I am playing with a climate controlled (heated/cooled) seat out of a Range Rover, trying to get the heating & cooling working. This seat has a TEC (Peltier) device with an integral NTC* thermistor for feedback.
(*) NTC according to drawings
The thermistor (I believe, not confirmed) is not accessible without destroying the TEC to remove it. I am trying to resolve what thermistor it is, so that I know what resistance corresponds to what temperature.
The other day in ambient temperature of approximately 80f (27C) I measured the resistance at 1046ohms. I then turned on the fan (part of the heating/cooling assembly) and watched the resistance gradually go down to 1020ohms, where it settled and did not change for several minutes. The TEC had no power going to it. There should have been no heat generated apart from that of the fan itself, which would expect to have been more than canceled out by the air that it was moving over the TEC.
Then, with the fan still running, I applied 12V to the TEC and instantly (more accurately: in the time it took me to connect the wires and grab my DMM probes) the resistance shot straight to 1372ohms and did not change during the time i monitored it (about a minute, maybe a bit less).
Then, with the fan still running, I swapped polarity to the TEC and again "instantly" the value shot to 399ohms and remained steady there for nearly a minute.
I did not feel any change in the seat temperature during either test.
What I was expecting to see, was a gradual change in resistance in the first test, followed by another gradual change. The TEC (and the thermistor itself) must have some thermal time constant, so I am perplexed by the "instant" resistance changes and further by the steady prolonged readings.
I suspected maybe my thermistor is bad, so I did a bit of research and I find that one of the less common failure modes is "parameter change." I could not find any resource which was more specific than that, so I'm here asking for personal observations. Can "parameter change" include a NTC thermistor spontaneously gender changing into a PTC thermistor? Could the strange readings I saw be included in the broad description "parameter change?"
Or maybe I should be asking, are my reading even strange? Are my expectations flawed? Does a peltier TEC in fact make "instant" temperature changes? This Peltier is rated 5.5A, at (I believe) 12V. so not a small device.
(*) NTC according to drawings
The thermistor (I believe, not confirmed) is not accessible without destroying the TEC to remove it. I am trying to resolve what thermistor it is, so that I know what resistance corresponds to what temperature.
The other day in ambient temperature of approximately 80f (27C) I measured the resistance at 1046ohms. I then turned on the fan (part of the heating/cooling assembly) and watched the resistance gradually go down to 1020ohms, where it settled and did not change for several minutes. The TEC had no power going to it. There should have been no heat generated apart from that of the fan itself, which would expect to have been more than canceled out by the air that it was moving over the TEC.
Then, with the fan still running, I applied 12V to the TEC and instantly (more accurately: in the time it took me to connect the wires and grab my DMM probes) the resistance shot straight to 1372ohms and did not change during the time i monitored it (about a minute, maybe a bit less).
Then, with the fan still running, I swapped polarity to the TEC and again "instantly" the value shot to 399ohms and remained steady there for nearly a minute.
I did not feel any change in the seat temperature during either test.
What I was expecting to see, was a gradual change in resistance in the first test, followed by another gradual change. The TEC (and the thermistor itself) must have some thermal time constant, so I am perplexed by the "instant" resistance changes and further by the steady prolonged readings.
I suspected maybe my thermistor is bad, so I did a bit of research and I find that one of the less common failure modes is "parameter change." I could not find any resource which was more specific than that, so I'm here asking for personal observations. Can "parameter change" include a NTC thermistor spontaneously gender changing into a PTC thermistor? Could the strange readings I saw be included in the broad description "parameter change?"
Or maybe I should be asking, are my reading even strange? Are my expectations flawed? Does a peltier TEC in fact make "instant" temperature changes? This Peltier is rated 5.5A, at (I believe) 12V. so not a small device.