Go Down

### Topic: Wheatstone bridge? (Read 6099 times)previous topic - next topic

#### Cow faith

##### Aug 23, 2012, 10:13 pm

Hello,

I am looking for some calculations about the precision of a wheatstone bridge. Actually I have a thermistor 1kOhm with a 0.1°c precision and I would like not to loose that with wrong resistors in the bridge. I try to find something on google but no results...

Have you got a link? Do you think I need special resistors?

Thank you

#### Grumpy_Mike

#1
##### Aug 23, 2012, 10:21 pm
http://en.wikipedia.org/wiki/Wheatstone_bridge

However you don't gain or loose precision with a  wheatstone bridge. All it is, is a potential divider with an active element in one arm. The other side is just a regular potential divider to provide a reference. Generally the resistor to choose is the same as the active element at mid range.

#### retrolefty

#2
##### Aug 23, 2012, 10:21 pm
Quote
Have you got a link? Do you think I need special resistors?

Well the bridge resistors would have to have a accuracy tolerance better (X10?) then the thermistor if you want the sensor to be the limiting factor in overall measurement accuracy. And then there will be the needed bridge op-amp and it's resistors to deal with. And you will have to see what the arduinos only 10 bit precision factors into the whole system.

Lefty

#### bibre

#3
##### Aug 24, 2012, 01:31 amLast Edit: Aug 24, 2012, 02:34 am by bibre Reason: 1
Billy     http://www.z-world.com/operations/gbremer/

When you've eliminated the impossible,
whatever remains, however improbable, must be

#### Docedison

#4
##### Aug 24, 2012, 04:59 am
There is also the problem of calibration, A Wheatstone Bridge's output is bi polar in nature and depending on the actual values on either half... Extremely sensitive, Typically used for making matched pairs of resistors... to .001% or more. A Wheatstone Bridge isn't a really great idea unless you are after 10th's of a degree accuracy over a narrow range of temperatures
Use a current source fed from a constant voltage source... Independent of the Arduino's Vcc.
I usually use a pot and a TIL431 reference diode (read Variable Zener Diode) fed from a clean 9 to 12V source. I use a 10K 1% MF or better, use a stable one with a low tempco (10PPM/deg C.).
There is a "Table" of values for calibration by PH Anderson and a formula for doing the same thing "On the Fly" available freely on the Internet for final calibration. Don't forget 2 really important things... Bypass the regulated voltage source well and really important use a star grounding layout... 5 or 10 mV of combined ground drop voltage can destroy any real accuracy however if you pay attention to detail you can easily make a nearly "Traceable" and very accurate thermometer... for a few dollars extra... IMO

Doc
--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

#### Cow faith

#5
##### Aug 24, 2012, 10:19 pm

Hello,

Thank you all for you answers, I can now begin to work well on that thermistor. Docedison, it seems that I don't have all the electronic skills needed for the understanding of you answer, have you got a schematic? I really would like to understand, would you not use a wheatstone bridge at all?
What is the goal of the current source? What is the "calibration" that you told me?

I'm just in first year of electronic engineer school.

#### Docedison

#6
##### Aug 24, 2012, 10:46 pm
Well get out your books and reference the devices... a constant current diode is a jfet operated at it's "pinchoff voltage"... they are available in 2 lead diode like packages... a constant voltage power supply feeding the diode will produce a constant set of conditions that the thermistor can react to.
The Wheatstone Bridge is a fine tool but because of it's bi-polar nature is much more difficult to directly interface to the A/D inputs on the Arduino.
Reference the "Star" method of power supply distribution in your texts and see what I was talking about as far as noise both distributed from the power supply and created by individual elements...
Getting back to the thermistor issue, a constant current through the thermistor will produce a variable voltage, temperature dependent and that will give you the information you need from a stable source.
I hope this helps a little. Unfortunately I need to go and do some "Honey Doo's" now and I will be busy for the next 3 or 4 hours... I think.

Doc
--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

#### Cow faith

#7
##### Aug 25, 2012, 12:07 am

Okay I begin to understand. So with a really stable current in the thermistor (with the advices you gave to me), the voltage around the thermistor will just have to be measured to have a good accuracy. And it will be maybe more stable than with a wheatstone bridge, without all the resistors problems. Is that right?

Last thing, you told me about calibration but I didn't find anything about PH Anderson and the tables on google, have you got another keyword please?

Thank you very much.
Sebastien L.

#### MarkT

#8
##### Aug 25, 2012, 12:43 am

The Wheatstone Bridge is a fine tool but because of it's bi-polar nature is much more difficult to directly interface to the A/D inputs on the Arduino.

I dispute that
Code: [Select]
`  int val = analogRead (1) - analogRead (0) ;`
is "much more difficult" than
Code: [Select]
`  int val = analogRead (0) ;`
?
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

#### Docedison

#9
##### Aug 25, 2012, 01:23 am
My Comment was based on the fact that a Wheatstone Bridge can swing negative and is equally likely to do so as to swing positive and on top of that... If you are after ~1% accuracy it still requires a Very Clean, Stable and known Reference Bias for the sensor... Then there is the issue of the accuracy of the Vref, nominally 5V or the chip Vcc. This forces a ratiometric measurement because the Vcc line might not be very stable or accurate. There are similar issues with the internal 1.1?V reference... it seems that it can vary somewhat. So I made these assumptions about the use of the thermistor because there are many ways to achieve that level of accuracy but the temperatures are lower, the thermistor can be limited only (reasonably) by the melting point of the solder... Not all or many but possible. So those thoughts first I thought to recommend a voltage source stable enough that it could through the proper voltage divider be the Vref for the measurements as well as the Thermistor Reference bias... that was to come after he gave me some indication that he understood my thoughts and was interested.

Doc
--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

#### Cow faith

#10
##### Aug 25, 2012, 05:39 pm

Hi,

I think I understand the pros and cons of the weathstone bridge. I will look for some information about the jfet as a current source, it seems to be quite useful. But indeed with a stable voltage source and if I use 1% resistors maybe I can have a quite good accuracy for the temperature measurement. I will make all the calculation needed with this page :

www.sanprotechmarketing.com/article_writing/ametherm/NTC_thermistor_wheatstone_bridge_oct31.pdf

And I will come back to tell you what accuracy I can have.
Thanks

#### Docedison

#11
##### Aug 26, 2012, 01:31 am

Doc
--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

#### Cow faith

#12
##### Aug 27, 2012, 06:04 pm

Ok I think I have understood the advantages of the two solutions. Indeed the jfet device seems to be quite easier than the wheatstone bridge, but you are right, i'm still in my studies and it will be very recommended for me to be able to use fastly a bridge network.
However thank you very much Docedison for all this information that you gave me and all the time you took to answer me.

->This forum is just perfect. Let go!

#### Docedison

#13
##### Aug 27, 2012, 08:48 pm
Well, Young man the knowledge was given to me for free... and the only way I can repay the patience of the people who took the time to explain things to me is to do the same for as many others as I can reasonably. You Are quite welcome and thank you for the opportunity to explain it to you...

Doc
--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

#### DaveX

#14
##### Apr 09, 2013, 10:49 pm
A 1K 0.1C thermistor seems like a bad use-case for a wheatstone bridge.    With a typical Beta of 3800, a 1K NTC thermistor at 25C will vary about 42 ohms (~4%) per degree of temperature change.  In a 5V Wheatstone bridge config with 1K resistors, the +/- 4% change in resistance per degree will make the voltage difference across the terminals of the Wheatstone about +/- 4V.  With a 10 bit ADCs on the terminals of the wheatstone, you might sense 0.1% differences, which would correspond to temperature differences in the +/-0.025C range.

But, with a 1K thermistor at 2.5V, you could be heating the thermistor significantly:  2.5V/1000ohm=2.5mA,  P=2.5ma*2.5V=6.25mW   With one particular 1K thermistor's dissipation constant of 1.7mW/C, it could self-heat by 3.67C, introducing a systematic error 147 times larger than the sensitivity of the wheatstone.

If you need to do this with an arduino, use a larger resistance thermistor and a very small reference voltage to minimize the self heating.  And figure out your operating range and what that would mean at the terminals.

The other problem is how you are going to to the +/- differential voltage measurement on the arduino.  If you hook the AREF to your wheatstone bridge's reference voltage, half of the wheatstone bridge is essentially useless-- you can do the inactive branch of the wheatstone bridge in math rather than measuring the terminals with ADCs.

Some AVRs can do differential +/- voltage measurements with some ADMUX magic: http://arduino.cc/forum/index.php?topic=147830.0

Go Up

Please enter a valid email to subscribe