controlled and
> 20 years old. It consists of a control board with the only real chip on it
> looking like it's the display controller, not a general micro. It has the
> thermocouple input, 3v and 23v AC inputs and 4 outputs to control a relay board
> for 2 elements, a fan, and a door lock (self clean).
>
> Early on the unit was flaky, often quiting with odd codes. Kicking it would
> help... Turned out a row of solder joints were cracked on the connector of the
> relay board. After resoldering, all has been fine for years. Seems it gets hot
> in there. ;) Until yesterday. The error code during use implied a bad
> controller board once, but that was just once. Even cold, it now says the
> thermocouple is out of range. It isn't, I tested. Wiring is fine too.
>
> I'm also getting some flickering segments on the display. It is probably the
> controller, although nothing seems visibly awry. No board schematics, and it's
> several boards mounted in a sandwich with almost all discretes.
>
> Before one thinks I'm looking for a repairman, I've always thought of
> re-designing the controller, since it has a separate relay board and low
> voltage control to it. I haven't had a chance to study the extras, like cycling
> during self cleaning, etc. Unfortunately, now it HAS to be done and time is
> important.
>
> So the question: given I've got a good thermocouple, but without knowing its
> temperature curve, and that I can get at controlling the elements, what would
> be the quickest, simplest, most creative way one could control oven temp with
> these resources? My PIC skills are not fast enough, and the time pressure has
> blunted my creativity. I'm hoping I'm overlooking a simple analog design that
> will allow me to control the elements to temperature using the current
> thermocouple or another component (to 550F approx or better). Time is urgent it
> seems (a hungry family), so parts count and availability are important.
>
> Any ideas?
>
> TIA, Skip
>
>
>
> --
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist