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hx711

How to read four load cells with a single ADC input?

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I am currently using strain gauge based load cells and want to weigh 1-5kg granular products on whatever container with 4loadcells beneath. I encountered a problem with connecting load cells in a parallel position. I thought it would work, but nothing showed up as I expected. A single load cell works well with a hx711, the amplifier, but when I link four load cells, parallel doesn’t work. The loadcell I use and a circuit are in the picture shown above.

What type of circuit junction box make all the loadcells in parallel contain? And how do I manage that?

Please tell me how to handle that issue most efficiently?

sensoradcload-cellwheatstone-bridge

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edited 5 hours ago

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Davide Andrea

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asked 13 hours ago

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Tumenbayar Munkhjargal

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  • 3had you connected two of them backwards? Choose one load cell as the reference, then check the polarity of each other cell to that. Then try again with them all in the same phase. – Neil_UK 13 hours ago
  • @Neil_UK I connected them, colour to colour like black to black red to red white to white green to green – Tumenbayar Munkhjargal 12 hours ago
  • I never thought about connecting 4 bridges in parallel, fighting each other. I always thought that I need 4 load cells, ie, 4 bridges, and 4 ADCs, and let the MCU calculate the weighted weights. HX711 is dirt cheap these days, so using 4 of them is a quick and dirty, but clean solution. – tlfong01 12 hours ago  
  • 1If they were a set, intended to be connected like that, then that would be the right thing to do. If they were a set intended to be connected to a specific PCB from some scales, then it might not be. Testing them in pairs to check phasing also checks whether any of them are shorted, thus stopping your whole quad from working. Look at it this way. You’ve tried something, it didn’t work, you’ve asked the question. I’ve asked you to do something that you haven’t done yet. Worth a try maybe? – Neil_UK 12 hours ago
  • @Neil_UK I got what you meant. I just tried two loadcells in parallel and got nothing again. Note that the two loadcell I’m using are working fine individually – Tumenbayar Munkhjargal 12 hours ago
  • Could you please be more specific? “got nothing”, “nothing showed up” and “doesn’t work” are not of much help for us. – devnull 12 hours ago 
  • @devnull reading data through the Arduino serial, it prompts 0 and means that some problem arises in the hardware part – Tumenbayar Munkhjargal 12 hours ago
  • 4Do you have a DVM? ‘Got nothing’ means nothing, you always get something. You’re putting 5 V on the red wire and 0 V on the black wire? With each load cell connected individually, measure the voltage on the green and white wires with zero deflection and full load deflection. Do the same for two cells in parallel. What are you using to power the cells? It’s possible that two or more in parallel are bringing your supply down. ‘Reading data through the arduino serial’ for debugging is like trying to eat spaghetti with chopsticks, wearing boxing gloves. Get down and dirty with the volts and wires. – Neil_UK 12 hours ago
  • 1Is your 5 V source capable of driving several load cells in parallel? Where is the datasheet of the load cells? What is the maximum current of the 5 V source? What is the resistance of each bridge resistor of the load cells? – Uwe 12 hours ago
  • @Uwe I did not physically measure the resistance of it, and I got 5v from Arduino so it might be around 40mA? – Tumenbayar Munkhjargal 11 hours ago
  • @Neil_UK I’ll try what you recommend, it seems like I did not do anything you mentioned, helpful info thanks! – Tumenbayar Munkhjargal 11 hours ago
  • 1Here is a datasheet ardu.dk/wp-content/uploads/2015/03/Vejecelle-YZC-1B.pdf The excitation voltage should be 10 to 15 V. Input Resistance may be as low as 400 Ohm. You probably need a amplifier to a decent voltage for the arduino, 20 mV may be too small for the ADC input. But I would not trust a datasheeet with MV/V instead of mV/V. They should know the difference between megavolt MV and millivolt mV. – Uwe 11 hours ago
  • @Uwe I already have HX711 amplifier actually, thanks for your feedback – Tumenbayar Munkhjargal 10 hours ago
  • The HX711 is designed for one load sensor only, there is no information about parallel connection of several sensors. – Uwe 10 hours ago
  • @Uwe HX711 is a differential voltage input ADC with a multiplexer, PGA, and optional voltage regulator that can be used for gage excitation – it’s entirely optional. It doesn’t care about source impedance as long as it’s low enough. Paralleling the load cells won’t affect it at all. I’m not sure what the OP is doing wrong, but I’ve just checked and it works fine for me with load cells on the WiiBoard. Turns out I’ve had such a thing put together a while ago and forgot. I paralled the load cells in pairs, and used both channels of HX711. – Kuba hasn’t forgotten Monica 4 hours ago 
  • @TumenbayarMunkhjargal Please edit the question with a link to the particular HX711 module you’re using, and add pictures of how that module is connected to the supply etc. – Kuba hasn’t forgotten Monica 4 hours ago
  • @Uwe The excitation can be anything you want as long as it’s not too high. Many low cost weigh scales drive the strain gages at 5.0 or 3.3V, or even directly from a rechargeable lithium cell, i.e. anywhere between 3.5 and 4.2V, or thereabouts. The ADC is ratiometric and directly measures in counts/V. As long as the rail is clean, the absolute voltage doesn’t matter if it only changes slightly. The major thing that degrades as you lower the excitation voltage is the noise level. The “10-15V” spec is the maximum to prevent heat damage to the sensor. – Kuba hasn’t forgotten Monica 4 hours ago
  • Could you excite each bridge sensor with a separate modulated current source? If you could, you could then mix the IN-AMP signals and analyze them in the frequency domain on the MCU. You would need a much higher sampling frequency. Definitely a brute-force approach – Luke Gary 4 hours ago

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This should work: the source resistances of the load cells are usually very close, and the Thevenin equivalent of this circuit is a voltage summer (averager).

There is a catch: while this may be OK at DC, it usually underperforms at AC, since the parasitics of each cell affect all the others. You’ll end up degrading AC CMRR versus using a dedicated in-amp or sigma-delta ADC on each cell individually.

Since you’re using a two-channel sigma-delta ADC connected directly to the strain gages, your best bet is to use two of them. You connect two load cells to each HX711, and can multiplex between them. This will still be way cheaper than using four instrumentation amplifiers and a single ADC. You can also buy other 4-channel ADCs that can take such low-level inputs: either multiplexing them, or sampling all four in parallel.

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edited 4 hours ago

answered 12 hours ago

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Kuba hasn’t forgotten Monica

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  • This should work only if there is enough current to feed 4 load cells in parallel instead of only one. The HX711 seems to be designed for one load cell only. – Uwe 8 hours ago
  • @Uwe Would you recommend some other amplifier that can manage? – Tumenbayar Munkhjargal 8 hours ago
  • like common in the market – Tumenbayar Munkhjargal 8 hours ago
  • I could not recommend any other amplifier. I would look for an amplifier with explicit specification of parallel load cells or an amplifier that may be used with a separate feed. – Uwe 7 hours ago
  • @Uwe HX711’s power supply regulation is optional and uses an external pass transistor. You can excite the load cells from a separate regulator if you wish, but that’s not usually necessary. Just select a pass transistor that has a good gain at the strain gage excitation current desired, and HX711 will control it just fine. In any case, the OP didn’t even intend to supply the gages using HX711’s built-in regulator it looks like. – Kuba hasn’t forgotten Monica 4 hours ago 

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