Circuit to output difference in temperature between two thermistors

That's two op-amps more than it took me. ;-)

Best regards, Spehro Pefhany

But I bet it took 80,000 transistors.

John

You'd lose. ;-) Nothing so gauche as that. A passive network that outputs two voltages, the ratio of which is proportional to the temperature in degrees C, linearized, corrected for leadwire resistance and filtered.

Best regards, Spehro Pefhany

Wow....I'm the OP. So many responses...

This circuit is for measuring the temperature drop of air across a huge intercooler I installed on my 1990 dodge daytona. I think it would be fun to have a gauge which shows just how cold the incoming air is, and then how cold it gets after exiting the enormous intercooler AND water injection.

It is not used in any control loop.

Accuracy to 2 degrees F would be great.

Response time needs to be less than 0.5 second to catch rapid changes in temperature as the boost increases rapidly.

I made up all the numbers in the preceding two sentences. Seems right though..

I chose thermistors because thermocouples seemed like a pain, but that chopper amp and thermocouple idea caught my attention so I might look into it.

I need to find some really small thermistors to get a good response rate.

Umm...I want it to be all analog JUST BECAUSE. Microcontrollers are cheating. Its good to know at least half of you didnt even mention MCU's (makes it easy to tell who REALLY knows circuit design).

Once I have the difference voltage I'll make a 7 segment display that is all analog too somehow. Fun!

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John,

So your idea doesnt need cold junction compensation because the difference in thermocouple voltage is what we care about, and the cold junction voltage will be the same for both thermocouples (since they will be thermally in the same place).

So lets say I have your spool(s) of thermocouple wire.

Somehow I weld up two thermocouples with 6 feet of wire

I mount each thermocouple in its temperature measuring location.

Now if I had two voltmeters and hooked one up to each thermocouple, I would get some voltage.

This voltage could not be converted directly to absolute temperature without cold junction compensation.

But the difference between the two voltages IS accurate immediately.

So thats how your idea works, right?

And why does the opamp need to have such a low offset?

Because the thermocouple voltages are in the uV's, and the offset will be interpreted directly as SIGNAL, i.e. temperature difference, right?

Is that all the opamp REALLY needs to be good for this, low offset?

Am I getting the whole picture here?

How good could I make this with two off-the-shelf thermocouples instead of making two on my own? (how much would that cost and how do I weld it up?)

See

My interest was slightly different - thermistors are Johnson-noise limited to a couple of microdegrees, while decent sized platinum resistance sensors dissiating at lot more power could go down another order of magnitude.

Don't remind me. My paper on our Peltier junction based milli-degree controller goes into this a bit - if we could have increased the gain at the cycling frequency of our air-conditioning system, we would have had even tighter control. Since we only needed to control to better than 10 millidegrees, we didn't bother to try.

Sounds like fun. One of my old friends was involved with a system that used Nitinol to control the focus - and it seems to have been something of a disaster. The phsyicist who invented the system wass too infatuated with his brain-child to see its defects.

-- Bill Sloman, Nijmegen

I dream of being Johnson-limited in a bandwidth of 0-1 Hz.

Nitinol is deceptively easy to use for low-precision stuff, e.g. Japanese greeting cards where a cutout waves to you and sings a song. For higher precision things you have to really design with it. Just using it to turn a focusing screw against a return spring is *not* a real design--you have to have a bloody great preload to avoid dying from vibration and shock. My gizmo's Nitinol ring was shaped like a washer, and went between the laser can and the collimator. The force required to pre-squash it was about 500 pounds--no vibration worries there.

Cheers,

Phil Hobbs

Late at night, by candle light, John Larkin penned this immortal opus:

The Foxboro E92 RTD to 4 - 20 mA converter slightly raised the excitation to the 3-wire bridge as temperature went up.

- YD.

Bah. Just use a logarithmic amplifier / converter. Build it if you have to, they are fairly simple (Never more than 4 bjt, some r's and an opamp).

Also a well designed bridge can linearize it over a moderate temperature range.

I have done this with analogue circuitry for bolometers, using a bridge circuit with excitation that was adjusted so that the bolometer sensitivity was less dependent on the ambient temperature. It was ok at room temperature but the sensitivity was not as stable over wider ranges of ambient temperature, nor as linear over bolometer input power as I could have achieved with a good ADC and a microcontroller. If you don't require very high accuracy over a wide range of ambient temperature, or you think that circuits containing several analogue multipliers and more than three interacting trimpots are a good idea then you might want to avoid the $5 microcontroller.

Chris

Nice linear analog multipliers are seven copses over and two copses forward in the forest. The sensor has an exponential property, the natural linearization is a simple logarithmic function, not a handful of complex analog IC's. Then the $5 uC with a 14 bit ADC can take care of auto calibration and second order effects.