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ericgibbs

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Hi I don't know by how much.If it shows 26.5C when it shift to 27.0C the display (LSD) is jittering.After 27.0C is comes up the display is stable.
Suraj,
What resolution does the program output to the display.?

Is it for example: 26.5C > 27.0 >> 27.5 OR 26.5 > 26.6 > 26.7 etc

The LM35 datasheet shows that the accuracy is only 0.5C. what accuracy are you expecting.?
 

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Suraj143

Active Member
Hi my program will increment on 0.5C steps not with 0.1C steps.

It will go like this 26.0,26.5,27.0,27.5 etc.............

I'm doing a direct conversion like you describe 100C = 5V
 
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ericgibbs

Well-Known Member
Most Helpful Member
Hi my program will increment on 0.5C steps not with 0.1C steps.

It will go like this 26.0,26.5,27.0,27.5 etc.............

I'm doing a direct conversion like you describe 100C = 5V
hi,
The conversion I suggested is in 0.1C resolution, using the full 10bit adc result.

If your code is in assembler, please post it and I will run it.
 

Suraj143

Active Member
Hi ericgibbs my circuit worked fine thanks for the help.

A small doubt in your op amp circuit.

Whats the purpose of that 4.7K input resistor & whats the purpose of that 470R output resistor? Do I need those? What actually they do?
 
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ericgibbs

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Hi ericgibbs my circuit worked fine thanks for the help.

A small doubt in your op amp circuit.

Whats the purpose of that 4.7K input resistor & whats the purpose of that 470R output resistor? Do I need those? What actually they do?
hi,
The 4.7K gives some limited protection to the non inverting input and the 470R is just in case the OPA goes well over +5V.
I know there are protection diodes on the pics adc input, but the output impedance of the opa is very low and could drive excessive current into the protection diodes.
 

ericgibbs

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Ok eric I'm not going to insert those two resistors (4.7K & 470R).My supply to the circuit is 12V.
hi,
Without the 470R, if the output for any reason greatly exceeds 5V, then the adc input could be damaged.!

I cannot understand why, for the sake of a 0.01pence resistor you are happy to risk damage to your MCU.:confused:
 

Suraj143

Active Member
Hi ericgibbs thanks for the comment that is very usefull I'll use it.

Can you tell me I'm supplying to your opamp single supply 12V (not dual supply).When there is 0V input whats the gain of that amplifier?

Some one told that is 1.2.

Your diagram shows it can show 2C onwards why it cannot display 0C,1C?
 
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ericgibbs

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Hi ericgibbs thanks for the comment that is very usefull I'll use it.

Can you tell me I'm supplying to your opamp single supply 12V (not dual supply).
When there is 0V input whats the gain of that amplifier?

Some one told that is 1.2.
Who told you that it would have a gain of 1.2, ask him to explain how he got that value.

Your diagram shows it can show 2C onwards why it cannot display 0C,1C?
Because the LM35 datasheet states that for the basic configuration the output is from +2C thru 150C
Which type of OPA are you using.??
 
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Suraj143

Active Member
Ok thanks eric I uses LM358, my friend is telling its the nature of OP Amps in single supply voltages when "0" input it will give you output 1.2

I told that is wrong.

Thanks for confirming & I thank you so much supporting me to sort out my question :)
 

ericgibbs

Well-Known Member
Most Helpful Member
Ok thanks eric I uses LM358, my friend is telling its the nature of OP Amps in single supply voltages when "0" input it will give you output 1.2

I told that is wrong.

Thanks for confirming & I thank you so much supporting me to sort out my question :)
hi Suraj,
The LM358 and CA3140 will saturate at approx 20mV with a 10K output load.
So for a 0V input you should see an output of approx 20mV.

As the LM35 is rated at 2C min and it outputs 10mV/C thats 20mV for 2Cdeg.

Assuming a OPA gain of 5, that will give an OPA output of 100mV, which is well above the 20mV OPA o/p sat.

Many of the older types of OPA, the 741 for example have an output Vsat of about 1 or 2 volts!... thats what your friend is probably thinking of.:)

For a test disconnect the LM35 and short the NON INV input to 0V and then measure the output of the OPA.
 

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Suraj143

Active Member
hi Suraj,
The LM358 and CA3140 will saturate at approx 20mV with a 10K output load.
So for a 0V input you should see an output of approx 20mV.

As the LM35 is rated at 2C min and it outputs 10mV/C thats 20mV for 2Cdeg.

Assuming a OPA gain of 5, that will give an OPA output of 100mV, which is well above the 20mV OPA o/p sat.

Many of the older types of OPA, the 741 for example have an output Vsat of about 1 or 2 volts!... thats what your friend is probably thinking of.:)

For a test disconnect the LM35 and short the NON INV input to 0V and then measure the output of the OPA.
Now I understood totally eric.I think he is talking about old opamps.

You have understood nicely.Thanks for the help & support.
 

Reid87

New Member
a little doubt with the equation for the op-amp LM358 circuit which measure the temperature from 2 deg C to 100deg C..

as i thought that the equation should be Vout = (1+R2/R1)Vsig...
if i use this equation, the R2's value should be 18.8kΩ and R1's value is 4.7kΩ to amplify the signal by the gain of 5.

but the equation for the circuit is Vout = 1+23.5/4.7k*5...

can anyone help me to explain it? thanks...

attachment of the circuit can be found at post #9
 
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