--- In Yan Seiner <> wrote:
>
> j.chitte wrote:
> >
> >
> >
> >
> > --- In <ts-7000%40yahoogroups.com>, Yan
> > Seiner <yan@> wrote:
> > >
> > > j.chitte wrote:
> > > >
> > > >
> > > >
> > > > This may also be a case of sinking current as well as sourcing
> > > > current. IIRC the 197 in unipolar mode is biased at half full
> > scale so
> > > > voltages less than that will probably need the sampled source to sink
> > > > current. Many applications supply current better than they sink it.
> > > >
> > > > As all correspondents suggested last time : buffer with an op-amp.
> > > >
> > > > (and check the amp's ability to sink).
> > > >
> > >
> > > Hehe... I am to hardware design as Bozo the Clown is to Shakespeare. :-)
> >
> > LOL. I think we'd guessed that by now.
> > As dirty Harry said: "a good man knows his limitations" ;)
> >
> > >
> > > I can wield a soldering iron usually without injury but that's about
> > > it. Further, this is purely a hobby project so I don't have any
> > > resources to draw on.
> > >
> > > However, googling around I discovered an LM324, which looks like it
> > will
> > > work - 5 VDC supply, 4 on a chip so I only need 4 chips, and a simple
> > > voltage divider to get the gain I need. Any hardware gurus confirm that
> > > choice?
> > >
> > > http://www.national.com/ds/LM/LM124.pdf
> > >
> > > I'm thinking the non-inverting DC Gain circuit on page 9 will work.
> > >
> > > No?
> > >
> >
> > Yep, with R1=R2, that's the voltage follower configuration I
> > recommended a few posts back. In fact the LM2902 which that data sheet
> > covers is what I use to buffer my thermistor temperature sensors.
> >
> > If you make the two resistors the same you will have a unity gain
> > buffer which if you use a large value (100k ?) will draw nearly
> > nothing from you LM24.
> >
> > However, current sink is an issue here again. These op amps can go
> > "near" the ground rail but cannot sink much current. See the data
> > section. They are not truly rail-to-rail output.
> >
> > I found I needed to "help" them a little with a 2k pull down resistor
> > on the output.
> >
> > Since your LM24 output is small, you'd probably be best biasing the
> > output by adding in half the supply voltage and subtracting this out
> > in software.
> >
> > Use the summing configuration also on page 9 , drop the negative
> > inputs add half the 5V rail to one input and your LM24 to the other.
> >
> > If you don't have a clean , stable 5V or you want more precision use a
> > bandgap reference like LM317. The applications notes show how to
> > increase the nominal 1.2V output. So you can double it to put your
> > op-amp in the middle of the 197 input range and avoid the current sink
> > limitations of the amp.
> >
> > Again, using high value input resistors you will be able to use one
> > reference voltage for several temp sensor circuits but watch out for
> > spikes if you are anywhere near the HT circuit of the car. Large
> > resistors will pick up EM spikes and spark plugs make lots of those.
> >
> > Use 1% resistors since all these errors add up quickly.
> >
> > I hope that's enough to get you started.
> >
>
> That's enough. I got to thinking about this... I have the LM34CZ,
> which can go from -40 to +240 degrees, or -0.4V to +2.4V, if provided
> with a negative supply. See Figure 2 in the spec sheet:
> http://www.national.com/ds/LM/LM34.pdf
>
> I can get a MAX1044 http://www.national.com/ds/LM/LM34.pdf and provide
> -5V to the LM34. Would this work better? It would certainly be simpler.
I don't understand the better and simpler bit in giving -5 to the LM34.
Presumably you don't need the neg range for the fridge. Even if it has a
freezer, the LM34 is a fahrenheit device so you get well below freezing without
going negative.
Anyway , yes a negative supply probably would help ensure that the op-amp can
swing to zero without fussing around. Just be sure to check it does correctly
follow low inputs during testing.
>
> >
> > If it's a secret, what's the project?
> >
>
> For now, it's some instrumentation for a fridge. I have a high-tech
> fridge that runs off of 12VDC. The manufacturer claims very low power
> usage. I want to instrument it so I can see just how much power it's
> using. Even with half my sensors not working, it's given me data that
> says the fridge it doesn't have sufficient ventilation, and thus is
> using about 4x the power it should be.
>
> In the long run, I want to look at instrumenting an incubator for
> reptiles, with a webcam and a live feed. Once I have the data
> acquisition figured out, doing some controls should be simple.
>
> There's other applications - temp alarms for animal housing, etc.
>
> --Yan
>
>
> --
>
> o__
> ,>/'_ o__
> (_)\(_) ,>/'_ o__
> A day may come (_)\(_) ,>/'_ o__
> when the courage of men fails, (_)\(_) ,>/'_
> when we forsake our friends and break all (_)\(_)
> bonds of fellowship, but it is not this day.
> It is not this day!
> This day we ride!
>
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