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 -----
From: "Olin Lathrop" <olin_piclist@EMBEDINC.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Monday, March 04, 2002 8:07 AM
Subject: Re: [PIC]: Beginner Q. How many K-type thermocouples can the 16F84A
h andle?


> > 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, olin@embedinc.com, http://www.embedinc.com
>
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