Dealing with thermocouples - and building thermocoupls

Rich M rsrich at cwcom.net
Fri Jun 16 21:36:17 GMT 2000


I've used a simple circuit to very good effect using the LM35 as a cold
junction compensator and a single good quality op-amp (Linear Technology
LTC1050) and a handful of passives to provide the gain.
One important consideration if you are doing your own op-amp circuit is to
ensure you use a good op-amp with very low input offset voltage (Vios)
specification. Consider that a 'K' type thermocouple generates a voltage to
the tune of 40 microvolts per degree centigrade, an input offset voltage
value which is significant with respect to this level can produce a
measureable error. The LTC1050 has a value of a handful of microvolts but
doesn't cost a packet (approx. 2.5UKP in small quantities). Anyone
interested is  welcome to a .pdf of the schematic if it's helpful.
As a side benefit, this circuit behaves predictably if the thermocouple
becomes disconnected; not designed that way, just happened!

Rich

> -----Original Message-----
> From: owner-diy_efi at diy-efi.org [mailto:owner-diy_efi at diy-efi.org]On
> Behalf Of C. Brooks
> Sent: 16 June 2000 21:05
> To: diy_efi at diy-efi.org
> Subject: Re: Dealing with thermocouples - and building thermocoupls
>
>
> Hi Roger, National has an IC "LM335" (Which is actually a temp
> sensor in and
> of itself) which can be used for cold junction compensation. Application
> Note 225 on the National site explains in detail how to go about
> accomplishing that. You can take the output signal from any of
> the circuits
> provided in the appnote and run it through an opamp to bring it up to
> whatever voltage your ADC requires.
>
> For signal conditioning on a budget, I HIGHLY recommend the "Active Filter
> Cookbook", "The IC OP-AMP cookbook", and a book titled (I believe) "The
> hobbyists guide to the 555".
>
> Between op-amps and 555 timers there isn't much you won't be able to
> accomplish as far as signal conditioning goes.
>
> Thermocouples are so inexpensive that  wouldn't bother trying to make my
> own. I've had custom thermocouples made by Gulf Sensor for past projects.
> You can find them at http://www.gulfsensors.com/
>
> I prefer the "Assembly Style 1A" sensor with the miniature connector "SM2"
> and using an extension cable made with a male TC connector on one
> end and a
> female on the other and whatever length of TC wire I need. It makes for a
> cleaner setup and easy replacement of TC's if they fail.
>
> Charles Brooks
> Mid-Atlantic Speed Shop
> http://www.massmotorsports.com
>
>
> ----- Original Message -----
> From: Roger Heflin <rah at horizon.hit.net>
> To: <diy_efi at diy-efi.org>
> Sent: Friday, June 16, 2000 1:03 PM
> Subject: Re: Dealing with thermocouples - and building thermocoupls
>
>
> >
> >
> > I found (on digi-key) a chip that is a Thermocouple cold junction
> > compensator and matched amplifier, it will supposely work for all
> > types of thermocouples and coverts to 10mv/dC.
> >
> > Does anyone know how hard it is to build a thermocouple from parts?
> > Omega seems to have everything needed to build one including
> > everything to contain the probe.
> >
> > Roger
>
>
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