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PICList Thread
'Dual slope converters'
1998\02\24@065455 by Leo van Loon

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I am planning to build a controller for a ceramic kiln.
I try to convert thermocouple mV with a TL501 dual slope converter that has
an accuracy of 0,05% FS. This converter is usually combined with and
controlled by a TL503 digital processor to obtain 41/2 digit BCD data.
Is there any experience (and code) for the control of this type of A/D
converters with PIC microcontrollers?

Leo van Loon
SBB simpeltronics
Netherlands
tel +31 (0481) 450034
fax+31 (0481) 450051
mail spam_OUTsbb.simpeltronTakeThisOuTspamtip.nl
url http://www.sbb-simpeltron.nl
SBB simpeltronics ontwikkelt technische projecten voor basisschool en
basisvorming.
SBB simpeltronics develops technical projects for children in primairy and
secondairy education.

1998\02\24@094348 by Joe Little

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    I use an AD595 from analog devices to convert the thermocouple output into
    something that the A/d can use.  The AD595 does the important job of 'cold
    junction compensation'.  Thermocouples generate a voltage proportonal to
    the temperature where its' two dissimilar metals are joined..... They
    generate this voltage at the end of the wire where you want to measure, but
    they also generate voltages where each of the wires are connected to the
    electronics.  To make it more fun, the temperature at the electronics
    causes these voltages to vary. Since the metals are different... Anyway the
    AD595 takes care of all this.

    The benefits of a super accurate A/D will be lost without the right kind of
    connectors, and cold junction compensation.   Anyone know the magnitude of
    these errors, and any ways to compensate for them without a 10 dollar chip?


    -----------
    I am planning to build a controller for a ceramic kiln.
    I try to convert thermocouple mV with a TL501 dual slope converter that has
    an accuracy of 0,05% FS. This converter is usually combined with and
    controlled by a TL503 digital processor to obtain 41/2 digit BCD data.
    Is there any experience (and code) for the control of this type of A/D
    converters with PIC microcontrollers?

    Leo van Loon

1998\02\24@121423 by lilel

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Leo van Loon wrote:


> I am planning to build a controller for a ceramic kiln.
> I try to convert thermocouple mV with a TL501 dual slope converter
> that has an accuracy of 0,05% FS.

Who makes the TL501 - Texas Instruments???

I'm very curious about how you get the thermocouple signal into this
chip.  I have heard you have to connect thermocouples with particular
metals and sometimes use cold junctions and other complex and
expensive measures.  How are you handling these issues?  Is the TL501
specialized for thermocouples or is it simply a general purpose A/D?




Best Regards,

Lawrence Lile

1998\02\24@143002 by Leo van Loon

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The AD595 comes before the A/D converter. I'll use one like that.
The measurement of the temperature in the kiln needs an accuracy better
than1%. The TL501 is the cheapest, cause its on the shelf.

-----Oorspronkelijk bericht-----
Van: Joe Little <.....Joe.LittleKILLspamspam@spam@SCIATL.COM>
Aan: PICLISTspamKILLspamMITVMA.MIT.EDU <.....PICLISTKILLspamspam.....MITVMA.MIT.EDU>
Datum: dinsdag 24 februari 1998 15:48
Onderwerp: Re: Dual slope converters


    I use an AD595 from analog devices to convert the thermocouple output
into
    something that the A/d can use.  The AD595 does the important job of
'cold
    junction compensation'.  Thermocouples generate a voltage proportonal
to
    the temperature where its' two dissimilar metals are joined..... They
    generate this voltage at the end of the wire where you want to measure,
but
    they also generate voltages where each of the wires are connected to
the
    electronics.  To make it more fun, the temperature at the electronics
    causes these voltages to vary. Since the metals are different... Anyway
the
    AD595 takes care of all this.

    The benefits of a super accurate A/D will be lost without the right
kind of
    connectors, and cold junction compensation.   Anyone know the magnitude
of
    these errors, and any ways to compensate for them without a 10 dollar
chip?


    -----------
    I am planning to build a controller for a ceramic kiln.
    I try to convert thermocouple mV with a TL501 dual slope converter that
has
    an accuracy of 0,05% FS. This converter is usually combined with and
    controlled by a TL503 digital processor to obtain 41/2 digit BCD data.
    Is there any experience (and code) for the control of this type of A/D
    converters with PIC microcontrollers?

    Leo van Loon

1998\02\24@145502 by David W. Duley

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In a message dated 98-02-24 12:16:14 EST, you write:

<<
 > I am planning to build a controller for a ceramic kiln.
 > I try to convert thermocouple mV with a TL501 dual slope converter
 > that has an accuracy of 0,05% FS.
  >>

Hello to everyone!
Ive been in the hospital for a few weeks but I'm back now.

I don't want to kill your fun but I specialize in Kiln control of industrial
ceramic kilns.  I highly recomend that you don't re-invent the wheel.  It is
rarely cost effective to build your own kiln controller these days.  It is
sort of like setting out to write your own accounting software.  Check out
omega engineering.  they have simple setpoint controllers starting at about
$79.
We use lots of them in our machines.  The ones we use are around $300 because
we control them via RS485.  Starting at about $500 you can get a decent
Ramp/Soak programable controller with 25 or so program steps. For this type of
controller we use the Watlow but there are many many others.  Most fit in a
1/4 or 1/8 DIN standard panel space.

Just a thought

Dave Duley

P.S. Sorry Lawrence for send this directly to you.  I forgot to check the TO
address.

1998\02\26@095332 by lilel

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> In a message dated 98-02-24 12:16:14 EST, DREITEK writes:
>
> <<
>  > I am planning to build a controller for a ceramic kiln.
>  > I try to convert thermocouple mV with a TL501 dual slope
>  converter > that has an accuracy of 0,05% FS.
>   >>
>

> I don't want to kill your fun but I specialize in Kiln control of
> industrial ceramic kilns.  I highly recomend that you don't
> re-invent the wheel.  It is rarely cost effective to build your own
> kiln controller these days.

It's also not cost effective to have eighth graders work Algebra
problems - they've all been done before too.

Most of us PICsters had to walk before we flew.  Going thrrough the
exercise of making a temperature controller, controlling a seven
segment display driving scheme, or some other task that was first
done half a generation ago is necessary for anyone who wants to get
good at this.

I just finished an cooking oven controller that used a thermistor
(Which was probably first done by Edison's assistant) and learned a
lot about oven performance.  I started out with a simple enough
control scheme - a sort of  on-off thermostatic control, that
resulted in broiling the tops of all the pies and muffins.  Back to
the Keyboard!  I ended up using a proportional control scheme that
kept the duty cycle on the heating elements low enough to prevent
them from getting cherry red.  This prevents burning the top of the
food, and also resulted in an extremely smooth temperature profile.

I'm sure I reinvented the wheel many times.  But I got paid for it!

Best Regards,

Lawrence Lile

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