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Help Needed with Load Cell Project and Op Amp gain configuration

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dtomazini

New Member
Hi Everybody,


I'm new in this forum, and new to Op Amp.

A two weeks ago, I'm trying to build a simple weight measure project, using a load cell from a 10Kg kitchen scale and a PIC18F4550. My project did not work. But i don't understood what is going wrong.

Searching in this forum, and Google, i found lot information about load cells and Op Amps. But in almost cases, the op amp used like INA125, AD620, AD623 is hard to find and much expensive here in Brazil.

The first step in my project, is know the load cell that I'm using. For that i use a multimeter, to measure the resistance between connectors, and the voltage in load and unload:

Resistance in terminals:
"Excitation+" to "Excitation-" = 1K
"Signal+ " to "Signal -" = 1K
"Signal+ " to "Excitation-" = 750R
"Signal-" to "Excitation+" = 750R

2013-08-05_23-51-39_814.jpg
2013-08-05_23-51-50_963.jpg


Voltage (using a 5.45v power source) in terminals SIG+ and SIG-:

Without load: 0,001v
At full load 10kg: 0,055v

2013-08-05_23-45-58_576.jpg

To confirm the measuring scale voltage, i repeat the test using a 9.4v power source, an result was:
Without load: 0,001v
At full load 10kg: 0,094v


So after this tests, the conclusive schematic for my load cell is:

2013-08-06_14-35-51_107.jpg


Please, can someone tell if this schematic is correct ?. And presume the scale of 1mv/V ?.


The second step, was try to use a Op Amp in "Differential" to amplify the voltage between SIG and SIG- for the PIC18F4550 ADC module .

Now is the problem...I try a LOT of schemas, formulas, Inverting, Non-inverting, Differential, Differential Instrumental, etc... and have nothing.

In my lab (house), I have the following Op Amps:

Single:
UA741CN
TL081
OP07
LM318
LM301AN

Dual:
TL082
LM358

None of them work's as expected... or the voltage is saturated at 4.8v, or stuck in 1.28v, or simply did not respond for the load cell load alternations.

Tried Schematics:

Diferential Op Amp ("SIG+" - "SIG-"), Gain Vo/Vi = 10:

2013-08-06_17-22-57_40.jpg


Similar project from this link: https://www.galant-e.ru/images/upload/1875/ru/an_651x_011%20-%20rev%201-3.pdf
 
Last edited:
hi Daniel,
Looking at your circuit diagram you are showing the OPA 0V power pin as Not Connected N/C...??? is this correct?

So which OPA type is being used in the circuit diagram.??

Eric
 
Last edited:
hi dt,
This is a LTSpice simulation of your circuit.


Vsig = 0mV thru +10mV

E
 

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Last edited:
hi Daniel,
Looking at your circuit diagram you are showing the OPA 0V power pin as Not Connected N/C...??? is this correct?

So which OPA type is being used in the circuit diagram.??

Eric


Sorry ericgibbs, the unconnected pins are: 1, 8 and 5.

The correct diagram are:

**broken link removed**




Tks;
Daniel
 
Last edited:
hi D,
I do not see a diagram.??

Have you tried your original circuit using dual supplies to the OPA.?? +5V and -5V.?

The OPA's require dual supplies.

E
 

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Better with dual supplies, but if using a single supply as shown then try it without the 1k5 at pin 3.
 
So, i supply with +5v and 0v (using a simple 110/220v AC/ to DC 5v, 2A converter).

I do this, because in simulator (ElectroDroid, OpAmp Calculator from android market), when i try with +5v and 0v, the simulation work´s fine.

Humm, i need buy a LM78xx negative 5v regulator.
 
Did you try without the 1.5k at pin 3?
i need buy a LM78xx negative 5v regulator.
I think the negative regulator you mean is the LM7905? But what will you power it with?
 
So, i supply with +5v and 0v (using a simple 110/220v AC/ to DC 5v, 2A converter).

I do this, because in simulator (ElectroDroid, OpAmp Calculator from android market), when i try with +5v and 0v, the simulation work´s fine.

Humm, i need buy a LM78xx negative 5v regulator.

hi D,

If you want to have an output of 0V from the OPA when the load cell is unloaded you will have to either use a dual supply for the OPA or apply an Offset voltage to the OPA.

If you remove the 1.5K from pin 3 of the OPA the output will be +2.5V when the load cell is unloaded, for a 0V output an Offset voltage will be required.

Also the +5V supply to the OPA and load cell bridge will have to be stable else the OPA will change with any change in the +5V.

E
 
Hi friends!,

This morning i set new prototype, following the instructions, on article: Bridge Circuits, USING A SINGLE OP AMP AS A BRIDGE AMPLIFIER FOR A SINGLE-ELEMENT VARYING BRIDGE.

". The key redeeming feature of the circuit is that it is capable of single supply operation and requires a single op amp.
Note that the RF resistor connected to the non-inverting input is returned to VS/2
(rather than ground) so that both positive and negative values of DR can be
accommodated, and the op amp output is referenced to VS/2."


Now i got better results !!!, at last the Vo is not satured.


To get a gain (Vo/Vi) about 100x, i use 1K(considering 500R from load cell) and 150k resistor.

Placed a 1uf poliester capacitor on OPA power source, and 100nf capacitor on SIG+ and SIG-.

Tested Schematic:

**broken link removed**

New board with much less wires:

**broken link removed**


Test results from OPA Vo (in volts):

CI_________No Load____2Kg________10Kg_______Diference 0-10Kg
OP07_______2.61_______2,94_______4,94 _______1,63
TL081______1,509______1,512______1,533______0,024
LM301______3,43_______3,68_______4,52_______1,09
LM318______2,34*______2,67_______3,56_______1,22
UA741______2,19_______2,51_______3,81 _______1,62


* this case is very strange, if i aproximate my hand of CI, the voltage variables from 2,34 to 2,64. Just aproximate at 1 or 2 centimeters.


Now i must understand why the Gain is not linear. But so was told to me, the single op amp configuration has poor gain accuracy.


Thank´s
Daniel
 
hi D,

With a 150K, the gain is too high, the OPA is saturating, see image3.

With 100K, see image4, the gain is 'linear'

Most common OPA types will not give a Vout close to +Vsupply,, often its at least 1V to 1.5V lower.

ie: Vout max= 3.5V to 4V max

E
 

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Humm, curious i'm a bit confused now...

When I use a 1K resistor, it´s expected the included 500R from load cell (westone bridge model), to result in 1.5K from SIG+ and SIG-.

Using a 150K resistor, the gain should be 100x. Using a 100K resistor, the gain will be 66.667x ?, that is correct ?.

Assuming:
R2 = 150K
R1 = 1.5K

V2 = SIG+
V1 = SIG-

Vo = ( R2 / R1 ) * ( V2 - V1)

Vo = ( 150000 / 1500 ) * ( 2,50 - 2,49)
Vo = 100 * 0,01
Vo = 1v

Each 1mv results in 1v, in this case you are right !!!, is too high. I could use a 15K for R2, to get just 10x gain ??.


This simulation was made in LtSpice ?.
 
hi,
Join the LTSpice Yahoo user group.. **broken link removed**

Its a free sign up, also download and install LTSpice, we can then post to you our LTS 'asc' files, which you can use.

E
 
hi D,

With a 150K, the gain is too high, the OPA is saturating, see image3.

With 100K, see image4, the gain is 'linear'

Most common OPA types will not give a Vout close to +Vsupply,, often its at least 1V to 1.5V lower.

ie: Vout max= 3.5V to 4V max

E


Using OP07, and 100K resistor:

@0kg - 2,60v
@2Kg - 2,81v
@5kg - 3,14v
@10kg - 3,65v


Using OP07, 150K resistor and +3v OPA power source:

@0kg - 2,32v*
@2Kg - 2,32v*
@5kg - 3,32v*
@10kg - 3,33v*


* My multimeter (Every Green EG-2200 F) have a voltage scale of 200m/ 2v/ 20v. The 20v scale used, has a low accuracy to detect the changes in Vo.


Att;
Daniel
 
hi D,
This is a spread sheet plot of your [100k] values.

It looks OK to me.:rolleyes:

E
 

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hi D,

It looks as though you are getting there OK.

Post your final results when ready.
Your drawings are difficult to read, very feint lines.

E
 
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