Multiple Sensor Array

[Terminal Crazy]

Hi guys,
I'm building a temperature monitoring system.

I've got about 8 Thermistors reading into a networked IO device.

I'm feeding the Thermistors from a 5V supply (LM317) through a potential
divider 47k resistor which works fine.

Q1 How many can I reasonably feed of the one supply without affecting the
other results

ie

5V_____________________________________________________.......
/ / /
\ \ \
/ 47K / 47K / 47K
\ \ \
| | |
------IO Input ------Input ------Input
/ / /
\ \ \
/Thermistor /Thermistor /Thermistor
\ \ \
0V___/__________________/________________/______________......


Also I need to mount this small circuit board onto DIN Rail along with the
IO Module & relays etc in the cabinet.

Q2 what would you use ? I havn't found much. My best thought was to
mount it onto a DIN Terminal and fix it with that. Any suggestions please.

--
Terminal_Crazy

Mitch - 1995 Z28 LT1 M6 terminal_crazy@sand-hill.freeserve.co.uk
Lancashire England http://www.sand-hill.freeserve.co.uk/terminal_crazy/

 

[Jasen Betts]

On 2010-10-10, Terminal Crazy <Terminal_Crazy@sand-hill.freeserve.co.uk> wrote:

Hi guys,
I'm building a temperature monitoring system.

I've got about 8 Thermistors reading into a networked IO device.

I'm feeding the Thermistors from a 5V supply (LM317) through a potential
divider 47k resistor which works fine.

Q1 How many can I reasonably feed of the one supply without affecting the
other results

5V_____________________________________________________.......
/ / /
\ \ \
/ 47K / 47K / 47K
\ \ \
| | |
------IO Input ------Input ------Input
/ / /
\ \ \
/Thermistor /Thermistor /Thermistor
\ \ \
0V___/__________________/________________/______________......

Until the total resistance in the common wires approaches 10 ohms you're unlikely
to notice any interferance between devices. (assuming 10 bit DAC)


--
ɹǝpun uʍop ɯoɹɟ sƃuıʇǝǝɹ⅁

 

[Terminal Crazy]

In article <i8s0mr$6a3$1@reversiblemaps.ath.cx>,
Jasen Betts <jasen@xnet.co.nz> wrote:

Q1 How many can I reasonably feed of the one supply without affecting
the other results

Until the total resistance in the common wires approaches 10 ohms you're
unlikely to notice any interferance between devices. (assuming 10 bit
DAC)

Can i ask how you came up with that figure and why ?

TIA

--
Terminal_Crazy

Mitch - 1995 Z28 LT1 M6 terminal_crazy@sand-hill.freeserve.co.uk
Lancashire England http://www.sand-hill.freeserve.co.uk/terminal_crazy/

 

[Rich Webb]

On Sun, 10 Oct 2010 09:54:07 +0100, Terminal Crazy
<Terminal_Crazy@sand-hill.freeserve.co.uk> wrote:

Hi guys,
I'm building a temperature monitoring system.

I've got about 8 Thermistors reading into a networked IO device.

I'm feeding the Thermistors from a 5V supply (LM317) through a potential
divider 47k resistor which works fine.

Q1 How many can I reasonably feed of the one supply without affecting the
other results

Feed your A/D's Vref from the same source as the top of the voltage
dividers. That obviates any effect from regulator variation with load or
temperature (assuming it stays within the allowable window for Vref).

If you're feeding them daisy-chain style, as you've shown, rather than a
star configuration, then you may see the effects of the voltage drop in
the supply and ground rails. Whether that's significant is something
you'll have to calculate, based on the number of bits you need, how many
drops there are, and the contributions of other error sources.

E.g.,

V5 ___Rwire ___Rwire
o---.----|___|----.----|___|---- ... ----.
| | |
.-. .-. .-.
| | | | | |
| |47K | |47K | |47K
'-' '-' '-'
| | |
.--o .--o .--o
| | |
.-. .-. .-.
| | | | | |
| |RT | |RT | |RT
'-' '-' '-'
| ___ | ___ |
o---'----|___|----'----|___|---- ... ----'
GND Rwire Rwire

(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

--
Rich Webb Norfolk, VA

 

[Terminal Crazy]

In article <3ti3b69mt8db7lc3pu5gum2shmha06b0dv@4ax.com>, Rich Webb
<bbew.ar@mapson.nozirev.ten> wrote:

Feed your A/D's Vref from the same source as the top of the voltage
dividers. That obviates any effect from regulator variation with load or
temperature (assuming it stays within the allowable window for Vref).
I've got seperate voltage divider feeding one channel so I can monitor the

voltage used to calc the temperature of each channel.

If you're feeding them daisy-chain style, as you've shown, rather than a
star configuration,

Just the Thermistors are fed on a long wire pair (about 20 - 30 metres max
), everything else is built & powered from 1 circuit board next to the IO
Device.

Any ideas on the best way to mount a PCB to DIN rail ?

Thanks

--
Terminal_Crazy

Mitch - 1995 Z28 LT1 M6 terminal_crazy@sand-hill.freeserve.co.uk
Lancashire England http://www.sand-hill.freeserve.co.uk/terminal_crazy/

 

[amdx]

"Terminal Crazy" <Terminal_Crazy@sand-hill.freeserve.co.uk> wrote in message
news:5163ef1891Terminal_Crazy@sand-hill.freeserve.co.uk...

Hi guys,
I'm building a temperature monitoring system.

I've got about 8 Thermistors reading into a networked IO device.

I'm feeding the Thermistors from a 5V supply (LM317) through a potential
divider 47k resistor which works fine.

Q1 How many can I reasonably feed of the one supply without affecting the
other results

ie

5V_____________________________________________________.......
/ / /
\ \ \
/ 47K / 47K / 47K
\ \ \
| | |
------IO Input ------Input ------Input
/ / /
\ \ \
/Thermistor /Thermistor /Thermistor
\ \ \
0V___/__________________/________________/______________......


Also I need to mount this small circuit board onto DIN Rail along with the
IO Module & relays etc in the cabinet.

Q2 what would you use ? I havn't found much. My best thought was to
mount it onto a DIN Terminal and fix it with that. Any suggestions
please.

--
Terminal_Crazy
Hey Terminal,

I have no answer to your question, but here's a screenprint from my
temperature
monitoring system. I have it set so if any sensor goes above 5*F it the
display will
turn red as an indicator there may be a problem. As you see I have 10
freezers
functioning properly and 3 that are empty and off. The program is always
running
on my computer, so a quick glance several times a day lets me know things
are OK.
When a freezer is empty the program allows me to move that freezer number
into the other group.
I have two groups, Full Freezers and Empty Freezers, I could have many more.
http://i395.photobucket.com/albums/pp37/Qmavam/LampomitarriScreenprintJPG.jpg
I'm using Maxims 1 wire system, I have 13 sensors but it will allow many,
many more.
The display program I chose is Lampomitarri, it fulfilled my need
perfectly, there are many
other display programs that may fit your need better.
MikeK
Helpful 1 wire forum.
http://www.cocoontech.com/forums/index.php?act=SF&s=&f=41

 

[Jamie]

Terminal Crazy wrote:

Hi guys,
I'm building a temperature monitoring system.

I've got about 8 Thermistors reading into a networked IO device.

I'm feeding the Thermistors from a 5V supply (LM317) through a potential
divider 47k resistor which works fine.

Q1 How many can I reasonably feed of the one supply without affecting the
other results

ie

5V_____________________________________________________.......
/ / /
\ \ \
/ 47K / 47K / 47K
\ \ \
| | |
------IO Input ------Input ------Input
/ / /
\ \ \
/Thermistor /Thermistor /Thermistor
\ \ \
0V___/__________________/________________/______________......

Lets assume you load the 317 to 50%.. We need a Total R load of 10 ohms.

TotalUnits = (1/10)/(1/47k) = 4700;

I don't think you have much to worry about..

 

[Michael A. Terrell]

Terminal Crazy wrote:

In article <3ti3b69mt8db7lc3pu5gum2shmha06b0dv@4ax.com>, Rich Webb
bbew.ar@mapson.nozirev.ten> wrote:

Feed your A/D's Vref from the same source as the top of the voltage
dividers. That obviates any effect from regulator variation with load or
temperature (assuming it stays within the allowable window for Vref).
I've got seperate voltage divider feeding one channel so I can monitor the
voltage used to calc the temperature of each channel.

If you're feeding them daisy-chain style, as you've shown, rather than a
star configuration,

Just the Thermistors are fed on a long wire pair (about 20 - 30 metres max
), everything else is built & powered from 1 circuit board next to the IO
Device.

Any ideas on the best way to mount a PCB to DIN rail ?

Plug in octal housing and a DIN relay socket?


--
Politicians should only get paid if the budget is balanced, and there is
enough left over to pay them.

 

[Jasen Betts]

On 2010-10-10, Terminal Crazy <Terminal_Crazy@sand-hill.freeserve.co.uk> wrote:

In article <i8s0mr$6a3$1@reversiblemaps.ath.cx>,
Jasen Betts <jasen@xnet.co.nz> wrote:

Q1 How many can I reasonably feed of the one supply without affecting
the other results


Until the total resistance in the common wires approaches 10 ohms you're
unlikely to notice any interferance between devices. (assuming 10 bit
DAC)

Can i ask how you came up with that figure and why ?

I didived your 47K by about 500 but it's just a guess.
figuring half the noise wouldn't make it back to the DAC

I was only figuring one device effecting the reading from another,
upon re-thinking it I reaslise it's likely that several thermisters
will show the same trend ans so their effects will add.

if you have multiple devices then divide the 10 by (n-1)

If you want to be sure do up a circuit model and apply kirchoff's laws.

another option is to run the bias current up the sense wire and switch
that current to only power one thermistor at a time

Perhaps you could use a 4017 decade counter, put the 47K bias resistor in series with the
VCC supply measure the voltage at the VCC pin and connect the thermistors between
the outputs and ground.

your micropcontroller would need to emit clock and reset pulses to
drive the 4017 and read one anaglogue input...

HEF4017 needs 3V to operate reliably so the bias resistor may need to
be reduced until there's sufficient current through any thermistor
for 3V drop.

TIA

--
ɹǝpun uʍop ɯoɹɟ sƃuıʇǝǝɹ⅁

 

[Jasen Betts]

On 2010-10-10, Terminal Crazy <Terminal_Crazy@sand-hill.freeserve.co.uk> wrote:

In article <3ti3b69mt8db7lc3pu5gum2shmha06b0dv@4ax.com>, Rich Webb
bbew.ar@mapson.nozirev.ten> wrote:

Feed your A/D's Vref from the same source as the top of the voltage
dividers. That obviates any effect from regulator variation with load or
temperature (assuming it stays within the allowable window for Vref).
I've got seperate voltage divider feeding one channel so I can monitor the
voltage used to calc the temperature of each channel.

If you're feeding them daisy-chain style, as you've shown, rather than a
star configuration,

Just the Thermistors are fed on a long wire pair (about 20 - 30 metres max
), everything else is built & powered from 1 circuit board next to the IO
Device.

If you can run each thermistor on a separate loop that will reduce
crosstalk too, another option is to use thicker wires for the +5 and
the ground return,


or even put the 5V supplky closer to the thermistors.

+ ----------------------[7805]--+...
9VDC (isolated) | |
- ------------------------+ [R1] others
| | wired in
--------------------------------+ parallel
sense 1 | | like original
| [T1] diagram
analogue ground | |
--------------------------+-----+...

for the 9V you could use something like a YCL101 from a old thin coax
ethernet card. or a transformer with two secondaries...
it's important that the 9V is lsolated from the main supply so that
thermistor current doesn't flow in the analogue ground wire

--
ɹǝpun uʍop ɯoɹɟ sƃuıʇǝǝɹ⅁