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'[EE] creative fix?'
2008\09\25@180315 by Dr Skip

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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



2008\09\25@182440 by Richard Prosser

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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 <spam_OUTdrskipTakeThisOuTspamgmail.com>:
{Quote hidden}

> -

2008\09\25@182953 by Grant Brown

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Easy answer,   Go buy a new oven.  ;-)

Grant

Dr Skip wrote:
{Quote hidden}

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Grant Brown

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2008\09\25@183301 by Danny Miller

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That thermocouple would be either Type J or Type K.  The voltage to temp
profiles are inherent to the type and not dependent on the mfg.  You can
look them up.

The voltage is low and requires a low offset amplifier to read.

A thermocouple creates a voltage based only on a temp difference between
the hot junction and the "cold junction" where the special alloy wires
meet copper wires or PCB traces.  Typically the controller has a
thermistor or diode or whatever to read the absolute cold side temp to
accurately gauge what the hot side temp is.  Otherwise you have to just
assume the cold side is 72F or whatever which may be far off at times.  
The board heats up from the oven heat and the room temp will fluctuate too.

Danny

Dr Skip wrote:
{Quote hidden}

2008\09\25@202214 by Forrest W Christian

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Dr Skip wrote:
> 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.
>  
Put a 'scope on the power supply rails and see if you're getting some
ripple.   My guess is a bad electrolytic somewhere.

-forrest

2008\09\25@210521 by Apptech

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> Even cold, it now says the
> thermocouple is out of range. It isn't, I tested. Wiring is fine too.

Any chance that's there's a cold junction reference that has died, making
all values look off scale?


  Russell

2008\09\25@210733 by Mark Scoville

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> Dr Skip wrote:
> > 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.
> >  
> Put a 'scope on the power supply rails and see if you're getting some
> ripple.   My guess is a bad electrolytic somewhere.
>
> -forrest

Or if you don't have a scope just measure the DC rails with a multimeter set
to mV AC. You won't be able to "see" the waveshape, but if you measure
something with the meter set for AC you've likely got some ripple (and
probably a bad elctrolytic like Forrest suggests)

-- Mark


2008\09\25@230418 by Dr Skip

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Good idea. I'm no expert in thermocouples, and that's what the manuals say it
is, but it's measured in ohms, not volts, and the resistance changes with temp.
It seems to be fine per the test specs.

Everything is clean, and reseating a few times didn't change anything, but in
going to check the ripple (non present BTW) it came up nice and stable and
bright and now is working again. No visible solder flaws, so maybe it's just
bad connector contact. It took a lot of re-insertions though. I'll use the time
to come up with specs for the probe and driving the relays now. :)

Thanks for the help. I'll probably make a 555 board just in case. ;)

And as for just buying a new one - that's really not the EE spirit, is it now? ;)


Thanks again.
Skip



Richard Prosser wrote:
{Quote hidden}

2008\09\25@231650 by jim

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A thermocouple generates a voltage as a result of the interaction of heat
being applied to ma junction of dissimiliar metals.

If you are measuring resistance, then you don't have a thermocouple.  You
probably have an RTD,
ie...Resistive Thermal Device.

Jim

{Original Message removed}

2008\09\29@155232 by Clint Sharp

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In message <48DC0A8A.4030000spamKILLspamgmail.com>, Dr Skip <.....drskipKILLspamspam.....gmail.com>
writes
> Time is urgent it
>seems (a hungry family), so parts count and availability are important.
>
>Any ideas?
>
>TIA, Skip
>
>
>
Given that the board is years old and gets warm, I'd be considering
doing a blanket replacement of all the electrolytics before I even
considered designing a replacement. That's likely to take under half an
hour and if it works should stop the family having to forage for nuts
and berries. Once you have the original controller working again then
you can document and study it.
--
Clint Sharp

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