Measuring temp via ADC & NTC & Loooong PDS Cable

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Hi Guyzz

I have some 10K NTC's here , and have used them to measure temperature , with a circuit like this:

VCC---NTC---10K---GND

And the ADC connected to the NTC/10K point.

If i put the NTC at the end of a 10..20 Meter cable ,would it function there ??

I mean , the few ohms in the cable is nothing , but there might be some problem with capacitance and the ADC , or the VCC level at the end of the cable.

Any hints/warnings ???

Or should i just get a DS18B20 :-)

/Bingo

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You can use long cables with NTC termistors. I don't know the limit though.
I have finished a project with NTC termistors of the same sort you use (Isuppose you mean 10K at 25 degrees C). I connected one end of termistor to ground and the other end to ADC pin. Then I had a 3K3 resistor connected between ADC pin and Vcc. Doing that I had stable 10 bit values from ADC over most of the 0-1023 range. My goal was to measure temperatures 0-99 degrees C.
I don't know if my way of connecting NTC is how a "real pro" would do. I did it this way because I didn't like the thought of having Vcc run in such long cables, so I used shieded 1-wire cables were GND was connected to the shield. I have after this was finished peeked into the circuit it's replacing. They had one end of NTC connected to GND so I believe I used a good method. :D The manual for this thingie says cables as long as 20 meters can be connected.
It would be interesting to hear opinions from someone with professional experiences about NTC termistors...

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We put a 1k series resistor and a pair of diodes to the rails between the connection point and the AVR pin when there is field connection in case of miswiring and static and such.

Our designer also does the circuit a bit differently: Vref --- Bias -+- NTC --- Gnd

with the "+" being the sampling point leading to the 1k resistor and protection diodes.

Now, the "ideal" bias resistor value depends on the temperature range of importance. I've got a spreadsheet that I use to help me choose, but with good selection of the bias resistor I can use a linear transfer function that has very little error over, say, a 40 degree C range. If you need full-range of, say, -40 to +80 degrees C then there is no way to straighten it out completely. ;) But many of our apps have an "area of interest".

For a linear segment near the bottom of the temperature range we use a bias of about 3x the thermistor nominal value. For a 10k we use 31.5k ohm.

Now, back to your question. Will there be voltage drop over that distance? Hmmm--not too much; maybe it can be calibrated out. If you want to run extra wires and use another channel you could use 3-wire or 4-wire techniques to try to cancel out some of the drop and noise.

Summary: We have had customers extend wires on 10k and 30k thermistors up to those kinds of distances, and it has worked OK. But it isn't an app with high accuracy or precision requirements.

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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Thanks for the good advices Lee :D
Obviously I did the most right, except for the serial !K and the diodes. I didn't really understand how you connected those diodes. Would you care to clarify?

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Thanx for the advices Lennart & Lee

This is just for measuring some "Temperatures" at my cottage.

So i expect -15 to +40 degrees Celcius would be of interest

/Bingo

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Quote:

I didn't really understand how you connected those diodes.

One diode from the signal line "pointing to" the +V rail. This dumps signal overvoltage onto Vcc before it gets to the AVR pin.

One diode from Gnd "pointing to" the signal line. This dumps negative voltages onto Gnd before it gets to the AVR pin. (Well, of course not really that--but the opposite of the overvoltage going to +V.)

They often come in "packs" of dual-diodes just for this purpose. A common connection for the +V. A common connection for the -V (Gnd). Individual connections for each signal to be protected. So an 8-pin device can do 6 signals, for example.

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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Quote:

So i expect -15 to +40 degrees Celcius would be of interest

Remembering that all thermistors are not created equal, for the Murata devices that I have in my spreadsheet a 10k to 15k bias resistor should give nearly linear values over that range--the Excel-fitted line has an R2 of over .99.

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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Last questions :D
I only found SMD diode arrays where both diodes connect to GND. Do you have a part number for a DIL or SIL?
Should they be connected before 1K resistor?

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We used to use arrays that are now obsolete, so I won't give you those part numbers. ;)

Recent schematics show almost exclusive use of BAV99 3-pin diode pairs, often in SOT23 package.

Example: DigiKey 568-1624-1

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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I did make an acdemy entry with the 2 types i have
https://www.avrfreaks.net/index.p...

There is a conversion table witch hopefully should kill non liniarity

/Bingo

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Bingo

I looked at the code. The values provided is from -55 to +125 degrees C. If you don't need to measure that temperature span in your home :D , you will have less values to represent temperatures of your interest = less good resoution.