Wheatstone Bridge Question

[Nevjc]

Hi
Could anyone tell me how I can add the two output voltages from two seperate wheatstone bridges without adding any further loading to each individual output.

Nevjc

[nicholasdark]

Hi Nevjc,
Generally you would use an instrumentation amplifier http://en.wikipedia.org/wiki/Instrumentation_amplifier to read the voltages.
You could either build one from scratch using opamps and precision resistors, or use a dedicated chip that has all of the hard stuff done for you http://uk.farnell.com/jsp/search/browse ... 1%2B203942

Any questions, just ask

Nick

[Nevjc]

Thanks Nick I will have a look at the dedicated chip and give it a try. I am trying to set up a device where I can read two seperate weights individually and then sum them together to give a total. I have just bought some cheap bathroom scales using one for each individual weight and then the display from the third to give the totals.

Thanks again
Nevjc

[nicholasdark]

Hi Nevjc,
I would start by reading one bridge accurately first, then get the second running. Once you can read weights accurately on both using a multimeter as your display, you then have a choice...

You could use an opamp as a summing amplifier to produce one combined analogue signal from the two, or you could read them both with a microcontroller and do the addition in software.

Personally I would go for the microcontroller, as it it much easier to add calibration factors, display the reading in a variety of units etc. I also think it would it will make driving the display easier. That said, you may be able to hack the parts from the scale units to have to avoid having to write any software.

Kind regards,
Nick

[Nevjc]

Thanks Nick,
I think I will try the former first as it is quick and easy and then look at the latter at a later stage.

Thanks for the advice

Kind Regards
NevJC

[nicholasdark]

Hi NevJC,
I've just re-read your original post and realised that my advice wasn't quite right.
The instrumentation amplifier will amplify the signal from a single raw bridge (i.e. from the 4 resistors). once you have amplified each bridge, you can then use the summing amp, micro-controller etc. on the outputs of the instrumentation amplifiers.

I hope that makes sense, if not let me know and I'll draw up a quick diagram.

Sorry for the confusion,
Nick

[Nevjc]

Thanks Nick,
I would be gratefull if you could draw up a quick diagram. It would help

Thanks NevJc

[nicholasdark]

https://dl.dropboxusercontent.com/u/87836270/noname.png

Hiya, sorry about that.
Ok, so the 4 resistors represent a bridge, so you have bridge 1 at the top of the image and bridge 2 at the bottom.
U1A would be the instrumentation amplifier for bridge 1, (the chip I linked to in a previous post), U2A would be the instrumentation amplifier for bridge 2. I've drawn them as simple opamps in the image.

So, I would build and test bridge 1 and U1A first, then build and test bridge 2 and U2A. Once you have them working independantly, you could add in a summing amplifier U3A, again drawn as just an opamp for ease.

This should be a good starting point for the summing amp: http://www.circuitstoday.com/summing-amplifier

Kind regards
Nick

[Nevjc]

Thanks for the Diagram Nick that is a great help. I will order up the bits and try to get it built this week.

Thanks for all your help.

Kind Regards
Nevjc

[nicholasdark]

You're more than welcome, I hope it all works for you

Any questions, just let me know.

Nick