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Topic: Decoding thermistor characteristics table (Read 2 times) previous topic - next topic

synfinatic

Actually, looking at the tables, am I confused or is that not for a NTC?  Looks like the resistance is *decreasing* as temperature increases.  Seems backwards.  Or am I just really confused?

mowcius


Actually, looking at the tables, am I confused or is that not for a NTC?  Looks like the resistance is *decreasing* as temperature increases.  Seems backwards.  Or am I just really confused?

Seems right to me.

Like an LDR, the resistance gets less with more light, with a thermistor, resistance gets less with more heat.

retrolefty

#7
Jan 30, 2011, 06:51 pm Last Edit: Jan 30, 2011, 06:53 pm by retrolefty Reason: 1
Thermistor are available with either negative temperature coefficient or positive temperature coefficient factors. The NTC is a negative variety. How the thermistor is wired in the voltage divider (either top of bottom element) also effects the direction of voltage change Vs temperature change.

Lefty

synfinatic

Yep, you're right.   I just measured the resistance of the thermistor using my multimeter, and it's definitely increasing as the temp decreases (ice water vs room temp).

So if V = IR, I'm guess current must be the same regardless of temperature, so V is proportional to R.  As R increases, so does the voltage.  Hence, as temp drops, the resistance increases which increases the voltage towards the max, which in this case is the input: 5V.  That' what I had wrong- I thought V & R were inversely proportional.

So, I guess the last thing to figure out is which column to use in the tables.  The information about the thermistor says the beta value (100/200 - what does that mean?) is 4300k.  The 2nd to last column has a label of 4300 at the bottom (labeled b value @ 25/50C).  I guess that must be it?  So at 0C it should read 352.4 * 100Kohm (.3524M Ohm)?  

Doesn't seem to match that or any column though.  My multimeter (+/-(%2 + 4) accuracy) is showing .75M @ 0C and .28M at 18C.  Or am I confused again?

Thanks!

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