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'[PIC]: Beginner Q. How many K-type thermocouples c'
2002\03\03@232656 by Von Snarski, Matthew

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Dear all,

I am (very) new to the world of PIC's but not to the general world of
electronics.  My question is: how many K-Type Thermocouples can I connect to
a PIC 16F84A?

I am wanting to use the PIC to display the readouts of various temperature
probes (all K Type), on one single 20x4 LCD screen.  Is this possible with
this chip or do I need to move up to one with an ADC incorporated?

I am aware that I will need one AD595 for each K-Type thermocouple.  Also,
how can I extend the leads of the thermocouples without creating errors in
the voltage created by the difference of the original 2 metals?

Thanks for your time,

Matthew.

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2002\03\04@052758 by Dave Dilatush

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Matthew wrote...

>I am (very) new to the world of PIC's but not to the general world of
>electronics.  My question is: how many K-Type Thermocouples can I connect to
>a PIC 16F84A?

That's like asking, "How many freight cars can I pull behind my
ten-speed bicycle?"

>I am wanting to use the PIC to display the readouts of various temperature
>probes (all K Type), on one single 20x4 LCD screen.  Is this possible with
>this chip or do I need to move up to one with an ADC incorporated?

The PIC16F84A has no innate ability whatsoever to deal with thermocouple
signals, as it does not have any on-chip A/D converter.  You must either
choose a PIC that has an A/D converter of the appropriate resolution
(they come in 8-bit, 10-bit, and 12-bit flavors), or use an external A/D
converter along with the PIC.

As I hinted in my bicycle analogy, the PIC16F84A is also rather
underpowered for the job: it has only 1K of code memory, which is going
to pose quite a challenge to you if you are very new to PICs--especially
if you need to perform linearization (i.e., if you need accuracy better
than about 2 degrees C).  While an experienced and clever PIC programmer
might be able to shoehorn everything into 1K of code space, you, as a
beginner, would be better off choosing something that won't make your
introduction to PICs a trial by fire.  The PIC16F877, for instance, has
8 channels of 10-bit A/D conversion on-chip, as well as 8K of code
space.

>I am aware that I will need one AD595 for each K-Type thermocouple.

Another way to deal with multiple thermocouples, perhaps less expensive,
would be to use a multiplexer ahead of a single AD595.

>Also, how can I extend the leads of the thermocouples without creating
>errors in the voltage created by the difference of the original 2 metals?

This question is answered quite nicely in Analog Devices' application
note AN369, "Thermocouple Signal Conditioning Using the AD594/AD595"; it
also gives a good example of using a multiplexer with the AD595, along
with what's called an "isothermal block" to cancel out the errors that
would otherwise result from thermocouple lead extension.

Another good resource is Linear Technology, Inc. application note AN28,
"Thermocouple Measurement", which provides a good introduction to the
basics of dealing with thermocouples.

National Instruments, Inc. has still more thermocouple measurement
information on their website; there are some very good application notes
and tutorials there.

Hope this helps a bit...

Dave

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2002\03\04@090903 by Olin Lathrop

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> I am (very) new to the world of PIC's but not to the general world of
> electronics.  My question is: how many K-Type Thermocouples can I connect
to
> a PIC 16F84A?

First, check out the 16F628.  It is a newer replacement for the 16F84 that
does more and costs less.

The 16F628 only has a comparator, no A/D.  Thermocouple signals are slow, so
you could use a low pass filtered PWM output as one input to a comparator
and the thermocouple as the other.  The 628 has two analog comparators built
in, so that's how many thermocouples it can handle without external
multiplexing or additional external comparators.

> Also,
> how can I extend the leads of the thermocouples without creating errors in
> the voltage created by the difference of the original 2 metals?

The thermocoulple will measure the temperature difference between the two
ends of the two-conductor wire formed by the two wires of the dissimilar
metals.  One end of this two-conductor wire is "shorted", and is the
temperature probe end.  The other end of the two-conductor wire eventually
transitions to two conductors of the same metal, usually copper.  The
temperature of that junction needs to be controlled or known.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, spam_OUTolinTakeThisOuTspamembedinc.com, http://www.embedinc.com

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2002\03\04@092354 by Lawrence Lile

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Thermocouple signals are very difficult to work with, because of some of the
problems Olin has  hinted at.

They are millivolt level signals, nonlinear, fraught with noise sensitivity,
have wierd low impedance levels, and connecting to them introduces more
thermocouple junctions with more problems.  I use them every day, with lots
of cussing.

Analog devices makes an AD595AD device that is designed to process
thermocouple signals.  I would be very reluctant to try a roll-your-own
thermocouple amplifier with only a PIC.  A PIC A/D port probably won't move
one bit (speaking literally!) in response to the timy signals out of a
thermocouple.  Look into the AD595 as a front=end processor.


--Lawrence Lile

{Original Message removed}

2002\03\04@094719 by David Covick

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All this talk is making me cringe!

Try the MAX6675, it works beautifully.

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2002\03\04@103236 by Spehro Pefhany

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At 08:21 AM 3/4/02 -0600, you wrote:

Another issue that's not been mentioned is that you want to use multiple
T/C's, which means that conventional grounded-junction thermocouples
may cause you problems. Floating junction T/Cs are more expensive and
have much poorer response times. If you ground the input and the junction,
you'll short out one leg of the T/C and get very strange errors, averaging
part of temperature differences and that sort of thing.

I've worked all of my career with thermocouples (including the lovely
platinum ones that put out about 1/6 the microvolts per degree as type K,
doing scores of designs, I am the 'resident' thermocouple expert at two
companies, and I can definitely say that there are plenty of things to
catch up the unwary. Type B is my favorite, it's actually non-monotonic
around room temperature (there are two temperatures that give the same
number of microvolts output). If you don't care much about accuracy, it's
easier, but you still have to pay some attention to thermal issues in
the packaging. The problems may be severe if you have significant heat
sources in your product.

To some extent, you can get around this by throwing money at it. If you
buy one loop-powered (transformer) *isolated* transmitter per input, you
can just build a display unit that reads 4-20mA signals. That will be
relatively costly, but it will work and will be reliable, even in a real
industrial environment. Or specify isolated T/Cs, and live with the lower
performance.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
.....speffKILLspamspam@spam@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
9/11 United we Stand

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2002\03\04@195855 by miked

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You may want to look at;
http://dbserv.maxim-ic.com/view_press_release.cfm?release_id=504

>
> Dear all,
>
> I am (very) new to the world of PIC's but not to the general world of
> electronics.  My question is: how many K-Type Thermocouples can I connect
> to a PIC 16F84A?
>
> I am wanting to use the PIC to display the readouts of various temperature
> probes (all K Type), on one single 20x4 LCD screen.  Is this possible with
> this chip or do I need to move up to one with an ADC incorporated?
> I am aware that I will need one AD595 for each K-Type thermocouple.  Also,
> how can I extend the leads of the thermocouples without creating errors in
> the voltage created by the difference of the original 2 metals?
Use thermocouple extension wire.
>
> Thanks for your time,
>
> Matthew.

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