I have a balanced Wheatstone Bridge connected to a HX711, that is connected to the Arduino Mega 2560. The purpose of the project is to measure the temperature with the resistance variation of a wire. The precision has to be approximately 4uV.
Reading the voltage on the bridge with a microvoltmeter without the USB connected to the PC gives the right values and changes as expected when the temperature variates. When I connect the USB to the PC to peform the readings, an offset voltage around 1mV appears on the bridge. I say offset, but no variation occurs on the bridge when the temperature changes.
I'm not reading on the PC yet. All the measures I've taken were from the microvoltmeter.
I don't have a diagram, but I will make one as soon as possible.
Here it is. The only difference between the two attempts is that in the first the USB of the arduino is not connected and in the second it is. The arduino is fed by an external power supply.
That could be an issue. How much current does the HX711 consume and what are the values of the resistors in the WS-bridge?
What if You dropped the 7.5 volt supply and use a powerpack, such one charging phones, instead?
I fear that the Arduino 5 volt converter does not manage the load.
The HX711 itself should not consume much, definitely not as much as to cause a problem - and at 7.5V input the regulator has quite some spare capacity - it has to drop only 2.5V. Nonetheless it's generally recommended to run the Arduino directly off a 5V power supply (to the 5V pin).
A quick Google search showed me that there are four connections for the wheatstone bridge on a typical HX711 module, your schematic shows you're using only two, leaving B- and B+ disconnected. That doesn't sound right.
I remember the HX711 produces its own excitation voltage of 4V or so - meaning no 5V or GND connections to your bridge, but the B+ and B- instead. Do have a good look at how others connect their HX711, as most schematics I found do not match yours.
You may have a layout problem - it’s easy to create a small voltage drop along power supply and ground connections - I’d look at those and see where you have voltage drops .
Generally you want all 5v and 0v connections to go to a single point rather than daisy changed .
wvmarle:
I remember the HX711 produces its own excitation voltage of 4V or so - meaning no 5V or GND connections to your bridge, but the B+ and B- instead. Do have a good look at how others connect their HX711, as most schematics I found do not match yours.
You picked that blunder correctly! Start by connecting the bridge properly and work from there.
Thanks guys, I tried all your suggestions but the problem persists:
-First I connected the supply of the Wheatstone Bridge on the E+ and E- ports of the HX711, as in the image below. The problem persists: after connecting the USB to the PC an offset appears on the bridge.
-After that I connected the USB to a laptop, and it worked. Also worked when the supply of the bridge was from the Arduino 5V regulator. But when I put the laptop to charge, the offset comes back. This is bad, because I need to measure the temperature for long periods, so an external battery supply or the laptop must be the last option.
The ports B+ and B- are just other differencial input of the ADC that I am not using, so it can float.
So, do you have another idea of what is causing the offset?
Trying to solve this, I realized that by only touching the outside parts of the USB (I'm using an extension to connect to the PC) the ofsset comes back. The outside is the shielding.
wvmarle:
This doesn't make sense - unless you forgot to mention using a 7-12V power supply.
I cutted off the 5V cable of the USB to power the Arduino with an external power supply (7.5V), but the offset continues to appear.
wvmarle:
What happens if you power the Arduino from a regulated 5V supply?
