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The resistance temperature characteristics of a temperature transducer is show in the figure below,

question

The transducer in question is which of the following? A) Nickel RTD B) NTC thermistor C) Platinum RTD D) PTC thermistor

Note this question is from the 2000 GATE question paper on the Instrumentation subject.

My attempt at a solution

It does not look like Nickel or Platinum RTD,

RTD

Can't be NTC since the slope is positive.

I thought it must be the PTC since it has a "critical temperature" after which Resistance shoots up, but switching PTC thermistors have a slightly negative temperature coefficient up to the point of minimum resistance. While the graph shows a positive slope until the "jump off" temperature.

PTC thermistor

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    \$\begingroup\$ D) is best choice and you are correct about NTC before PTC on polyfuse. bad question Looks more like a thermistor in series with a polyfuse \$\endgroup\$ Jan 23, 2019 at 17:00
  • \$\begingroup\$ But the PPTC isn't a thermistor right? Thanks a lot for putting me on the right track, though. I realized it was D by a process of elimination but I wanted to be sure that the RTD did not act weird at high temperatures. \$\endgroup\$
    – Aditya P
    Jan 23, 2019 at 17:09
  • \$\begingroup\$ options did not include a PPTC which has this threshold effect and thermistors do not have this threshold effect. so its a dumb question \$\endgroup\$ Jan 23, 2019 at 17:13
  • \$\begingroup\$ Right, I totally agree that the question seems wrong. But even if the options had PPTC, I think the question wouldn't be correct because even the PPTC has a small negative slope and then the sort of sudden increase in resistance. \$\endgroup\$
    – Aditya P
    Jan 23, 2019 at 17:45

1 Answer 1

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Remember with multi-choice questions the instruction is to choose the answer which you think is best,

PTC thermistor is a generic term that includes metal RTDs and Polyswitches, - anything with a mostly positive slope.

The curve on that graph is surely some sort of PTC device that two RTD answers do not match this curve, leaving PTC thermistor as the best choice.

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  • \$\begingroup\$ Thanks, I didn't know PTC thermistor was a catch all term, I guess it's hard for them to frame "perfect" questions. It's upto us to choose the best option among them. \$\endgroup\$
    – Aditya P
    Jan 24, 2019 at 7:49

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