Configuration of load cell on digital scaling

Question

UPDATED! Anyone can tell me how to configure the load cell (like mine is MLC902) on digital scale? How is the calculation voltage output when is loaded by 50kg or 100kg?

![this is my wheatstone bridge][2]

Updated : the second schematics is my wheatstone bridge with 4 load cell. The load cell have 3 wires, red, white(green) and black. R1(red wires), b1(black wires) and w1(white wires) on first load cell. R2(red wires), b3(black wires) and w3(white wires) on second load cell. R3(red wires), b3(black wires) and g3(green wires) on third load cell. R4(red wires), b4(black wires) and g4(green wires) on forth load cell. i know little bit about wheatstone bridge, just like to calculate the output Vo= R1/(R1+R4) VB - R2/(R2+R3) VB. But the wheatstone bridge have 4 load cell on each side, which is have a spesification output voltage 1,15mV/V and resistance 1K for each strain gauge on MLC902. Anyone can help me? or give me some reference?

Answer 1

This is a better view of a fairly generic load cell comprising 4 strain gauges: -

A simple single-ended power supply can be used (such as 5V) and this is applied between red and black wires with black being connected to 0V.

The green and white wires will nominally be centred at about 2.5 volts with no load. One wire's voltage will rise with load whilst the other wire's voltage will fall. Ideally you need to interface these wires to an instrumentation amplifier. The inamp has a transfer function like: -

Output = gain*(in_pos - in_neg) + reference.

The single output from the inamp feeds your microcontrollers ADC input. Here's a typical example using an external ADC: -

Circuit taken from here and it includes an anti-alias filter based around the OPA350 but you would get away with a couple of single order RC filters for simple weighscales.

How is the calculation voltage output when is loaded by 50kg or 100kg?

The 50 kg load cell has an output of 1mV per volt and this means if you excite it with 5 volts the output voltage will be 5 mV on full load. This, in effect means that the green wire has risen 2.5mV higher than its nominal "neitral" voltage of 2.5 volts and the white wire has fallen 2.5mV from its nominal neutral voltage. The difference is of course 5mV and it is this difference that has to be measured i.e. we don't care about the neutral voltage.

This is why it is important to use an instrumentation amp. But, you'll also need to feed the inamp with a reference voltage. For a 5V supplied ADC/micro this should ideally be 2.5 volts.

Note also that when using a single ended excitation voltage you need to ensure that the inamp's inputs can cope with the full range of voltages that may appear on green and white wires. I don't see it as a problem using a 5V supply - green or white will generally be no less than 2.45 volts and most single ended inamps will cope with that. However, the inamp in the circuit above is "true" rail-to-rail meaning it will cope with inputs all the way down to 0V.

Because the inamp's inputs are very high resistance you don't need to worry at all about loading effects. One more tip - put a 100nF capacitor across green and white to cut down any RF pickup.