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Using Arduino => Sensors => Topic started by: Naesstrom on Jan 10, 2017, 02:34 pm

Title: Bypass HX711 VCC and power the load cells directly?
Post by: Naesstrom on Jan 10, 2017, 02:34 pm
I'm working on a project with multiple load cells going through some muxers to a HX7110 like this:
(http://i.imgur.com/MvhqA6c.png)

Question is if I can take 5V directly from my ATX powersupply and power each of the 16 (or 32) load cells instead of using the Excitation+from the 711, or wont it be a problem connecting that many load cells to a single amplifier otherwise?
Title: Re: Bypass HX711 VCC and power the load cells directly?
Post by: pylon on Jan 10, 2017, 06:40 pm
I hope I understand you correctly in that you want to remote the multiplexer with the white wires in your picture.

This is not possible you'll understand why by looking at Fig.1 of the HX711 datasheet (https://cdn.sparkfun.com/datasheets/Sensors/ForceFlex/hx711_english.pdf). The load cell is Vcc and GND input and a positive and a negative output. Both outputs must be connected to the HX711 to get a correct measurement.
Title: Re: Bypass HX711 VCC and power the load cells directly?
Post by: Wawa on Jan 10, 2017, 11:19 pm
Supplying the cells from the raw 5volt rail will make readings unstable.
The HX711 chip uses an external PNP transistor to make a stable 4.3volt E+ excitation voltage.
I don't think that regulated supply can power more than four load cells.
Leo..
Title: Re: Bypass HX711 VCC and power the load cells directly?
Post by: Naesstrom on Jan 11, 2017, 07:07 am
Thanks for the answers, when looking at the same figure it looks like E+ is powered with the line in but also going through the S8850 transistor, that has a max A of 125m (http://www.datasheets360.com/pdf/-252002789868889517) and since each load cell only uses 5mA it shouldn't be a problem running 16 on it, right?
Title: Re: Bypass HX711 VCC and power the load cells directly?
Post by: Naesstrom on Jan 11, 2017, 07:18 am
I hope I understand you correctly in that you want to remote the multiplexer with the white wires in your picture.

This is not possible you'll understand why by looking at Fig.1 of the HX711 datasheet (https://cdn.sparkfun.com/datasheets/Sensors/ForceFlex/hx711_english.pdf). The load cell is Vcc and GND input and a positive and a negative output. Both outputs must be connected to the HX711 to get a correct measurement.
No the white wires is correct, that is one of the signal wires, the Green one is the other signal. Using the multiplexers should work since they will switch between the 16 load cells and each time send a new measurement to the 711.
I was referring to the red and black cables that supports the load cells with power and ground, but as @wawa said I'm afraid the load cell needs to know exactly what voltage the load cells are getting at E+/E-. I'm just not sure how many load cells the HX711 can power?
Title: Re: Bypass HX711 VCC and power the load cells directly?
Post by: Wawa on Jan 11, 2017, 07:49 am
Thanks for the answers, when looking at the same figure it looks like E+ is powered with the line in but also going through the S8850 transistor, that has a max A of 125m (http://www.datasheets360.com/pdf/-252002789868889517) and since each load cell only uses 5mA it shouldn't be a problem running 16 on it, right?
That depends on how much base current the HX711 can provide to saturate that transistor (Hfe is almost meaningless with <0.7volt across the transistor). Can't find that in the HX711 datasheet.

That transistor circuit is basically a low dropout voltage regulator, with R1,R2 providing the feedback.
The boards I know are set to 4.3volt.

You could measure E+ with one load cell, and again with 16 load cells.
It should be close to 4.3volt in both cases.
Leo..
Title: Re: Bypass HX711 VCC and power the load cells directly?
Post by: boboye on Dec 28, 2017, 05:14 pm
Hello, I also have a similar question. So my load cell is a 20kg and it needs 10v for its excitation. Whereas  my hx711 board can only take 5v max based on the day a sheets. I hope it won't be a problem to by pass the hx711 E- and E+ and power the load cell from  normal battery. But I will still connect the load cell outputs to the hx711 A+ & A-.
Please help!
Title: Re: Bypass HX711 VCC and power the load cells directly?
Post by: DaveEvans on Dec 28, 2017, 06:26 pm
If you pwr the cell with 10v you will violate the common mode limit of the hx711.   Just use the hx711's excitation.  Should be fine.
Title: Re: Bypass HX711 VCC and power the load cells directly?
Post by: Wawa on Dec 28, 2017, 11:15 pm
...my load cell is a 20kg and it needs 10v for its excitation.
No it doesn't.
Load cells work fine on lower voltages than they are specified for.
Bridge output voltage is just lower with a lower excitation voltage.
Not a problem for the HX711.
Leo..
Title: Re: Bypass HX711 VCC and power the load cells directly?
Post by: shmulik on Dec 14, 2018, 02:49 pm
Hi
I am trying to do something similar.
can you share your final connection between all the electronic parts ?

best regards