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'[PIC]: Using LM35 ??'
2002\01\10@011922 by

Hi everyone,

I´m trying to read negative temperature using LM35, and one 16F877.
It gives me 10mv/ºC, so at -55º, I´ll have -550mv.
But I want to have 0 Volt at -10ºC, How do I change the zero position ??

It it for being able to read this temperature with the 16F877.

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A) Use a diferential amplifier ( I hope you are familiar with the
configuration), Vout = vp - vn where vp is the potential on
+ input and vn is the potential on - input. On Vp put a reference of +550mV, then for -55C the output will be 0V.
For positive temperature the output will be +550mV +temperature potential.
B) If you can go only to -45...-50 C then no external operational is need.
Take a look to my site:
http://www.geocities.com/vsurducan/electro/PIC/f877.html

regards, Vasile

On Thu, 10 Jan 2002, Gabriel Caffese wrote:

{Quote hidden}

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part 1 2194 bytes content-type:text/plain; charset=ISO-8859-1 (decoded quoted-printable)

The circuit as you described it, with +550 mV on the (+) input of the
diffamp and the LM35 output connected to the (-) input of the diffamp,
would give 0V output at +55C, not -55C.  The diffamp output would go
positive for temperatures below 0C, and negative for temperatures above
0C.  Also, the output of the LM35 can't go below ground (at 0C) without
some help.

The LM35 data sheet (http://www.national.com/ds/LM/LM35.pdf) recommends
a couple of ways for dealing with this.  One way, shown in Figure 7 of
the data sheet (copy attached as Image2.gif), allows temperature sensing
below 0C by placing a couple of diodes in series with the ground lead of
the LM35, and lightly pulling down the LM35 output with a resistor to
ground to allow the LM35's output to go a bit negative with respect to
its ground pin.

This method requires that two of the PIC's analog inputs be used; one to
read the LM35's output pin, and the other to read the potential on the
LM35's ground pin.  After taking the two A/D readings, a simple
subtraction yields the temperature result.  The only components required
are two 1N4148 (or equivalent) diodes and an 18K resistor, which should
be somewhat less expensive than a differential amplifier.

Dave

Vasile Surducan wrote...

{Quote hidden}

part 2 6983 bytes content-type:image/gif; name=Image2.gif (decode)

part 3 136 bytes
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At 02:13 1/10/2002 -0300, you wrote:
>Hi everyone,
>
>   I´m trying to read negative temperature using LM35, and one 16F877.
>   It gives me 10mv/ºC, so at -55º, I´ll have -550mv.
>   But I want to have 0 Volt at -10ºC, How do I change the zero position ??
>
>    It it for being able to read this temperature with the 16F877.
>
>

Hi Gabriel,

You'll need to scale it with an op-amp to get zero at -55. Just subtract
550mV from the LM35 voltage.

We've used them at very cold temps. here and they work great. We use a look
up table instead of an op-amp to scale the temps. Much easier in our
situration.

Good luck

chuck

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"Erare humanum est, perseverare diabolicum ! "
You have right Dave, I've screw up with signs, he need a -550mV reference
indeed. But the floating structure for LM35 is not very nice, even it use
only one supply. I'll go on fig.2 application, using a negative voltage
too. 877 must have -Vref=-500mV,( minimum -50C, but I've guess at -55C you
are already dead)  +Vref=+2.5V ( the lowest value it can be ). LM35
output to one analog channel input. All negative inputs to PIC must be
protected like I've point in the schematics. Of course will not be able
to use the whole AD 10 bit resolution.
Or a diferential amplifier with gain greater than 1 to improve your
solution from image2.gif and use only one analogic input.

regards, Vasile

On Thu, 10 Jan 2002, Dave Dilatush wrote:

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Thanks Dave and Vasile.

Gabriel Caffese
----- Original Message -----
From: Dave Dilatush <dilatushHOME.COM>
To: <PICLISTMITVMA.MIT.EDU>
Sent: Thursday, January 10, 2002 9:37 AM
Subject: Re: [PIC]: Using LM35 ??

The circuit as you described it, with +550 mV on the (+) input of the
diffamp and the LM35 output connected to the (-) input of the diffamp,
would give 0V output at +55C, not -55C.  The diffamp output would go
positive for temperatures below 0C, and negative for temperatures above
0C.  Also, the output of the LM35 can't go below ground (at 0C) without
some help.

The LM35 data sheet (http://www.national.com/ds/LM/LM35.pdf) recommends
a couple of ways for dealing with this.  One way, shown in Figure 7 of
the data sheet (copy attached as Image2.gif), allows temperature sensing
below 0C by placing a couple of diodes in series with the ground lead of
the LM35, and lightly pulling down the LM35 output with a resistor to
ground to allow the LM35's output to go a bit negative with respect to
its ground pin.

This method requires that two of the PIC's analog inputs be used; one to
read the LM35's output pin, and the other to read the potential on the
LM35's ground pin.  After taking the two A/D readings, a simple
subtraction yields the temperature result.  The only components required
are two 1N4148 (or equivalent) diodes and an 18K resistor, which should
be somewhat less expensive than a differential amplifier.

Dave

Vasile Surducan wrote...

{Quote hidden}

??
>>
>>     It it for being able to read this temperature with the 16F877.
>>

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Of course you could also use the LM335, which gives 10mV/K instead of
10mV/C, so your output is always positive. 0C is at 2730mV, -10C at 2630mV.
Then do the offset in software. No opamps required.
/\/\/\/*=Martin

{Original Message removed}
Martin Peach wrote...

>Of course you could also use the LM335, which gives 10mV/K instead of
>10mV/C, so your output is always positive. 0C is at 2730mV, -10C at 2630mV.
>Then do the offset in software. No opamps required.

Damn!  Why didn't I think of that?  <KICK><KICK><KICK>

Nice part, not too expensive either: US\$1.19 at 100 ea. in Digi-Key.

Dave

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Thanks, Chuck.-

----- Original Message -----
From: Chuck Mancuso <aecsysMOSQUITONET.COM>
To: <PICLISTMITVMA.MIT.EDU>
Sent: Thursday, January 10, 2002 11:51 PM
Subject: Re: [PIC]: Using LM35 ??

> At 02:13 1/10/2002 -0300, you wrote:
> >Hi everyone,
> >
> >   I´m trying to read negative temperature using LM35, and one 16F877.
> >   It gives me 10mv/ºC, so at -55º, I´ll have -550mv.
> >   But I want to have 0 Volt at -10ºC, How do I change the zero position
??
{Quote hidden}

look
{Quote hidden}

--

But also you'll have the same problem regarding full scale resolution...
Without external op-amp you'll never have it ! And the software offset
will not solve the problem...
regards, Vasile

On Fri, 11 Jan 2002, Dave Dilatush wrote:

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Vasile Surducan wrote...

>But also you'll have the same problem regarding full scale resolution...
>Without external op-amp you'll never have it ! And the software offset
>will not solve the problem...

Well, I'm not sure this is a "problem", as the OP didn't specify what
resolution he needed over what temperature range; he simply asked for a
way to read temperatures below 0C.  With a 10mV/K sensor (LM335) and a
10-bit PIC A/D operating from a 5.0V Vref, he gets a resolution of
roughly 0.5C which may or may not suit his needs.  If 0.5C resolution is
sufficient, it doesn't particularly matter that the entire A/D input
range isn't utilized.

Obviously, if he wants to get the maximum available resolution over a
specific temperature range, the LM335 output would have to be offset and
scaled appropriately; but he didn't say anything about that.

If that's the case, a simple circuit consisting of a RRIO opamp (e.g.,
1/2 of a National Semiconductor LMC6482) and three resistors will do the
trick, as follows:

1) Connect the opamp (+) input to the output of the LM335;
2) Connect the opamp output to the PIC A/D input pin;
3) Connect one of the resistors (R1) from Vcc to the opamp (-) input;
4) Connect resistor R2 from the opamp (-) input to ground; and
5) Connect resistor R3 from the opamp (-) input to the opamp output.

Let Tlow be the temperature at which you want the opamp output to be
zero volts (i.e., the lowest temperature you want to measure); at this
temperature the LM335 output is Vlow = Tlow/100, where Tlow is expressed
in Deg. K.  Let Thi be the temperature at which you want the opamp
output to be 5V (your upper temperature limit); at this temperature the
LM335 puts out Vhi = Thi/100, where Thi is expressed in Deg. K.

6) Choose an arbitrary, convenient value for R2, say 10K ohms.

Since the opamp will act to keep its (-) input equal to its (+) input,
we can write a pair of nodal equations for the (-) input--one at Tlow
and one at Thi--that will allow us to solve for the other two resistors,
R1 and R3:

7)    (5 - Vlow)*(1/R1) - Vlow*(1/R2) - Vlow*(1/R3) = 0; and
8)    (5 - Vhi)*(1/R1) - Vhi*(1/R2) + (5 - Vhi)*(1/R3) = 0.

Plug Vlow, Vhi, and the chosen value of R2 into 7) and 8) above and
solve them simulaneously to give the required values of R1 and R3.

Voila.

Dave Dilatush

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OKI-DOKI, I'm satisfied now !
best,best,

Vasile

On Mon, 14 Jan 2002, Dave Dilatush wrote:

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