Thermistor in self heating mode

Then the temperature will be at the mercy of the liquid.

What are you trying to predict or design?

It's perhaps not a bad way to make a level switch. You should be able to predict the steady-state temperature with and without the liquid being present for a given current or series resistor (given a constant and (let's say) equal temperature for the liquid and air). Then play with that single variable (the current or resistance) and see what limits you run afoul of, and then you can play with variations in ambient temperature.

Best regards, Spehro Pefhany

Can anyone point me to a GOOD ref for running a thermistor in self heating mode? How do you figure out how much power you need to be putting through it??

All the instrumentation texts describe this, but not at a real practical level. I think I'm sneaking up on the current from the low side, but I want to avoid smoking these things.

What is your reason to be heating a thermistor? The search for assistance depends a lot on your reason.

In the mean time, perhaps one of these general thermistor application notes will help.

Are you talking about a PTC or NTC thermistor? PTCs typically have a snap characteristic, almost like a thermostat, where their resistance increases about 3 orders of magnitude at some temperature between about

NTCs have a resistance that decreases about 3-4% per kelvin, which makes them more settable but much more of a puzzle to drive. Constant-current drive will always be stable, because an increase in temperature will always result in a decrease in dissipation--P=I**2 R, so dP/dT = I**2 R**2 dR/dT, which is always negative. On the other hand, constant voltage drive will be unstable above some current--P=V**2/R and dP/dT = -V**2/R**2 dR/dT, which is positive. Heat loss Q-dot tends to be proportional to delta-T, as does the dissipation in the thermistor, so when you crank up the voltage past the point where dP/dT + d(Q-dot)/dT = 0, it'll run away.

Soooo, if I really had to build a temperature control loop with an NTC thermistor in a self-heating mode, I'd start with a constant-current drive and put a very slow P-I feedback loop around it. Figuring out what the set point should be is left as an exercise for the reader. ;)

Good control results when P = Q-dot and dP/dT is large compared with the thermal conductance G_th=d(Q-dot)/dT. (At short times, you also have to contend with the fact that G_th isn't frequency-independent and that the thermistor itself has some thermal mass.) You can't achieve both of those things at the same time with a barefoot NTC unless your insulation is really really amazing--the temperature coefficient is just too low.

Cheers,

Phil Hobbs

I like running in self heating a PTC, well I guess it does not matter. I like them in water baths. When the water runs dry, it auto shuts down. You can get an excellant self heating differential by dipping in water and comparing results with the air.

greg

Spehro Pefhany wrote in news: snipped-for-privacy@4ax.com:

At the moment, just trying to understand thermistor self-heating through experience. No urgent practical use, but lets just call it measuring liquid flow at constant temp. It's not going to be in a control loop. I know its not the only, or best way to do it, but I'd like to understand the method.

I have a feeling I'll just end up finding out why its not particularly easy or reliable, but I'd like to know.

Or my Bachelor's Thesis...

"THERMISTORS AS BLOOD FLOW-RATE TRANSDUCERS",

MIT, June 1962... with cooperation of Peter Bent Brigham Hospital, Harvard University Medical School

...Jim Thompson

I have this in my "things to do file when I am smarter".

I attended a seminar at the EAA Convention in Oshkosh in 1982(?) where there was a presentation by a scientist from Langley AFB who built an aircraft autopilot based on the principal. Two thermistors were in a bridge with an air jet (tiny) that drove an RC servo on a trim tab on one aileron. There was a professor from Ohio State University (Walt(?)) who had it working on his Thorpe.

Apparently it worked very well but Gyro's won out in that application.

John Ferrell W8CCW Beware of the dopeler effect (pronounced dope-ler). That's where bad ideas seem good if they come at you fast enough.

Stolen for personal use ;-)

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at

Jim Thompson wrote in news: snipped-for-privacy@4ax.com:

Thanks. I'm trying to put together a classroom demo or lab exercise for a sensors and instrumentation course. I had a nice thermodilution lab, but a self heating setup just never came to be, so the class never saw one beyond reading about it.

Fluid level sensors usually use NTC thermistors... in air their resistance plunges... in the fluid, they cool and their resistance rises.

...Jim Thompson

Jim Thompson wrote in news: snipped-for-privacy@4ax.com:

Thermodilution is the method currently used to measure cardiac output, which isn't necessarily easy to do. A catheter is inserted into the right side of the heart, and a balloon-like thing is used to float a thermistor into the pulmonary artery. A cold water bolus is injected into the heart, and the integral of the temp change is proportional to flow

My thesis was based on two thermistors on the end of a catheter, one with current flow to heat it, one simply to measure blood temperature. This was back in the days of Germanium transistors, so I built my own chopper-stabilized amplifier ;-)

...Jim Thompson

It's not on your web page? ;-)

Best regards, Spehro Pefhany

I'd have to scan it... it dates to the days of trypewriters, except I used a flexowriter + PDP-8 + punched paper tape ;-)

...Jim Thompson

Jim Thompson wrote in news: snipped-for-privacy@4ax.com:

How many Ph.D candidates back in the day do you think got married just for the cheap typing??

Quite a few ;-)

I was already married, had a typewriter, but decided the PDP-8 was ideal for generating a correctable copy, and I could spit out an additional copy at will. (I think I still have the punched paper tape around here somewhere ;-)

...Jim Thompson

In 1962 my 'Chopper' was a Rat Terrier and he was self stabilizing. ;-)

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