RTD and NTC sensors circuit

meereck · Joined: 09 Nov 2006 Posts: 173

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.

Humberto · Posted: Thu Oct 11, 2007 3:15 pm

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

crystal_lattice · Joined: 13 Jun 2006 Posts: 164

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

meereck · Joined: 09 Nov 2006 Posts: 173

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

Humberto · Posted: Fri Oct 12, 2007 9:42 am

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: