Trimmer position

[bee]

This is my circuit for strain guage measurement it works
But i cannot find a place to put the trimmer resistor R15,So that i can get enough resolution need at least 2 turns, I am currently using 25 turn trimmers
I have tried different values of trimmers,and configurations around the bridge ,The AD623 has a gain of around 500
Any suggestions appreciated

cheers
bee

 

[ericgibbs]

This is my circuit for strain guage measurement it works
But i cannot find a place to put the trimmer resistor R15,So that i can get enough resolution need at least 2 turns, I am currently using 25 turn trimmers
I have tried different values of trimmers,and configurations around the bridge ,The AD623 has a gain of around 500
Any suggestions appreciated

cheers
bee

hi,
Its present position is a poor choice, it will effect the linearity of the strain element.
You need a multiturn10R variable in the top of R1 and R2 to the +Vx rail, if thats not possible use a 500R variable connected from the AD623 REF pin to 0V.
Depending upon the sense of amplified zero offset voltage at the output of the AD623, the top of the 500R will require at +Vref or -Vref voltage.

Do you follow that.?

 

[bee]

Thanks eric
I will give that a go,, I think i follow ,,
I will have to try an get a 10R variable
Many thanks mr gibbs

bee

 

[ericgibbs]

Thanks eric
I will give that a go,, I think i follow ,,
I will have to try an get a 10R variable
Many thanks mr gibbs

bee

hi bee,
This is your IA in LTspice.
1st image is the strain gauge output and the 2nd the AD623 and transistor output.
Gain is 500.

What are you expecting the circuit to do.???

 

[bee]

I am trying to read the strain guage and give me 0~5v or 5v~0 for adc in 12f675, then output on s/seg 0~9
It is all working okay was just looking to trim the bridge before the amp
Can you see any improvements with the circuit
It is a steep learning curve for me
Couldnt get a 10R trimmer multiturn all i could get of the shelf today was 100R and i have put it in and it works a lot better,, Will have to try and find 10R for better res
I have added updated eagle schematic

Cheers
bee

 

[cobra1]

is this the same type circuit used in scales???

 

[bee]

cobra1

This is a wheastone bridge circuit used for strain guage measurements ,,, So yes it would be used in scales

bee

 

[ericgibbs]

hi bee,
The output of the transistor 'out2' is working as a switched output, either +0.7V or 5v, nothing in between, which isn't suitable for adc conversion

I cannot open your Eagle schematic.

 

[bee]

eric
I have added the file in pdf format
The output from the transistor is linear
It is not on or off it drops as the output from the amp rises
what do you suggest as i have one circuit on test and one on the bench to play around with always looking to improve

cheers
bee

 

[bee]

Eric
I have just noticed on the spice sim R6 should be 10K not 22K

bee

 

[ericgibbs]

eric
I have added the file in pdf format
The output from the transistor is linear
It is not on or off it drops as the output from the amp rises
what do you suggest as i have one circuit on test and one on the bench to play around with always looking to improve

cheers
bee

hi bee,
I have expanded the plot to show the Vout into the adc, its not linear.

What voltage range do you want for the adc input.??

For the 100R add, add a 10R in parallel with each end of the 100R

 

[bee]

eric
I am looking for 0~5v would be ideal
That is linear either rising or falling as i can alter the code to accomadate
How can i make my circuit more linear

bee

 

[ericgibbs]

eric
I am looking for 0~5v would be ideal
That is linear either rising or falling as i can alter the code to accomadate
How can i make my circuit more linear

bee

hi bee,
You cannot get a +5v output from a AD623 powered by 5V.
Do you have a 9v supply available for the AD623.?
The output transistor should be removed.
Do you want a circuit posting.???

 

[bee]

Hi eric
I am using a 9 volt battery but would prefer to keep it at 5v
I can use +.5v to +4v that was linear as long as i can get around 3volts change up or down
If you have an idea for a circuit that would be good
I am only getting from 80mv to 580mv from the output of the ad623 with a 500 gain (200ohm res) so not much signal to play with

thanks
bee

 

[ericgibbs]

Hi eric
I am using a 9 volt battery but would prefer to keep it at 5v
I can use +.5v to +4v that was linear as long as i can get around 3volts change up or down
If you have an idea for a circuit that would be good
I am only getting from 80mv to 580mv from the output of the ad623 with a 500 gain (200ohm res) so not much signal to play with

thanks
bee

hi,
A quick fix would be to replace the transistor with a rail to rail OPA.
Example MCP6001 or equivalent, this would give the extra gain required...OK.?

 

[ericgibbs]

hi,
Something like this circuit.
Use R6 to set the gain.

 

[bee]

Thanks eric
Is it not possible to amplify the signal with the transistor
I have a mcp 6292 in my junk box but didnt want to use another ic if possible

cheers
bee

 

[ericgibbs]

Thanks eric
Is it not possible to amplify the signal with the transistor
I have a mcp 6292 in my junk box but didnt want to use another ic if possible

cheers
bee

hi,
You could set the AD623 for a gain of 1000.
Using a single transistor on the output is not a viable solution.

 

[bee]

Eric

I have been playin around all night with this thing and i have come up with 2 transistors together(darlington pair not sure?) ,,And iam getting from 3.5v down to about .5v and its pretty good linear wise in spice anyway ,Check it out and see if i can squeeze a bit more from this configuration
I can use this in my circuit now it is a lot more stable and easy to get parts
Thanks for making me think about it a bit more

bee

 

[ericgibbs]

hi,
I would not call that linear.

I would suggest you use an OPA.