@Adam Equipment Company 2004
5
6.0 PRINCIPLE OF OPERATION
The strain gauge load cell is a method of measuring the amount of stress put
onto a metal structure due to a weight being supported by the structure. The
stress is measured using 4 strain gauge sensors mounted in a particular way
on the metal structure.
These 4 strain gauges are connected in such a way that they form a
Wheatstone Bridge. See the figure below. In the simplest load cells there are
only the 4 resistances of the strain gauges to consider. Normally without a
load on the load cell, all the resistances are the same. However, when the
load cell has a weight on it, two of the strain gauges will be in compression (A
and D) and the resistance will decrease and the other two will be in tension (B
and C) and their resistance will increase.
350
oh
m
35
0 o
hm
350
oh
m
35
0 o
hm
A
B
C
D
+E
-S
-E
+S
BALANCED BRIDGE
10 VOLT POWER
SUPPLY
Vout = O VOLTS,
BS AND -s
351
oh
m
34
9 o
hm
351
oh
m
34
9 o
hm
A
B
C
D
+E
-S
-E
+S
UNBALANCED BRIDGE
FULL CAPACITY LOAD
ON THE LOAD CELL
10 VOLT POWER
SUPPLY
Vout = 0.028 VOLTS (28mv),
BS AND -s
This will cause the bridge to become unbalanced. When the bridge has a
voltage across it from +E to -E then the signal output at +S and -S will show a
voltage of zero volts with no load and a small voltage proportional to the load
as the load is increased. Typical load cells show 10mv of signal if the
excitation voltage is 5volts and the load cell is fully loaded.
To check the load cell, apply power to the scale and measure a voltage
between the S+ and S- wires when de-soldered from the PCB. The voltage
should be less than 2millivolts with no load on the scale, increasing as the
load is applied. The voltage between the E+ and E- wires should be 5V.
