| Electronics > Projects, Designs, and Technical Stuff |
| HX711-based milliohm meter |
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| dannyf:
I just found a HX711 in my box of junk parts. It is a 24bit adc, primarily designed for weight scales. The module I have seems to have two imports wired out, but not symmetrical for some reason. The plan is to configure the two channels in serial, with the appropriate padding. One channel would be a precision resistor and another would be the dut. The adc chip has fairly low spec, so I wouldn't expect miracles but it would be interesting to see if it works out. More to come. |
| jaxbird:
--- Quote from: dannyf on December 26, 2014, 10:44:37 pm ---I just found a HX711 in my box of junk parts. It is a 24bit adc, primarily designed for weight scales. The module I have seems to have two imports wired out, but not symmetrical for some reason. The plan is to configure the two channels in serial, with the appropriate padding. One channel would be a precision resistor and another would be the dut. The adc chip has fairly low spec, so I wouldn't expect miracles but it would be interesting to see if it works out. More to come. --- End quote --- Usually I throw out these obscure Chinese chips because of lack of documentation. But if you got sufficient docs then I guess it could make a nice ratio based meter :-+ Otherwise I have at least 5 8051f350 in my uC box, I'll send you one if you want it. |
| dannyf:
I have used F350s before, very nice chips, particularly with both Vrefs out. |
| dannyf:
Here is a quick schematic. The module I used was sourced from ebay, and was poorly designed and poorly documented. It has six pins to interact with your circuit, 1) E+/E- are the positive / negative engerizing terminals. 2) A+/A- are for channel A (128x and 64x gain). the pins are unbalanced in that only A+ has a 1.2k resistor. 3) B+/B- are for channel B (32x fixed). The pins are unbalanced in that only B= has a 1.2k resistor. The original module was meant for weight scales, so it is designed to work with a bridge, and with only 1 channel (typically Channel A). I want mine to be ratiometric so I have to use both channel A and channel B. So here is my wiring: |
| dannyf:
The chip has some interesting spec: 1) padding: it requires minimum and maximum common mode on the adc pins. I think it is from GND+1.2 to AVDD-1.3. As the module can only be run from a 5v source (due to a poorly designed Vfb circuitry where a 20k/8.2k divider ratio was used -> AVdd=4.3v). So I have to 'pad" the circuit and to keep the adc pins to be reasonably close to the middle (about 2.1v). 2) channel selection: the full scale voltage is very low. For Ch A, it is either 4.3v/128=33mv or 4.3v/64=67mv (or half of that for bipolar readout). For Ch B, it is about 4.3/32=134mv. So it makes sense, in order to maximize resolution, to put the precision resistor on Ch B, and the dut on Ch A. 3) precision resistor selection: The current going through the resistor string is about 4.3v/20k=0.2ma. The maximum bipolar reading on Channel B is 134mv / 2 = 70mv. So the maximum value of Ch B resistor is 70mv/0.2ma=350ohm. I happen to have a 82ohm resistor so that's what I am going to use. |
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