LM35 and Op-AMP

[ekonoflow]

Hi All.

After searching through the forums i can see this is a common problem - but i cant seem to work out what i need to buy to resolve my issue. I have a Velleman USB 8005 board which has an onboard ADC (8 bit) which takes 0-5v

Ive interfaced an LM35, but due to the low outputs i cant get the results i need ( im measuring temps circa 40-60 degrees celsuis ) - im only getting circa .4 to .8 volts for my temps so im unable to get any meaningful results from the ADC.

I see that i can use a 2 resitors across + and ground and take a reference of 2.5v which may increase the output scale by 2.

I also see that i can use an Op-Amp and scale the output from the LM35 by a factor of 5 which would give me the resolution required by both my application and the ADC.

My (rather long winded) question is - what op-amp and components would i need to buy to fabricate a 0-5v op-amp.

The LM35 and the ADC are all being fed from a 5v supply.

as you can tell, very new to all of this and still trying to get good electronics selection purchased to avoid the frequent trips to maplins.

thanks

David

 

[ericgibbs]

hi,
What tempr range do you want to cover.?

 

[ekonoflow]

Hi,

im really interested from 0 - through to 85 degrees C max

regards

David

 

[ericgibbs]

Hi,

im really interested from 0 - through to 85 degrees C max

regards

David

Hi,
From the LM35 you can see that its basic range is +2Cdeg to +150Cdeg.
At +2C the output is zero, changing by 10mV/C over its range.

The problem is, if the OPA are operating from a single supply, common OPA like the LM358 outputs will only go as low as about +0.5V

You can get rail to rail output OPA's, again they will not go to absolute zero output without a small negative bias on the output.

So for 85C max == 85 * 0.01 =+0.85V hence gain in OPA is going to be *5.88.

Assuming a low cost LM358,, with a +0.5V minimum output is equal to about +9Cdeg.

Is this lower limit going to acceptable.?

Do you follow.?

 

[ekonoflow]

Eric,

+9Cdef is fine, im more interested in the 30-85 degrees range, but will have another sensor for monitor outside airtemp and was hopging to use the same op-amp circuit on the other LM35.

For now though, +9 is absolutely fine as my typical readings with the multimeter have been no lower than 29CDeg.

regards

 

[ericgibbs]

Eric,

+9Cdef is fine, im more interested in the 30-85 degrees range, but will have another sensor for monitor outside airtemp and was hopging to use the same op-amp circuit on the other LM35.

For now though, +9 is absolutely fine as my typical readings with the multimeter have been no lower than 29CDeg.

regards

hi David,
Do you need a circuit design for the LM35 with a LM358 or equivalent.?

EDIT:
Attached a basic circuit, you could use a higher spec OPA.

 

[ekonoflow]

You star!

Could you clarify what the 5V1Z symbol is , and also the 10K0 with the line through it please? Im so new to all this that i dont want to get anything wrong as it takes days for the components to arive in the post!

If you dont mind, could you cast your eyes over https://uk.rs-online.com/web/search/searchBrowseAction.html?method=searchProducts&searchTerm=lm358

and let me know if the LM358 listed there are ok for my requirements?

once again, thank you!

 

[ekonoflow]

You Star!

is it just a basic LM358 i need? Could you also quickly clarify what 10K0 is as it has the line going through it, and also 5V1Z ( a diode ? )

once again, thank you for your quick and thorough advice and help.

David

 

[ericgibbs]

You Star!

is it just a basic LM358 i need? Could you also quickly clarify what 10K0 is as it has the line going through it, and also 5V1Z ( a diode ? )

once again, thank you for your quick and thorough advice and help.

David

Consider that you powered the OPA from a 12V supply, under fault conditions the OPA could drive the input of the ADC upto about +12V. Although the inputs to the PIC are protected, IMO its not a good idea to connect a 12V low impedance source to the ADC input.
The series resistor and 5V1Z, clamp any excessive output to about 5V.

The 10K with a line thru it is a variable resistor, its purpose is to set the gain of the OPA.
In this way you set a +5V output for the 85Cdeg point, OK.?

Do you follow.?

 

[ekonoflow]

Got it - filling in an order now for the parts.

thanks again

 

[rupen]

problem

hey ppl..can you please provide me the circuit diagram for LM35 and then amplifying it using LM358... the circuit that i am building in which LM35 gets really hot

 

[rupen]

hey i am not able to see the circuit diagram you have provided..please resnd it once more

 

[ericgibbs]

hey i am not able to see the circuit diagram you have provided..please resnd it once more

hi,
Here it is: