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meereck
Joined: 09 Nov 2006 Posts: 173
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RTD and NTC sensors circuit |
Posted: Thu Oct 11, 2007 2:10 pm |
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Hello,
i need to connect 2 temperature sensors to a microcontroller: RTD Pt100 and thermistor NTC.
I don't care about linearization, because it will be carried out at a microcontroller side (I have formulas for calculating temperature from resistance).
The only issue I have is a measurement circuit - how to measure resistance of the sensor?
Would anyone be so kind and help me out a little bit?
Thanks in advance, M. |
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Humberto
Joined: 08 Sep 2003 Posts: 1215 Location: Buenos Aires, La Reina del Plata
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Posted: Thu Oct 11, 2007 3:15 pm |
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To connect an NTC it is very straight forward, a voltage divider will do the trick. Connect
them in series (NTC + Metal Film Resistor 1% low drift) between +5V and Gnd, picking the signal in the union.
To connect an RTD, will be necessary "to adapt" the very small delta-temp levels
given by the sensor, to higher values range that can be handled by any MCU / ADC.
If you are only playing with, you will do the RTD level adaptor with an LM324 or better,
also you will find dedicated IC´s designed for this (Analog Devices, Maxim, Texas
Instruments and others) but if you need a reliable and industrial grade device, Google
"RTD conditioner" and you will find a lot of comercial "ready to use" modules with
different capabilities.
Humberto |
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crystal_lattice
Joined: 13 Jun 2006 Posts: 164
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NTC and RTD Sensor Interface |
Posted: Fri Oct 12, 2007 12:06 am |
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Hi there,
I would go for the Maxim range try the MAX66... parts The MAX6675 is for type K and the MAX6682 is for NTC.
I sampled some of them but only tried the MAX6675, not working(yet) using software SPI but think timing might be out.
But yes, check out the maxim range ![Wink](images/smiles/icon_wink.gif) |
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meereck
Joined: 09 Nov 2006 Posts: 173
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Posted: Fri Oct 12, 2007 2:40 am |
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thanks for the replies,
i have googled a little bit and found wheastone bridge, it might be helpful for the RTD.
As for NTC, can you give me a clue how to choose the value of the resistor if I want to measure the temperature between 0 and 70 degC? My NTC is :
0 degC=19590 Ohm
35 degC= 3918 Ohm
70 degC=1051 Ohm
Shall the resistor be 3918 or about 10000 Ohm?
Measurement doesn't have to be so accurate so I do not need any special IC for it.
best regards, meereck |
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Humberto
Joined: 08 Sep 2003 Posts: 1215 Location: Buenos Aires, La Reina del Plata
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Posted: Fri Oct 12, 2007 9:42 am |
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Wiring 680R to +5V and other end to the NTC and this to Gnd, modeling the NTC + R with a 10K input impedance (for the built-in ADC of the PIC) you will get the following values:
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0° 20K 2.62V
8° 16K 2.37V
15° 12K 2.04V
25° 8K 1.59V
35° 4K 0.96V
58° 2K 0.53V
70° 1K 0.28V
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To get a bigger span, - beyond 1/2 Vs - you should use a diferential amplifier.
You will find good info in the following Microchip Applications Notes:
1. AN 990 Analog Sensor Conditioning Circuits
2. AN 679 Temperature Sensing Technologies
3. AN 929 Temperature Measurement Circuits for Embedded Applications
4. AN 685 Thermistors in Single Supply Temperature Sensing Circuits
5. AN 684 Single Supply Temperature Sensing with Thermocouples
6. AN 844 Simplified Thermocouple Interfaces
Humberto |
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