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Kitchen balance load cells 10 kg

joniengr

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Hi,

I need to make a custom weighing machine for 10 kg and then I need to interface with PIC microcontroller having 10 bit ADC. I am looking for the load cells that can give signal when power up by a circuit. Are they resistive type ? I mean can I connect them in parallel with c voltage source to get a signal. I will use OpAmp to amplify the signal according to ADC input requirement but it should be fine if the signal from load cell come with sensitivity of 1 mV / gram.
 

danadakk

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There are solutions out there that would be single chip -



This was done using Correlated Double Sampling. Note DelSig was only
used at 12 bits, but is capable of 20 bits. Also most of chips resources
went unused, available for other tasks, see right hand window. Chip also
has DSP that can be used for any additional filtering needed.

1611667556559.png

Board to do this - https://www.cypress.com/documentation/development-kitsboards/psoc-5lp-development-kits

CY8CKIT-059

Compiler and IDE free - https://www.cypress.com/products/psoc-creator-integrated-design-environment-ide



Regards, Dana.
 
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wwfeldman

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what research on load cells have you done?
 

joniengr

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Hi,

What about bridge resisters, they are load cells, I guess. I am looking for load cells for 10 Kg that I can connect in a bridge. How to get them ?
 

joniengr

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I found these load cells but not sure what is the resistance change when I put 10 Kg on them ? What is the base resistance without any load and what is the resistance when 10 Kg load put on it. Do we also have similar load cells in kitchen balance ?


--- Updated ---

There are solutions out there that would be single chip -



This was done using Correlated Double Sampling. Note DelSig was only
used at 12 bits, but is capable of 20 bits. Also most of chips resources
went unused, available for other tasks, see right hand window. Chip also
has DSP that can be used for any additional filtering needed.

View attachment 167151

Board to do this - https://www.cypress.com/documentation/development-kitsboards/psoc-5lp-development-kits

CY8CKIT-059

Compiler and IDE free - https://www.cypress.com/products/psoc-creator-integrated-design-environment-ide



Regards, Dana.
@dana, where do I connect load cells on CY8CKIT-059 ?
 
Last edited:

FvM

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According to datasheets available for this half bridge load cell, the sensitivity is about 1 mV/V for rated load of 50 kg. Respectively it can't be used without a high gain differential amplifier.
 

KlausST

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Hi,
come with sensitivity of 1 mV / gram.
Did you do some basic calculations?
1mV/ gram * 10kg = 10V. Is this useful for PIC input?

10bits resolution is 1024 steps.
10kg / 1024 = about 10g step size. Is this useful resolution for a kitchen weigh?

Klaus
 

joniengr

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Hi,

Did you do some basic calculations?
1mV/ gram * 10kg = 10V. Is this useful for PIC input?

10bits resolution is 1024 steps.
10kg / 1024 = about 10g step size. Is this useful resolution for a kitchen weigh?

Klaus
The analog input of PIC is 5 V. I think the sensitivity which I can get is 0.5 mV/g.

0.5 mV /g x 10 kg = 5 V

I am also thinking to use PIC with 12 bit ADC.

10 kg / 4096 = 2.4 g per ADC step which is nice.
--- Updated ---

According to datasheets available for this half bridge load cell, the sensitivity is about 1 mV/V for rated load of 50 kg. Respectively it can't be used without a high gain differential amplifier.
Yes, I will use high gain differential amplifier but I don't understand the units of sensitivity 1 mV/V for load of 50 kg. What does mV / V mean ?

The scaling shows 5 mV / 5 V but at which load ?
 

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With a bridge supply voltage of 5V, you get 5 mV output for full scale load of 50 kg. Or 1 mV for 10 kg.
--- Updated ---

No load offset is however +/- 0.5 mV. To allow offset zeroing in software, you want some extra ADC resolution.
 

danadakk

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You complete the bridge with external Rs, and measure the
differential V across bridge, in the example in post #2 thats
connect to the two pins SensorPlus and SensorMinus.

Note the dashed lines and dashed components on the IDE screenshot are the external
stuff to psoc. Just done for completeness when user does the drag and drop resources
onto schematic. Note PSOC terminology a component is an onchip resource. You drag
components out of the chips component catalog and drop them onto schematic, wire
them up and config with wizards. Then do any additional code (from their API library for
each component) if you have to change config or do real time alterations to the compo-
nent.


Just curious, why are you going with half bridge load cells. "Normally"
full bridge sensors have laser trimmed R's to make sure bridge accuracy
is not compromised by unbalanced ref R's in the bridge.

Some useful ref material and a handbook on sensor errors and interface design. Its analog devices
but principles are same independent of HW parts.




Regards, Dana.
 
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joniengr

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With a bridge supply voltage of 5V, you get 5 mV output for full scale load of 50 kg. Or 1 mV for 10 kg.
--- Updated ---

No load offset is however +/- 0.5 mV. To allow offset zeroing in software, you want some extra ADC resolution.
Now I understand the meaning of sensitivity of 1 mV/V. If the bridge is powered up by 5 V then with the load of 50 kg the signal will be 5 mV that would need an amplification of 1000 to get 5 V that cover full ADC input range. I will use differential amplifier which will only amplify the difference in the bridge.

I am looking for 10 kg load cells actually because the loads I have is within 10 kg. I have searched but I did not find manufacturers that produce load cells for 10 kg with sensitivity better then 1 mV/V. Which load cells they actually use in kitchen balance ?

I have a plate of 20 cm x 20 cm approximately and I need to put load cells at four places. This means I will need four bridges because each load cell need to be connected in bridge configuration. Each bridge will have one load cell and three other balance resisters, right ?
 

danadakk

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Keep in mind the PSOC ADC has a PGA in its front end, G up to 8, and a Vref config
down to +/- ~ 78 mV full scale, with resolution to 20 bits.

For G = 1 thats a res down to .074 uV/bit. The onchip Vref, for absolute accuracy considerations,
is good to +/- .1%.

Here is an ap note on layout and error considerations -



Also you can config the onchip mux to handle 4 load cells....

Your biggest challenge is error budget, T & V errors, etc...., weather you care about absolute versus relative error.

A basic ap note that might help -



1611749448135.png


Regards, Dana.
 
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joniengr

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Any suggestion which manufacturer is producing kitchen load cells for 10 kg ? I will appreciate if someone come with the product link where I can order the load cells.
 

daraceleste

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Ive always used Load Cell Central. I usually call vs get a quote inline because they’re super helpful with questions and helping me figure out the best option. Website is www.800loadcel.com and the phone number is on there. I think it’s the same tho, 1-800-loadcel. I talk to Tom.
 

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