Selecting Temperature sensor IC

[sam_kevin]

Hi, Could some one help me out in selecting a temperature sensor IC for my project.

I need to sense the temperature of the refrigerator which will drop to minus 30 deg Celsius, say the sensing range should be -30 to +40 deg Celsius.

The device that I am using has only one ADC pin, and the distance between my sensor and the device will be about 10meters long, Kindly suggest me sensor IC or guide me how to overcome the wire resistance as the wire length is 10Meters. the accuracy can be +- 2%.

I am new to electronics I am wishing to do this project by my own, Kindly guide me with better solutions. Thank you

Yours faithfully
Sam.

 

[InNeedOfHelp]

do you have any specific uC that you need to use?

 

[barry]

What device are you interfacing to, a microprocessor? You can get an inexpensive semiconductor device that would have to be mounted on a pcb. You can get analog output or digital.

For starters, look at Texas Instruments and Maxim-IC website. Considering the distance your device will be located, I would personally go with a digital (SPI or I2C) output since the analog devices would probably require some amplification and would be more susceptible to noise.

 

[tpetar]

Use digital sensor for example DS18B20.

I dont think its good to use analog sensor over long wire.

Or you can use analog if you insist, but only localy, for data transfer use digital transfer, such as ADC for 1-wire,....

 

[sam_kevin]

NO, THE device is GSM, where i got one ADC pin and i got to connect the hardware in that pin to get the digital value in my data, I cant program the GSM device.

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@InNeedOfHelp

NO, THE device is GSM, where i got one ADC pin and i got to connect the hardware in that pin to get the digital value in my data, I cant program the GSM device.

 

[tpetar]

You should mention that in thread question. What is ADC specification (what voltage range) ?

 

[sam_kevin]

ADC voltage rage is 1-30V

 

[tpetar]

This ADC is probably determined for measuring some voltage, such as 12V battery.

You can try with LM358 OpAmp and LM35, make some voltage range on output.




Additional material:

https://learn.adafruit.com/tmp36-temperature-sensor
https://www.facstaff.bucknell.edu/mastascu/elessonsHTML/Sensors/TempLM35.html

 

[sam_kevin]

tpetar : LM35 sensor is not reliable the accuracy below -11*c is too low, so i discontinued using that IC

 

[tpetar]

You can use other sensor, principe is the same.

 

[FvM]

To see the bigger picture, classical analog temperature sensors like thermocouple or Pt100 have been since long and are still used over long wires (sometimes 50 m and more) in industrial automation. The essential point is to have interference immune sensor amplifiers respectively signal filters and to eleminate wire resistance effect (using three- and four-wire configurations) for resistive sensors.

 

[betwixt]

The wire resistance shouldn't be a problem as the current flowing into the ADC input will be very small.

I've just used MPC9701A sensors on 50m wires. It needed a lot of filtering on the output voltage but then it changes so slowly that isn't a problem. It's a nice sensor for use with 8-bit ADCs because it's output is 19.5mV/C which equates almost exactly to 0.25C per step on a standard 5V FSD ADC. It also has no trouble driving highly capacitive loads and is good from -40 to +125C.

Brian.

 

[sam_kevin]

Betwixt: Thanks for your reply, Datasheet of MCP9701A page -2 electrical chac and temperature chac says MCp9701A works from -10*c to +125*C, BUt i want to sense the temperature of -30*c

# Page 12, 4.3 Layout considerations has some recommendation, is that enough or we need more filtering to be used to output lines ?
# will sample the ADC pin at every minute, I hope by that time i can get the voltage at the other end.
# Where did you used your 5V supply regulator ? near to MCP9701A IC ? (I need to use regulator IC to regulate it from 12V)
# What kind of wire's you used at Vout ?(Shielded cable or twisted pair cable) ?


Thank you.

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THanks FVM, For using pt100 Thermocoupler i think i need to use opamp IC and i am not familier ith those opamps as i am beginer in electronics i am finding dificult to follow that unless someone guides me,

THE IC's like MCP9701, AD22100 has a fixed voltage raise for each *C. and I find it easy, The only thing is I need to design the power supply to regulate it to 5V from 12V. The only problem is my sensing distance is too long to 10MTs.

 

[betwixt]

It should still work down to -40C without problems. It may fall out of accuracy specification at extremes of temperature but it will still work. You can always use the 9700A instead of the 9701A, they are both very accurate when their zero offset voltage is adjusted.

I'm using standard four core unscreened alarm cable with an outside diameter of about 4mm. Two wires are ground (it only needs one but I doubled up on the ground wire), one carries a filtered 5V from the measurement unit and the other is the sense voltage. The 5V comes from a 7805 (from 12V) which also feeds quite a lot of other circuitry. Before going out to the sensors it passes through a 47 Ohm resistor and 47uF/100nF capacitor grounded at the ADC ground pin. The incoming sense wire connects through a 100 Ohm resistor to a 10uF/100nF capacitor, again grounded at the ADC. There is also a 100nF capacitor across VSS and VDD at the sensor itself. It seems to work very well.

In other 'long distance' applications I use a DS18B20 and PIC10F200 combination. They are mounted at the remote end on a small (about 5mm x 10mm) PCB with the sensor flat side overhanging the edge of the board so it lies flat against the measurement surface. The PIC takes care of taking the 1-Wire mesaurement and converts the result into some status information and the sensor reading then sends it as bi-phase serial data down the same alarm cable as before. I use differential signals because the unit is located where there is very sensitive radio monitoring equipment and it helps by minimizing EM radiation from the wires.

Brian.