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# Forenheit thermistor

Resistance = temperature
 (+1) [vote for, against]

This is an electronic integrated circuit which measures its own temperature and converts it to the ratio of voltage to current across it. The power to operate it is drawn from the meter (multimeter in ohms range) used to measure it.

An ohm-meter measures resistance by applying a known voltage (typically below half a volt) to the resistor and displaying the inverse of the resulting current as the resistance. Or, forcing a constant current through it and displaying the resulting voltage scaled as the resistance.

A multimeter is available in almost every technician's toolkit, and so this sort of a temperature sensor would make easy and error free measurement of temperature.

Thermistors already exist, but their resistance is some convoluted function of temperature, and require convoluted mathematical gymnastics to convert to a usable form.

 — neelandan, Nov 17 2013

LM34 Fahrenheit Temp Sensor http://www.jameco.c...s/ProdDS/155192.pdf
Suitable? [csea, Nov 18 2013]

4-Terminal Ohmmeter http://www.ko4bb.co...truction_Manual.pdf
Suitable? [csea, Nov 18 2013]

Does your idea boil down to "a plug-in temperature probe for a multimeter"? If so, it's baked - my multimeter has one, although it uses a thermocouple rather than a thermistor.
 — spidermother, Nov 17 2013

 //a plug-in temperature probe for a multimeter//

Yes, but two-terminal and without batteries ie, entirely passive, like a thermistor.
 — neelandan, Nov 18 2013

 My thermocouple-based one is exactly that - two- terminal and passive. A thermistor would work too, but it wouldn't be as precise.

The multimeter can also give an internal temperature reading (without the external probe connected), which is presumably derived from an internal thermistor. Good thermocouple thermometers need such an internal reference anyway.
 — spidermother, Nov 18 2013

 The thermocouple based meter has a goodish bit of circuitry inside it - an amplifier to boost the minuscule thermocouple voltage, cold junction compensation, etc., and is therefore more complex (and expensive) than an ordinary multimeter.

My proposal is to bring all that complexity inside a purpose-built sensor which can be used with any ordinary multimeter set to the ohms range.
 — neelandan, Nov 18 2013

 Be careful with thermocouples where the cold junction is potentially affected by the hot junction temperature. In that respect, it was correct to choose something else. But you should choose an RTD for your idea, I think, and a thermocouple has a much wider range.

The name of the circuit is a loop-powered transmitter, where the loop is not 24V as normal, but only 0.5V. This is a tough remit as, for example, a pn junction requires more than that.
 — Ling, Nov 18 2013

That is why it is a halfbakery idea rather than a commercial product.
 — neelandan, Nov 18 2013

An LM34 Fahrenheit sensor [link] is a three pin device (V+, Tmp, GND) where Tmp is 10mV/deg F. A DVM with a 4 terminal Ohms function [link2] should be able to power the device, and display. Source current to V+ ref GND, sense voltage between Tmp and GND.
 — csea, Nov 18 2013

Thanks for the explanation; your probe has more smarts than I was imagining. In effect, the multimeter is just an external power supply and display.
 — spidermother, Nov 19 2013

I wonder if it would be possible to create a MEMS thermometer that is basically an array of small bi-metalic switches that switch at different temperatures, connecting resistors in parallel as the temperature drops. This would create a completely passive two-terminal device who's resistance is the temperature in Fahrenheit. My guess is you'd have a few ohms of switch resistance, so you wouldn't be able to get accurate meaurements very close to 0.
 — scad mientist, Nov 19 2013

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