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Author Topic: Which thermistor to use??  (Read 2088 times)

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Offline cloneTopic starter

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Which thermistor to use??
« on: September 05, 2009, 05:09:00 PM »
I am making a digital thermometer using an arduino.  I was gonna base my code off of this to make it http://www.arduino.cc/playground/ComponentLib/Thermistor2

it says to use a 10k thermistor and a 10k resistor.  the electronics shop i went to didnt have 10k thermistors, so i got a 4k7, 30k, and 50k.

I want the thermometer to measure between 0-250 degrees F.  Which one should i use, or does it even matter?

thnx.

Offline Joker94

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Re: Which thermistor to use??
« Reply #1 on: September 05, 2009, 06:35:21 PM »
Well if it says 10k and has been proven by others go with 10k.

If you want thermisters have a look here

http://www.futurlec.com.au/test13.jsp?category=THERMISTORS&category_title=Thermistors&main_menu=OTHERS&sub_menu=THERMISTORS

Offline cloneTopic starter

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Re: Which thermistor to use??
« Reply #2 on: September 05, 2009, 09:33:17 PM »
well i already have 3, so im gonna end up using one of them.  plus i dont wanna purchase online because of the wait and shipping costs.

I guess i need to clarify what I want to know about thermistors.   what determines a thermistors temperature range?  what is the difference between a 1k thermistor and a 10k thermistor?  Does the 10k have more precision or something?  whats the difference between the various types of thermistors?  I have one thats labeled 'bead' another thats labeled 'disk' and another that isnt labeled but looks weird.. i also saw some that look like a disc capacitor.  attached are some pictures of the ones i have.


Offline Soeren

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Re: Which thermistor to use??
« Reply #3 on: September 06, 2009, 11:46:27 AM »
Hi,

what is the difference between a 1k thermistor and a 10k thermistor?
The resistance obviously ;D


Does the 10k have more precision or something?
Nope.


whats the difference between the various types of thermistors?  I have one thats labeled 'bead' another thats labeled 'disk' and another that isnt labeled but looks weird.. i also saw some that look like a disc capacitor. 
Bead thermistors are shaped like, well, small beads and disks are... You guessed it - Disc shaped.
The smaller the physical size (mass), the faster it reacts and v.v.
Some are like the head of a pin for fast reaction (used on eg. the early variety of "instant" body thermometers). Some are integrated into various shapes of bolts and tubes etc. it's a matter of adapting shape to purpose.

attached are some pictures of the ones i have.
Ahemmm?


You can use either of the ones you have for the schematic you linked to, just use a resistor of the same value in the voltage divider.
The nominal value of a thermistor is the resistance at 25°C.

NTCs are non-linear devices.
You can either use a table to correct for this response, or, if less sensitivity is OK, it can be linearized to a reasonable degree by adding a parallel resistor (which must be matched to the specific NTC and the main temperature region it's supposed to act in).

It's a bad idea to go 0°F to 250°F, as the non-linearity will be vast over such a range.
In a body thermometer for instance, you only need a range of 35°C to 42°C, making the linearity error very small.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline cloneTopic starter

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Re: Which thermistor to use??
« Reply #4 on: September 06, 2009, 05:20:05 PM »
i was planning on using the Steinhart-Hart Thermistor Equation to correct for non-linearity.

I actually only need a temperature range in the range of boiling water and alcohol for use in an automated still.

Offline cloneTopic starter

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Re: Which thermistor to use??
« Reply #5 on: September 09, 2009, 09:14:41 PM »
Well, I decided to use the 30k ohm bead thermistor, because i found a data sheet pdf of a similar one on the internet.  the data sheet showed the resistance for every 5 degrees celcius... so i ended up just using a lookup table type thing in my code instead of using the steinhart equation.  Here is a video of the finished project!

Digital thermometer arduino project

Offline Joker94

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Re: Which thermistor to use??
« Reply #6 on: September 09, 2009, 09:35:37 PM »
nice job clone, any chance of a shematic

Offline cloneTopic starter

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Re: Which thermistor to use??
« Reply #7 on: September 09, 2009, 09:55:01 PM »
just drew this up super quick.  i hooked up both 7 seg displays together.. pin for pin.. and the code in the arduino triggers the transistors to switch between the displays.. it does it super quick so u dont even notice that its only lighting up 1 display at a time.  enjoy!
« Last Edit: September 09, 2009, 09:57:36 PM by clone »

Offline Razor Concepts

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Re: Which thermistor to use??
« Reply #8 on: September 09, 2009, 10:29:01 PM »
LEDs did not burn out without resistors??

Offline cloneTopic starter

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Re: Which thermistor to use??
« Reply #9 on: September 10, 2009, 03:17:09 PM »
nope.. didnt need any resistors.  the leds aren't that bright as it is!

Offline Soeren

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Re: Which thermistor to use??
« Reply #10 on: September 10, 2009, 03:58:08 PM »
Hi,

PIC outputs are internally limited to... around 25mA max. IIRC.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

 


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