Yan Seiner wrote:
>
> --- In .com <ts-7000%40yahoogroups.com>,
> Jeff Cunningham < > wrote:
> >
> > Yan Seiner wrote:
>
> > > The document that Jeff quoted shows two ways to wire a three-wire
> > > setup, both on page 7. I can't quite figure out what the difference
> > > is, which one I should use, and why there are 2 resistors in parallel,
> > > one 499 1% and the other 22K 5%.... Are they just trying to adjust
> > > the range of the sensor with the 22K resistor?
> > >
> > > What I have is 12 bit A/D that is currently set up for 0-10V; I can
> > > reduce that down to 0-3.3 if need be. I need to measure normal room
> > > temps; between 60 deg. F and 110 deg. F to an accuracy of about a
> degree.
> > >
> > > The sensors will be powered from the TS7300 5v supply. It will be
> > > remotely mounted (up to 10') in a low-noise environment. I have
> > > shielded twisted pair available which I'd like to use.
> > >
> > > Could someone give me clue?
> > >
> > > Thanks,
> > >
> > > --Yan
> > >
> > If you are powering your sensor from the same supply as the sampling
> > system, you will want to use the left circuit. The one on the left is
> > missing the comment that the 20k resister should be a pot. The whole
> > point of that resister pair is to allow you to calibrate the device.
> The
> > 499 ohm resister gets it in the ball park and the 20k (pot) is used to
> > fine tune. If absolute accuracy is not that important, you can omit the
> > 20k resistor. They are pretty close as is.
>
> I think what has me confused about that one is the + / - on either
> side of the top 499 resistor. The way I read it, I should bring +5v
> at the top, read the sensor where the schematic shows -, and
> shield/ground at the bottom. Right?
>
> Thanks for all your help.
>
> --Yan
>
Actually, I used the simpler 4-wire circuit of Figure 2. The sensor was
at the end of a 40-ft shielded 4-wire cable, mounted outside under my
deck when I lived in Pennsylvania. I was logging temperature once a
second for a couple years just for the hell of it. I calibrated it by
measuring two extreme temperatures with an accurate digital thermometer
and modeling error as a linear offset which I removed in my software. It
worked surprisingly well. And it was very sensitive. At one point I was
seeing these slight temperature fluctuations - little spikes - but only
early in the morning and late in the evening. I thought it was
interference until I realized that they coincided with trips through a
sliding glass door to get firewood. It was during a winter cold spell
(over a week the temperature never rose above 7F and went down as low as
-22F at night). The sensor was about 20 feet to the side and slightly
above that door, but the heat released in passing through it was
sufficient to affect it for a few seconds.
--Jeff
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