Circuit to output difference in temperature between two thermistors

Im sure this has been done before, right?

Two NTC thermistors.

10k @ 25C Beta = 3380

I'd like a circuit which outputs a voltage which is the difference in temperature between the two resistors.

But it needs to be linear.

I.e.

1 degree difference = 0.1V 10 degree difference = 1V 100 degree difference = 10V

At ANY location in the thermistors operating range.

Im trying to get my head around this....what is the mathematical function for what Im trying to do? Since the thermistor is non-linear?

Im thinking a circuit which linearizes each thermistors resistance vs. temperature, and then another which simply does a voltage difference.

Sound right?

I can do the voltage difference, but how do I do the linearizer?

Reply to
acannell
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Tricky. You seem to want to cover a wide temperature range, and keeping good accuracy over the whole range would be difficult.

The function you are looking for is the Steinhart-Hart fitting function

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Getting the three fitting parameters for a particular thermistor isn't all that easy, and working out temperature from resistance isn't trivial either.

The most practical way of solving your problem would be a tolerably precise A/D converter - Linear Technology have a nice range of nominally 24-bit sigma delta A/D converters starting with the LTC2400 - on each thermistor. You'd then feed both digital outptus into some kind of microcontroller, linearise in the digital domain, subtract the temperatures in the digital domain, and feed the digital difference into a D/A converter.

Remember not to dissipate too much power in the thermistor anywhere in the temperature range - 10uW is what I've used - because even "interchangeable" thermistors tend to drift if you give them too much power, and they all go unstable if you give them much more. I think we saw instability at 100uW, but it's while since I've worked with thermistors.

--
Bill Sloman, Nijmegen
Reply to
Bill Sloman

You can do a reasonable approximation over a small range of temperatures using analogue circuitry but for a general solution that works for all temperatures, I believe that you should use a microcontroller to do the maths. You could digitise the thermistor resistances either by connecting them in some kind of resistive divider with an ADC, or connect them into R-C oscillators e.g. with 555 timer chips, or you could measure the time constant of a R-C network made with the thermistors as the resistors, using a microcontroller port pin and a comparator.

Chris

Reply to
Chris Jones

Why thermistors? Why not some more linear sensors? RTDs, ICs, thermocouples?

There are some nice thinfilm platinum RTDs that look just like surface-mount resistors. Thermocouples are nicely suited to delta-T measurements reasonably near room temp.

John

Reply to
John Larkin

Why not use those temperature sensing IC's that provide a linear output. Example:

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D from BC myrealaddress(at)comic(dot)com British Columbia Canada

Reply to
D from BC

To what accuracy?

As Bill Sloman mentioned, it's Steinhart-Hart. There's another (that's named "senior moment" right now, apparently ;-) that uses the thermistor beta but the S-H equation follows the behavior more closely.

Modulo your un-stated accuracy, you can probably get "good enough" linear behavior over a *portion* of the range by paralleling the thermistor with a larger resistor.

For a similar thermistor that I have a handy listing for on the hard drive, its S-H parameters work out to be 8.626294E-4, 2.586585E-4, and

1.396712E-7 (which is a 25-to-50 Beta of about 3440), then using a 47K in parallel and a 4.7K "on the bottom" yields linear results to within 0.5 C from about 15 to 65 C and fits nicely in the middle between Vcc and ground. YMMV and you'll need to do the math to check, of course, and don't forget that the resistors etc. affect the accuracy as well.
--
Rich Webb     Norfolk, VA
Reply to
Rich Webb

Thermistors give you by far the best temperature control of anything. See ABSE for a picture.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

Still, there's the linearity issue.

A pair of back-to-back thermocouples, from the same hunk of wire, driving a chopamp, would be a nice diff-temp sensor. One could easily get dt offset below 0.1K with no calibrations. That would be hard to do with thermistors.

John

Reply to
John Larkin

Providing you can keep the offsets in the rest of the system down to 1 uV or so, over temperature. I'd rather fight the thermistor problems.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

One other thing: YSI also makes interchangeable glass bead thermistors that track to the

0.1 degree level, iirc. I've used those with great success in the past.

They publish their thermistor curve, and for a delta-T application you only have to worry about the scale error, so the OP should be able to do a good job that way.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

Starting with a 40 uV/K thermocouple, and using a sub-uV offset, nanovolts per degree drift, cheap opamp, it's easy!

This stuff used to be hard.

John

Reply to
John Larkin

hook them both to a microcontroller.

why do you need to use thermistors?

can't you use something linear like thermocouples, or themometer chips, or diodes?

Reply to
Jasen Betts

On a sunny day (Fri, 09 Jan 2009 21:56:04 -0500) it happened Phil Hobbs wrote in :

Why should that be? Sound like a religious statement. A LM135 will give you nice linear temp in Kelvin.

10mV / degree Kelvin. 155 to +150 °C, I use them.
Reply to
Jan Panteltje

By far the best way, 1 ptc in each arm of the bridge gives you the difference directly.

Reply to
cbarn24050

What is the thermistors' operating range? Range of temperature difference? Desired accuracy? Resolution? Allowable self heating?

In general, the easiest way would be to digitize, linearize and then subtract the two numbers, and convert back to analog if you must.

Last time I did something like this (covering the full rated range of the thermistor) I used three sets of Steinhart-Hart coefficients to digitally linearize the thermistor.

If you're trying to measure something like heat flow, I'd likely go with Pt RTDs or thermocouples, depending on cost and accuracy constraints.

Thermistors have an initial tolerance, and a tolerance in the 'beta' as well, so even ones that match well at room temperature may not match so well at the high and low extremes. That translates into an error in the difference output.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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Reply to
Spehro Pefhany

Use a PIC.

Microchip have an app note for measuring temperature using the RC time constant of the thermistor with a known cap. Once you have the resistance a look up table works well for getting temperature. I normally put in R for every 5C and do a simple piece wise linear approximation to find the actual value.

Do that for both thermistors, take the temp difference and use software pwm, or a PIC with pwm, into a RC filter and op amp to give you the linear output.

Parts cost for the above is under £2 or $3.

Works well over a wide T range. If you need an extra wide range the spare pins on the PIC can switch in different ref Rs and caps for the initial conversion.

Reply to
Raveninghorde

The App note is AN512:

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Reply to
Raveninghorde

Doing it verifiably is still hard. Just the difference between copper alloys can be worse than that. Also those trimmed-to-the-eyeballs CMOS op amps usually have horrible worst case drift specs--like 4 uV/K.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

Doing it verifiably is still hard. Just the difference between copper alloys can be worse than that. Also those trimmed-to-the-eyeballs CMOS op amps usually have horrible worst case drift specs--like 4 uV/K.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

Look at the picture in ABSE.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

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