Thermometer Sensor Circuit Explained with Op Amp and NPN Transistor

Thermometer Sensor Design and Operation are explained in this video. A single Op Amp, an NPN BJT transistor, two Zener Diodes and two potentiometers are used to design this thermometer sensor circuit. The PN junction of the NPN transistor is serving as temperature probe in this circuit. The value of variable resistors R1 and R2 are computed in this video so that the output voltage of the circuit is approximately zero when the PN junction temperature is at zero degree Celsius. And increases 100 mV per degree Celsius increase in junction temperature. Effectively the output voltage of this thermometer circuit is proportional to the measured temperature. The series of two carefully selected Zener diodes provide us with a robust 8.2v reference voltage with a very small temperature sensitivity of 25 micro Volt per degree C to support excellent temporal thermometer accuracy. A combination of virtual short for the operational amplifier operating in linear region, KCL and KVL are used to find the needed two equations to solve for the two unknowns which are the desired values of the two potentiometers in this circuit.

[@whatever3041]

Hi, I love your videos, can you please make a very thorough video on BJT CE amplifier. Thanks 🙏

[@STEMprof]

You're welcome! Glad that this channel is useful. Sure, regarding BJT Amplifier please see:
Class A BJT Amplifier Design with Transformer ua-cam.com/video/gKlJrqGqeCI/v-deo.html
Differential Amplifier Design with BJT & JFET transistors ua-cam.com/video/zKgSYvk7JAM/v-deo.html
Feedback Amplifier: How to find DC Bias and AC Gain ua-cam.com/video/6dLMxpLNKv4/v-deo.html
How to find Gain of Feedback Amplifier Example with JFET & BJT Transistors ua-cam.com/video/NB1-mYglXsY/v-deo.html
I hope these videos are helpful. 🙋‍♂️

[@shakaibsafvi97]

Hi,
Was waiting for this ..... Cheers !!!

[@STEMprof]

You are welcome! Thanks for your interest in my videos. Please see more sensor circuit videos in ua-cam.com/play/PLrwXF7N522y7Ut9bm8TXAOhIWqL__FGlj.html which is sensor video playlist.

[@Chupacabras222]

Very interesting. Thanks for valuable content. I like the "trick" with those 2 zeners :)

[@STEMprof]

You are welcome 🙏 Glad that you like this video and the technique of series of two Zener Diodes that counteract their temperature dependencies. 🙋‍♂️

[@LousyPainter]

Excellent content!

[@STEMprof]

Thank you! Glad that this video is useful. For more sensor circuits & amplifiers please see ua-cam.com/play/PLrwXF7N522y7Ut9bm8TXAOhIWqL__FGlj.html a

[@byronwatkins2565]

Actually, 4.3(0.055%)-3.9(0.06%)=0.0025% so V = 8.2V(1+0.0003% (T-25C)). Also, by varying the Zener current we could even trim this out to have zero temperature coefficient.

[@STEMprof]

Well said! Thank you for the follow-up accurate calculation of the overall temperature coefficient for the series of two Zener Diodes described in the video. The Zener current adjustment is best done via using a Potentiometer instead of the fixed 1k Ohm resistor. For an alternative Temperature sensor design technique pls see Thermometer Circuit Design with Op Amp and BJT transistor ua-cam.com/video/55YsraFE0rg/v-deo.html

[@STEMprof]

Thermometer Sensor Circuit Operation is explained in this video. For more sensor amplifiers see ua-cam.com/play/PLrwXF7N522y7Ut9bm8TXAOhIWqL__FGlj.html and the following videos
EKG ECG Amplifier with Right Leg Drive Explained ua-cam.com/video/1c7KGXPs4do/v-deo.html
Thermometer Circuit Design with Op Amp and BJT transistor ua-cam.com/video/55YsraFE0rg/v-deo.html
PhotoDiode Amplifier with Data Compression Explained ua-cam.com/video/hqrRx2ufAwg/v-deo.html
Thermometer Current source ua-cam.com/video/Ggf0yCaTTiY/v-deo.html
Strain Gauge Wheatstone Bridge Instrumentation Amplifier Explained ua-cam.com/video/io1yBcCsP-Y/v-deo.html
Temperature-Compensated Programmable Current Source Circuit Design with Zener Diode, BJT Transistors ua-cam.com/video/QY48IQXJIRI/v-deo.html
Temperature-Independent Current Circuit Design with Op Amp, BJT, Zener, Schottky Diodes ua-cam.com/video/hFbnjbddUvs/v-deo.html
Instrumentation Amplifier with Electronic Gain Control ua-cam.com/video/C4tghZ-q6Zs/v-deo.html
For more analog circuits and signal processing examples see: ua-cam.com/play/PLrwXF7N522y4c7c-8KBjrwd7IyaZfWxyt.html
I hope these sensors and thermometer circuits videos are interesting. 🙂

[@alexloktionoff6833]

Can two zeners be replaced with a single TL431 ?

[@STEMprof]

That's a good question. Yes, you can as long as the precision reference voltage is set so that it's temperature variation is negligible compared with -2.1mv per degree voltage change of PN junction. Otherwise, more precise Shunt Voltage Regulator is recommended.

[@shakaibsafvi97]

-15V required :(

[@STEMprof]

-15v is not required. You can change it to -10V and reduce 1k Ohm to say 470 Ohm. Or alternatively we can design the same circuit using +15V instead of -15V. Here is also an alternative implementation: Thermometer Circuit Design with Op Amp & BJT transistor ua-cam.com/video/55YsraFE0rg/v-deo.html

[@TeslaFactory]

Could snap ground for +15V and snap-15V for ground to get the same potential difference, non?

[@STEMprof]

From Voltage difference perspective, yes but we won't then get zero volt at output when PN junction temperature is zero degrees Celsius. We will need a level shifter or further circuit modification.