Require explanation for temperature sensor(LM35) internal circuit.

[sysysy]

hi,

below is the internal circuit of LM35, may i anyone can give me the details explanation regarding this circuit?

I wish to understand the circuit

**broken link removed**

There is some description for this circuit that i not really understand, i already copy and paste below.

why is the transistor use for? how it operate?
what is the op amp use for? amplifier or comparator?

Description:

The circuit diagram is shown above. Briefly, there are two transistors in the center of the drawing. One has ten times the emitter area of the other. This means it has one tenth of the current density, since the same current is going through both transistors. This causes a voltage across the resistor R1 that is proportional to the absolute temperature, and is almost linear across the range we care about. The "almost" part is taken care of by a special circuit that straightens out the slightly curved graph of voltage versus temperature.

The amplifier at the top ensures that the voltage at the base of the left transistor (Q1) is proportional to absolute temperature (PTAT) by comparing the output of the two transistors.

The amplifer at the right converts absolute temperature (measured in Kelvin) into either Fahrenheit or Celsius, depending on the part (LM34 or LM35). The little circle with the "i" in it is a constant current source circuit.

The two resistors are calibrated in the factory to produce a highly accurate temperature sensor.

The integrated circuit has many transistors in it -- two in the middle, some in each amplifier, some in the constant current source, and some in the curvature compensation circuit. All of that is fit into the tiny package with three leads.

<end>

Thanks.

 

[crutschow]

The description you copied seems to explain things pretty clearly.

The transistors are used as a band-gap reference to generate a voltage proportional to temperature.

The amp on the top is not necessarily an op amp, it is just a differential amp.

 

[sysysy]

For A1 comparator, how they compare the output of the two transistors?
A2 is functioning as comparator as well?
I don't know how the A2 work as comparator.
Hope someone could explain to me

 

[ericgibbs]

For A1 comparator, how they compare the output of the two transistors?
A2 is functioning as comparator as well?
I don't know how the A2 work as comparator.
Hope someone could explain to me

hi,
They are not comparators they are configured as amplifiers.

A comparator is used when the output switches from a High to Low or Low to High output depending upon the state of the two input pins. [acts like a switch hi/low,,, on/off]

An amplifier OPA is configured to give a linear output over a range of outputs depending upon the voltages at the two input pins.

Its important to note that the diagrams shown are 'block' diagrams and do not show all the internal components

 

[sysysy]

Hi,

there are two transistors in the center of the drawing. One has ten times the emitter area of the other. This means it has one tenth of the current density, since the same current is going through both transistors. This causes a voltage across the resistor R1 that is proportional to the absolute temperature

From the statement above, may i know why the same current going through the transistor, then the voltage across the resistor R1 that is proportional to the absolute temperature? how the transistor take influence or use for in this situation?

 

[ericgibbs]

Hi,



From the statement above, may i know why the same current going through the transistor, then the voltage across the resistor R1 that is proportional to the absolute temperature? how the transistor take influence or use for in this situation?

hi,
As the notes say the 10e transistor has 10 times the base area of the other transistor, so the current density thru the 10e is ten times smaller that the other.

The Vbe of the transistors 'diode' changes by -2mV/Cdeg rise in temperature so the current flowing thru the transistors will change, the 10e transistor about 10 times less than the 1e.
This imbalance will be amplified by the top OPA and used as feedback to the maintain the voltage to the resistor divider feeding the bases.

Can you say why you need to know this detail.??

 

[sysysy]

hi,

thanks for the reply

Because the lecturer require student understand the sensor operating theory instead just simply implement the project.
There is no point to in learning, he said that.

but, since i have no good in fundamental of electronics, so it bring troublesome to me.

Do u mind to give an overall explanation for this circuit?
I will be very appreciate if you willing to help me.

Thanks.

regards,

sysysy

 

[ericgibbs]

hi,

thanks for the reply

Because the lecturer require student understand the sensor operating theory instead just simply implement the project.
There is no point to in learning, he said that.

but, since i have no good in fundamental of electronics, so it bring troublesome to me.

Do u mind to give an overall explanation for this circuit?
I will be very appreciate if you willing to help me.

Thanks.

regards,

sysysy

hi,
Because you have not got the basics or any fundamental of electronics its not possible for me to explain to you how the circuit works.

Re read the notes with the datasheet,

Its basic operting principle is the change of -2mV/Cdeg change in Vbe of the transistors with increasing temperature.

 

[marivini]

Working principle of lm35 ..and how to detect the heat

 

[atferrari]

Working principle of lm35 ..and how to detect the heat

Why to waste your time and forum space with a such useless post?

 

[schmitt trigger]

Why to waste your time and forum space with a such useless post?

He may have the same lecturer, which 5 years later posted the same project to his pupils. One of them which did a google search and found this old thread.

But in contrast with the original poster which at least made an attempt to understand the circuit, and copied some of the text and images he studied, this next poster didn't even bother to put a question mark at the end of the sentence.

 

[kubeek]

He may have the same lecturer, which 5 years later posted the same project to his pupils. One of them which did a google search and found this old thread.
But in contrast with the original poster which at least made an attempt to understand the circuit, and copied some of the text and images he studied, this next poster didn't even bother to put a question mark at the end of the sentence.

Well the original poster at least asked a question. Marimini just presented a meaningless statement, yet he most likely expects an answer. Also he didn't care to notice that what he presumably wants to know is already written in this thread.

This style of writing reminds me of a math exam, where the test paper said "answer all the questions below and show how you found the answers", but all the "questions" were in the form of "define this" or "calculate that" and not a single question mark.

 

[spec]

Any help?
http://www.ti.com/lit/an/snoa748c/snoa748c.pdf
http://www.spelektroniikka.fi/kuvat/LM35ap.pdf
http://www.introni.it/pdf/Bob Pease Lab Notes Part 9.pdf

 

[large_ghostman]

Well done spec at least some try and help.

 

[MEET2222]

marivini the question asked by some one for how to detect heat is not An usless question if we are working on LM 35 than we should know each an every thing so if you dont know the answer than please dont say it that it is usless

 

[spec]

marivini the question asked by some one for how to detect heat is not An usless question if we are working on LM 35 than we should know each an every thing so if you dont know the answer than please dont say it that it is usless

Yes, agree and, in general, all information is useful/interesting, to me anyway.

Very often a seemingly useless piece of information will come in useful.

The operation/use of band-gaps is fundamental to modern electronics and was a major breakthrough in semiconductor design in the 1960s.

spec