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From: email@example.com (Nico Coesel)
Subject: Re: LM35 Temperator Sensor Design Help
Date: Mon, 23 Dec 2002 23:11:00 GMT
NNTP-Posting-Date: 23 Dec 2002 23:11:09 GMT
X-Newsreader: Forte Free Agent 1.21/32.243
"Antony N. Lord" wrote:
>Seasons Greetings People!
>I am wondering if someone could help me interpret the information on the
>LM35 Precision Centigrade Temperature Sensor and assist in design of
>suitable electronics around it.
>I need to install several remote temperature sensors (LM35), run over
>some spare Cat 5 cable to a PC interface (Weeder Technologies Analog
>* Runs to sensors are less than 10m over Cat 5 ethernet cable (4 - 8
>* Interface has 5V, 12V and a variable 3-12V supply rail available
>* Interface board input range 0-4095 mA (source impedance 8k ohm max)
>* My temperature measuring range is from -5°C (fridge) to about 45°
>(hottest day here) perhaps even 80° for measuring some electrical items
>My greatest problem is correct selection of component values for the
>right temperature range. The ranges shown in several figures of the
>"Typical Applications" section of the data sheet are not adequate.
>Figures 6 & 8 appear to be good starts.
I think the problem lies in your interface board. The LM35 is a device
with a variable voltage output and your interface board expects a
current. That's not going to match.
Besides all this, the LM35 is not something you want to use. I
strongly recommend using the LM335. This device has a Kelvin
calibrated output and needs no negative supply.
The LM335 works just like a zener diode. Run a current (constant from
a current source or within range from a resistor) trough it and it
will keep the voltage accross itself in track with temperature. 0
Degrees Celcius means there will be 2.73V accross the device (273
You'll still need an A/D converter which measures input voltage
instead of current. Or a voltage to current converter. Here is link
with a circuit at the bottom of the page that does voltage to current
Reply to nico@nctdevpuntnl (punt=.)
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