j.chitte wrote:
>
>
>
>
> --- In <ts-7000%40yahoogroups.com>, Yan
> Seiner <> 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.
>
> 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
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