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From: Terminal Crazy on 8 May 2010 12:30
Hi guys,
quick question for the experts.

I'm playing with a system of thermistors feeding an RS485 Networked ADC to
my computor.

My supply is 5V with a 47k resistor as a Voltage divider with the
thermistor acting as the low side.

Q: Can i use the same resistor to feed multiple channels or is it better to
use a seperate one for each channel?.
I'm only useing std cheap resistors but they vary ie 46.1k and 46.3k for
47k resistors.
If i feed off the same one I can use the same values in my temperature
calculation rather than having to measure each one.

Any thoughts please.
TIA

--
Terminal_Crazy

Mitch - 1995 Z28 LT1 M6 terminal_crazy(a)sand-hill.freeserve.co.uk
Lancashire England http://www.sand-hill.freeserve.co.uk/terminal_crazy/
From: Sjouke Burry on 8 May 2010 14:00
Terminal Crazy wrote:
> Hi guys,
> quick question for the experts.
>
> I'm playing with a system of thermistors feeding an RS485 Networked ADC to
> my computor.
>
> My supply is 5V with a 47k resistor as a Voltage divider with the
> thermistor acting as the low side.
>
> Q: Can i use the same resistor to feed multiple channels or is it better to
> use a seperate one for each channel?.
> I'm only useing std cheap resistors but they vary ie 46.1k and 46.3k for
> 47k resistors.
> If i feed off the same one I can use the same values in my temperature
> calculation rather than having to measure each one.
>
> Any thoughts please.
> TIA
>
Use a current mirror(google) to feed each termistor.
From: Terminal Crazy on 8 May 2010 14:01
In article <51144654d4Terminal_Crazy(a)sand-hill.freeserve.co.uk>, Terminal
Crazy <Terminal_Crazy(a)sand-hill.freeserve.co.uk> wrote:
> Q: Can i use the same resistor to feed multiple channels or is it better
> to use a seperate one for each channel?.

Sorry stupid question... all the inputs would be connected to the same
point. Duh.

--
Terminal_Crazy

Mitch - 1995 Z28 LT1 M6 terminal_crazy(a)sand-hill.freeserve.co.uk
Lancashire England http://www.sand-hill.freeserve.co.uk/terminal_crazy/
From: whit3rd on 8 May 2010 15:33
On May 8, 9:30 am, Terminal Crazy <Terminal_Cr...(a)sand-
hill.freeserve.co.uk> wrote:
> Hi guys,
>  quick question for the experts.
>
> I'm playing with a system of thermistors feeding an RS485 Networked ADC to
> my computor.
>
> My supply is 5V with a 47k resistor as a Voltage divider with the
> thermistor acting as the low side.
>
> Q: Can i use the same resistor to feed multiple channels or is it better to
> use a seperate one for each channel?

Best is to use a Wheatstone bridge, and amplify the imbalance between
two
legs (sense leg = +V through 47k pullup to tap, from tap to GND
through thermistor).
The reference leg of the bridge is at the ADC (so resistors match in
temperature)
and the sense leg is at the to-be-sensed temperature. That way, your
sensors can be individually tuned for a common 'zero' point with
adjustments to
their pullups. The bridge amplifier (just about any slow op amp
will do fine)
will boost the temperature signal to, ideally, a few volts so the ADC
has good
temperature resolution.

This requires three wires, though, to the sensor. Other sensor types
that
make a two-wire solution, like AD590 or a diode-connected transistor,
might be suitable, too.
From: Jasen Betts on 9 May 2010 06:13
On 2010-05-08, Terminal Crazy <Terminal_Crazy(a)sand-hill.freeserve.co.uk> wrote:
> Hi guys,
> quick question for the experts.
>
> I'm playing with a system of thermistors feeding an RS485 Networked ADC to
> my computor.
>
> My supply is 5V with a 47k resistor as a Voltage divider with the
> thermistor acting as the low side.
>
> Q: Can i use the same resistor to feed multiple channels or is it better to
> use a seperate one for each channel?.

yes, use a CD4016 or similar to steer the current to the desired
thermistor before reading it. (unless you're interested in measuring
how well insulated the thermistor is)

if you don't have control over when the device takes readings from the
thermistors this probably won't work.

> I'm only useing std cheap resistors but they vary ie 46.1k and 46.3k for
> 47k resistors.
> If i feed off the same one I can use the same values in my temperature
> calculation rather than having to measure each one.



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