op amp resistors

[bee]

Tested again today with better dmm changes by 1 ohm found website on s/g will check out later for spec sheet thanks for your input got me thinking
regards
bee

 

[audioguru]

Your R1 is 1m which is only 0.001 ohms. Maybe it should be 1M which is one million ohms.

 

[bee]

Sorry it is 1M I did the drawing in a hurry
regards
bee

 

[simplicio]

Note sure I understand the "GF 2.07 +- 1%" part?

Strain gauge - Wikipedia, the free encyclopedia

Apparently it means "Gauge Factor" - I'm no expert though

 

[ericgibbs]

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

hi bee,

If the diagram you have posted is correct, the circuit will not work.

I have added some rough calculations to your dwg.

The best amp IMO for this type of bridge is the AD623 or any other Instrumentation OPA.

 

[bee]

Hi Eric
wouldnt the 120ohm res and the strain gauge (120ohm+10k in parallel) form a voltage divider
regards
bee

 

[ericgibbs]

Hi Eric
wouldnt the 120ohm res and the strain gauge (120ohm+10k in parallel) form a voltage divider
regards
bee

hi,
Perhaps I maybe misreading your circuit, are you saying the strain gauge is connected across the 10K.?

I thought the label 'strain gauge' near the 120R was indicating the 120R was representing the strain gauge.?

Is it like this image.?

 

[bee]

I have updated the schematic the other was a bit misleading. And added the voltages at the points 'A' and 'B' and Vout under loads seems to be working . Do you recommend any improvements to this circuit . ie where would be the best place to place a trim resistor to balance the bridge.
regards
bee

 

[ericgibbs]

I have updated the schematic the other was a bit misleading. And added the voltages at the points 'A' and 'B' and Vout under loads seems to be working . Do you recommend any improvements to this circuit . ie where would be the best place to place a trim resistor to balance the bridge.
regards
bee

hi bee,
look athis circuit.

 

[bee]

Thanks for the schematic i will try that next week as out of town on weekend
thanks again
regards
bee

 

[bee]

Hi eric
Changed the circuit(see attached) to lower current through strain gauge seems to be working better but having trouble zeroing the bridge.Not sure where to put the trimming resistors with this circuit tried a few ways but was to coarse in adjustment.Found AD623 amp for $14 to dear as the 6292 is $3.95 still might try it then find cheaper supply if it works well.
Any help much appreciated
Regards
bee

 

[ericgibbs]

Hi eric
Changed the circuit(see attached) to lower current through strain gauge seems to be working better but having trouble zeroing the bridge.Not sure where to put the trimming resistors with this circuit tried a few ways but was to coarse in adjustment.Found AD623 amp for $14 to dear as the 6292 is $3.95 still might try it then find cheaper supply if it works well.
Any help much appreciated
Regards
bee

hi,
Look at these changes to your circuit.
Remove the two resistors feeding the 120R and Sg, they are reducing the sensitivity of the bridge.
Add the 100K's and pot for zeroing, you may have to swop the bridge zero tap junction, it depends upon the polarity of the zero error.

Reduce the gain of the OPA.

Why dont you use the 2nd half of that dual OPA, so that you have better overall gain and control.?

 

[bee]

Hi Eric
I have played around with it today it is working to a degree. As you said the 1.5k resistors in the bridge reduce the sensitivity but i am trying to reduce the current through the s/g and the overall circuit as it is powered by battery.I can use the other amp but unsure about connecting one to the other i am having enough trouble with one hey.Can you help with connections and do you think this would give better control.If you think there is a better circuit i will give it a go

 

[ericgibbs]

Hi Eric
I have played around with it today it is working to a degree. As you said the 1.5k resistors in the bridge reduce the sensitivity but i am trying to reduce the current through the s/g and the overall circuit as it is powered by battery.I can use the other amp but unsure about connecting one to the other i am having enough trouble with one hey.Can you help with connections and do you think this would give better control.If you think there is a better circuit i will give it a go

hi,
The correct OPA for a bridge is a Instrumentation Amplifier, I use the AD623, its an 8 pin DIP device.
Do you know what the V excitation voltage is for that strain gauge.
For a 120R bridge a 5Vex is commonly used, for a 350R bridge I have seen 5,6 and 10Vdc used.

Even if you get your present configuration to work, it will not be stable and will not give reliable readings.
What are you going to use it for.?

 

[bee]

The strain gauge is measuring the flex of crank arms on racing bicycle.I will try to find the AD623 for right price maybe this would be easier.Cant seem to find what voltage/current recommended for this gauge.
Regards
bee

 

[ericgibbs]

The strain gauge is measuring the flex of crank arms on racing bicycle.I will try to find the AD623 for right price maybe this would be easier.Cant seem to find what voltage/current recommended for this gauge.
Regards
bee

hi,
With the present configuration the resistors will have to well matched in order to maximise the common mode rejection [CMR].
Its also difficult to make changes to the gain with respect to the CMR.
The output nonlinearity can be double that of the bridge circuit alone.

Its possible to configure a Inst OPA circiuit using individual/dual OPA's by using good spec OPA's.

 

[bee]

I will do some more thinking about this problem. I might attach another s/g on other side of crank arm so one is in compression and other is in extension this will double the change in resistance as i am running out of ideas.Thanks for your help let me know if you think of anything else
Regards
bee

 

[ericgibbs]

I will do some more thinking about this problem. I might attach another s/g on other side of crank arm so one is in compression and other is in extension this will double the change in resistance as i am running out of ideas.Thanks for your help let me know if you think of anything else
Regards
bee

hi,
Adding that extra SG element will double the sensitivity.

This image clip is from one of my weighing scale projects.
Using an AD623, the bridge is 10mV output for max load, amplified upto 5.00V for input to a A2D convertor.

 

[bee]

I will add the other s/g and try it out. They where only sold in packs of ten so nothing to lose.The drawing looks similar to what i need. Futurlec has the ad623 but never used them to buy anything yet so dont know what they are like?
Regards
bee

 

[ericgibbs]

I will add the other s/g and try it out. They where only sold in packs of ten so nothing to lose.The drawing looks similar to what i need. Futurlec has the ad623 but never used them to buy anything yet so dont know what they are like?
Regards
bee

hi bee,
Depending on the alignment on the crank where you mount this second SG. will determine which 'arm' of the bridge it uses.