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Topic: voltage divider for temp sensor (Read 3042 times) previous topic - next topic

jnohs

I am having a hard time figuring out why i need a 2nd resistor to measure resistance from a thermosister. ok so if we change the resistance of a 12 volt supply we end up varying the current. so in order to read it as volts we need to ad a resistor co the change we get is in volts not in current. how does this happen though, i can not see why adding a resistor of known value  in series will now make the voltage vary instead of the current.....

Wow i think i just figured it out..... please tell me if i am right,   what we are doing is  trying to make a tempature sensor, we will start with a supply voltage of 3 volts then in to the thermosister witch will change the resistance according to tempature and as a result we will see on the trailing end of the thermosister a change in current  while still providing 12volts since they are all independent of each other v=I*C  if v is constant and we change  I then c must change also.  so in order to measure the resistance of the thermosister that is now supplied with a varying  current and a constant resistance witch results in a varying of the voltage.

Grumpy_Mike

Quote
please tell me if i am right

Yes bang on. Well done.  :)

It helps you think about the problem when you try and ask the question.

jnohs

ok how do we know witch resistor to use..? what is the math associated with it?

Grumpy_Mike

It is ohms law.
Voltage = Current times Resistance
you can rearrange it so that if you know any two you can find the third.

arduinoadrian

#4
Apr 07, 2014, 05:19 pm Last Edit: Apr 07, 2014, 05:35 pm by arduinoadrian Reason: 1
This is why you need a bias resistor and how to calculate it.
CAUTION: This method will give you a highly nonlinear voltage output with temperature, because of both, the thermistor is a nonlinear device for which its resistance changes nonlinearly with temp and because of the voltage divider used for bias. If you don't know the transfer function of the thermistor with temperature you will be in deep trouble to read the temp and making the linearization. I suggest you use a device giving an already linear output with temp like the LM35 for instance.
Good luck.
Perseverance is 90% of the solution. The remaining 10% is more perseverance.

mrburnette

#5
Apr 21, 2014, 03:30 am Last Edit: Apr 21, 2014, 03:32 am by mrburnette Reason: 1
A general rule of thumb, is to select a series resistor that gives a 50% volktage with the thermistor at the critical temperature you wish to measure: Ex, for room ambident a 10K NTC would have a 10K 1% or better resistor.

Again, for room temp, my experience suggests that using the
http://en.m.wikipedia.org/wiki/Steinhart%E2%80%93Hart_equation
Tracks to within 0.5F at room temp based on 4 separate didital thermometers.  Not bad.

Be aware of the self-heating aspect of thermistors!  This can break the rule of thumb easily!


Ray

arduinoadrian

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Again, for room temp, my experience suggests that using the
http://en.m.wikipedia.org/wiki/Steinhart%E2%80%93Hart_equation
Tracks to within 0.5F at room temp based on 4 separate didital thermometers.  Not bad.

Interesting; but that's when you know the coefficients in the equation and therefore the equation. The problem is when those coefficients are not known for the particular thermistor one is trying to use, in which case they have to be found and that's a complicated and time consuming process which requires special instrumentation to be used as standards.
I used a thermistor a while ago and I could get temp readings  within 0.1C; but that was  based on the manufacturer's provided coefficients and equation.
http://forum.arduino.cc/index.php?topic=194174.msg1434134#msg1434134
Perseverance is 90% of the solution. The remaining 10% is more perseverance.

mrburnette

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I used a thermistor a while ago and I could get temp readings  within 0.1C; but that was  based on the manufacturer's provided coefficients and equation.


Well, yes that is true, but thermistors are inexpensive and the data sheets for major many factors are easily found.  As always, if one goes through surplus junk to salvage parts, they are pretty much on their own to categorize the spoils.

For the above crowd, a late friend of mine posted another way to implement thermistor temperature readings.
http://www.phanderson.com/picaxe/lin_thermistor.html


Ray

arduinoadrian

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As always, if one goes through surplus junk to salvage parts, they are pretty much on their own to categorize the spoils.

That's exactly what I was telling the original poster.
Perseverance is 90% of the solution. The remaining 10% is more perseverance.

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