op amp resistors

bee · Dec 2, 2008

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

crutschow · Dec 2, 2008

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.

bee · Dec 2, 2008

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

Roff · Dec 2, 2008

bee said:
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

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

bee · Dec 2, 2008

Yes it is the pinout from a 741 in eagle
bee

bee · Dec 3, 2008

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

ericgibbs · Dec 3, 2008

bee said:
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.?

bee · Dec 3, 2008

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

eblc1388 · Dec 3, 2008

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.

ericgibbs · Dec 3, 2008

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

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.???

bee · Dec 3, 2008

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

Leftyretro · Dec 3, 2008

eblc1388 said:
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.

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

bee · Dec 3, 2008

That is a voltage divider so 5 volts
bee

bee · Dec 3, 2008

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

Leftyretro · Dec 3, 2008

bee said:
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

ericgibbs · Dec 3, 2008

bee said:
That is a voltage divider so 5 volts
bee

Hi,
Remember you are working at millivolt levels.

So if the top resistor was 101R and the bottom resistor is 99R [ total 200R]

Connected across 10V, gives a current of 0.05A.

Using ohms law on the lower resistor of 99R, gives a voltage drop of 4.95V, so you have a ZERO error voltage of 50mV.!!!!

As your amp has a gain say of 1000. then the Vout would be 50V.!!!

You should always consider a method for ZEROing the bridge offset , BEFORE the amp.

I have added an image of a single element bridge that may help your calculations.

bee · Dec 3, 2008

Sorry slow to post got called to work. What method of zeroing the offset can i use on this circuit .Also is this cicuit correct,and the voltage starts at 4.9v and goes down to .5v when load applied will change schematic and repost tomorrow thanks
regards
bee

bee · Dec 3, 2008

Hi lefty
The strain gauges that i have are made by bcm in belgium and the specs are 120.0 ohms
+- 0.1 / GF 2.07 +- 1% /grade A. I dont have data sheet but will do some searching today
regards
bee

bee · Dec 3, 2008

schematic update

This is the updated schematic for your perusal(big word) note that is not the correct pinout for 6292 it is all i could find in eagle.Thanks for taking the time to help me with this much appreciated.
regards
bee

Leftyretro · Dec 3, 2008

bee said:
Hi lefty
The strain gauges that i have are made by bcm in belgium and the specs are 120.0 ohms
+- 0.1 / GF 2.07 +- 1% /grade A. I dont have data sheet but will do some searching today
regards
bee

Note sure I understand the "GF 2.07 +- 1%" part? If it means that 2.07 grams (or grains?) of force equals 120 ohms +/- .1 then that is a single reference point and doesn't state what the resistance should be with 0 grams (or grains) of force. There needs to be known the slope of the gauge and that takes at least two points to categorize. Again a full spec sheet is what's required. I suggest that designing the op amp's gain and balancing requirements is premature without full knowledge of the sensors sensitivity and range specifications.

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