Hi
Trying to increase vout from 0v? to close to rail voltage but dont know what i am doing.
The op amp is a microchip mcp 6292 rail to rail amp I am using to measure strain gauge in bridge circuit it is working but not right. Dont know if correct resistor config or values please see attached schematic thanks
Regards
Bee

Attached Thumbnails

 

 

A bridge circuit has four resistors. Yours only has three (R4, R5, R6). To learn more about what you are doing, try reading some tutorials on bridge circuits and bridge amplifiers (just Google). You obviously need more understanding.

You need to configure the circuit like this bridge circuit with amp:


How can we tell you what gain is needed to get voltage near the rail if you don't tell us what the change in the strain gauge resistance is and what the amp operating voltages are? We're not psychic.

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Carl
Curmudgeon Elektroniker

 

The strain gauge is the fourth res The amp is operating on the same voltage as the bridge
circuit .The strain gauge resistance is very small i will measure it again after work .sorry for not enough info
regards
bee

 

Did you use a symbol for a different op amp? That is not the pinout of an MCP6292.

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Ron

 

Yes it is the pinout from a 741 in eagle
bee

 

The strain gauge resistance is (120.3 ohms no load)(120.5 full load) so not much change i have got it working better by changing a few resistors got rid of r2 and put 220k in parallel with r7 and 10k in parallel with s/gauge .The vout goes from about 4.9v to .5v .Imagine what i could do if i knew what i was doing
thanks

bee

 

hi bee,
What tolerance are the resistors in the passive sides of the bridge circuit.?

__________________
Eric " Good enough is Perfect "
I will NOT answer PM's requesting technical help, please use the Forum
PIC tutorials: Nigel's www.winpicprog.co.uk/ Bill's: www.blueroomelectronics.com/

 

They are 1% do you think there is a problem with these
bee

 

Hi bee,

I have re-stated your problem. See if you have any changes you want to add:

How do I convert a resistance change of 120.3 ohm to 120.5 ohm (no load and full load of a ground referenced strain gauge) into 0~5V output, using a rail-to-rail opamp operates using single supply of 5V. I understand that the 5V full load output might not be available but I am prepare to accept slightly less than 5V for the full load value.

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L.Chung

 

hi,
If you considered two 100R 1% in series and say the the top resistor was 1% high on 100R and the lower resistor was 1% low on 100R.

For explanation, assume the two resistor are connected across a 10Vdc supply.

What do you calculate the voltage at the junction of the two resistors to be when measured with respect to 0V.???

__________________
Eric " Good enough is Perfect "
I will NOT answer PM's requesting technical help, please use the Forum
PIC tutorials: Nigel's www.winpicprog.co.uk/ Bill's: www.blueroomelectronics.com/

 

I seem to have fixed the problem i will edit the schematic tommorow when at work and re post it thanks
regards
bee

 

I've worked with a few strain gauges and your range of resistance change from no load to full load just does not seem possibly correct. Does that match the spec sheet from it's manufacture? It seems to me that your other bridge resistors are never going to be temp. stable enough to be able to accurately utilize such a small change of resistance. Could you strain gauge be defective? Is there a link to a data sheet for the gauge?

Lefty

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Measurement changes behavior

 

That is a voltage divider so 5 volts
bee

 

Hi lefty
I measured with dmm no load and with full load but dmm not a good one. The circuit seems to be working .The s/g is from bestech in aus The application doesnt move much so that is what i expected
thanks
bee

 

If the strain gauge is not defective then at least it seems to be mismatched to your application range. They come in different sensitivity ranges (full scale to grams and up to full scale to tons of weight). I can't believe you will ever get good accuracy and repeatable measurement with the resistance range you are seeing.

Lefty

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Measurement changes behavior