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Author Topic: 5 Ohm NTC to Arduino ADC?  (Read 714 times)
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I need to use NTC 5D-20 to measure temperature.
It has 5 Ohm R0 and 3150 Beta. The temperature I'm interested in +20...+70C.
It appears that resistance range would be from 1-5 Ohm, which is very little. What bias resistor should I use? How tto connect to Arduino ADC with less non-linearities?
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Drive it with a known current source and sense the voltage.
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I understand that, but what value of bias resistor should I select? 10k? 100Ohm? The more is bias resistance value, the less precision I will get?
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Can you link to a datasheet for the particular thermistor?
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Sure, this is datasheet
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Where do you see on the datasheet the beta value of 3150.....or did you get that through your own empirical measurements?
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Sorry, must be this pdf
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I remember when I was working through a problem like this a while back, I set up a spreadsheet like the one that is attached and then played around with the resistor values until I got the resolution that I was happy with.  Maybe this will give you some ideas on how to set the problem up in a spreadsheet for yourself.

* BBQ Controller.zip (26.32 KB - downloaded 7 times.)
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The question is not spreadsheet, but how to connect it to Arduino ADC!
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Your question was "What bias resistor should I use?" 

Put together a spreadsheet like mine and play around with the bias resistor value until you have a resolution you are happy with.

In terms of connecting it to the Arduino that is easy.

5V---------bias Resistor--Thermistor----------Ground
                                   |
                                   |
                                   |
                          Arduino ADC PIN
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About the easiest method I can think of would be to use a Wheatstone Bridge It would take take 3 resistors and 3 pots.
The rest of this is based on the idea that you have to use that particular thermistor. {edit}
It is however the only easy way I can think of to utilize that value of thermistor without drawing a whole lot of current.
It would be a great easier if a more appropriate sensor could be used but this should work.
There is a way to sense the imbalance of the bridge by sensing the current it draws from the 5 V source. One pot is twice the value of the thermistor at it's highest and is used to balance the bridge at some temperature. The top half is the pot and thermistor in series the bottom is 2 47 ohm metal film (any value from 50 to 150 ohm would work  but the smaller values reduce sensitivity or range and the larger values can be more susceptible to drift) The center of the resistors joined together is ground and either end connects to the lose ends of the pot/thermistor. The center of the pot thermistor connects to a 1K pot and the remaining pot connects across the bridge on the 2 remaining unconnected points. When in balance the pot in the power leg and the pot across the bridge determine range and sensitivity and the balance pot to zero the bridge at the operating temperature.
It might be easier to configure the analog measurement to use the 1V1 Vref for greatest accuracy and keep the bridge configured for a 1 V max output.

Bob
« Last Edit: January 08, 2013, 04:45:11 pm by Docedison » Logged

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Quote
I understand that, but what value of bias resistor should I select? 10k? 100Ohm?

If you did understand that, you wouldn't be asking that question.
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Like Doc hinted, do you really have to use this thermistor for your project? This thing will draw 500mA even with a proper serial resistor. That is the upper limit of USB connection. You should be using this in line with large current applications. Any chance to use a good old 10K ohm thermistor instead?

The best accuracy can be obtained by finding the resistance of 20C and 70C, then find the average of the log of the two resistance and take exponent. One example, at 20C resistance is 1000, at 70C resistance is 10. Then the base 10 logs are 3 and 1. So average is 2. Take 10 exponent of 2 you should be using 100 ohm resistor. I remember this from deriving the errors of voltage divider resistance measurement for my electronics class. I also have enough data to show it.
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