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Voltage Amplifier circuit

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Jitesh

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Greetings all,

Im new to the Forum and I think you guys are doing an amazing job.
Keep it up.



I need some help with the project im working on.

I'm using a LOAD Cell connected to a Campbell Scientific Logger.

The excitation on the Load cell is 2.5 to 12 Volt.
The load cell has a rated signal output of 3mV.
So at a maximum load of 200 kg, with an excitation voltage of 2.5V, i should be getting a mV reading of 7.5mV from the load cell but im only getting 3.1mV.

I think I may get better results if I can increase the exciation voltage and thats where I need help.

How can I amplify the 2.5 V signal to 12V. Can any one help me with a circuit that can do that.

Thanks
 
hi,
Its usual to specify a mV/Volt signal at full load, so if its 3mV/V at full load, using a 2.5V excitation you should get 7.5mV as you expected.
I assume you are measuring across the +sig and -sig wires from the cell and not to 0v.?
The output from bridge would normally be connected to a differential amplifier in order to give a usable signal level.

Whats the bridge resistance?

EDIT:
What is the +sig/-sig from the cell with no load.?
 
Last edited:
You don't mention the make & model of the load cell, however:

The excitation on the Load cell is 2.5 to 12 Volt.
The load cell has a rated signal output of 3mV.
So at a maximum load of 200 kg, with an excitation voltage of 2.5V, i should be getting a mV reading of 7.5mV from the load cell but im only getting 3.1mV.

The signal output of the load cell is likely 3mV/Volt meaning at FS (Full Scale) of 200 Kg applied and an excitation voltage of 10.000 volts you would see 30 mV output from the load cell. Therefore 0 Kg = 0 Volts and 200 Kg = 30 mV.

Very important when using load cells of this type is that the excitation voltage be stable for obvious reasons. At this point generally a signal conditioner circuit is added that also provides a stable excitation and amplifies the mV / Volt output of the load cell bridge circuit. Typically you end up with something like 0 to 200 Kg = 0 to 10 Volts which makes for much better resolution as to data recording or plotting.

Hope that helps...
Ron

<EDIT> Eric is fast! :) </EDIT>
 
Last edited:
hi,
Its usual to specify a mV/Volt signal at full load, so if its 3mV/V at full load, using a 2.5V excitation you should get 7.5mV as you expected.
I assume you are measuring across the +sig and -sig wires from the cell and not to 0v.?
The output from bridge would normally be connected to a differential amplifier in order to give a usable signal level.

Whats the bridge resistance?

EDIT:
What is the +sig/-sig from the cell with no load.?


How do i measure the bridge resistance, Im not too sure.
I measured the resistance across the excit+ and excit - and i get 450 ohm. When I measured across the signal + and signal - i got 350ohm. those results are with no load!


I have connected the excit + to the excitation channel of my logger and the excit - to ground. And yes I have connected the signal + and - to the differential input of the logger.

So why am i not getting the full 7.5 mV at full load of 200 kg
 
What you read will depend on the bridge but a 350 Ohm bridge is common for load cells. The following applies using a high impedance ohmeter and the load cell not connected:

2.1 Check Bridge Circuitry and Zero Balance. (Numbers apply to standard 350 ohm bridges.)
Typical Connections Instrument required: Ohmmeter with 0.1 ohms resolution in the range of 250-400 ohms. Bridge Input Resistance: RAD should be 350 ±3.5 ohms (unless the cell has standardized output, in which case the resistance should be less than 390 ohms) Bridge Output Resistance: RBC should be 350 ±3.5 ohms Bridge Leg Resistances: Comparing the leg resistances at no load permits evaluation of the cause of any permanent damage in the load cell flexure. The computed unbalance of the bridge shows the general condition of the cell. The computed unbalance, in units of mV/V, is determined as follows: Unbalance = 1.4 (RAC - RAB + RBD - RCD) The Zero Offset, in units of % of Rated Output, is determined as follows: Zero Offset = 100 Unbalance ÷ Rated Output. If the ohmmeter resolution is 0.1 ohm or better, then a computed Zero Offset of greater than 20 percent is a clear indication of overload. A computed zero balance of 10-20% is an indication of probable overload. If the load cell has been overloaded, mechanical damage has been done that is not repairable, because overloading results in permanent deformation within the flexural element and gages, destroying the carefully balanced processing that results in performance to Interface specifications. While it is possible to electrically re-zero a load cell following overload, it is not recommended because this does nothing to restore the affected performance parameters or the degradation to structural integrity. If the degree of overload is not severe the cell may in some cases be used at the users discretion, although some performance parameters may violate specifications and the cyclic life of the load cell may be reduced.

The resistances you are seeing would indicate a problem. That would be my take anyway.

Ron
 
hi Jitesh,
I would agree with Ron, the resistances are not what I would expect.

Is it possible you can us the excited, off load mV is.?
Also as a quick check, measure the resistance of the 4 arms of the bridge.?

Like to help.
Which part of SA.?
 
Last edited:
Is it possible you can us the excited, off load mV is.?
Also as a quick check, measure the resistance of the 4 arms of the bridge.?

Like to help.
Which part of SA.?

I'm not sure what you mean by excited off load mV, .. The Load cell reads 0V when no load connected.
I've managed to increase the Excit Voltage to the Load Cell to +12V and the results were what i expected.

At full Load of 200kg I got 36mV - 12V excitation * 3mV/V ... which is correct.

I'm not sure why i didnt get 7.5mV that when i used 2.5 V excitation.


@ Eric, Im from Durban.
 
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