©
Adam Equipment Company 2011
7
6
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.
35
0 o
hm
35
0
oh
m
35
0 o
hm
35
0
oh
m
A
B
C
D
+E
-S
-E
+S
BALANCED BRIDGE
10 VOLT POWER
SUPPLY
Vout = O VOLTS,
BS AND -s
35
1 o
hm
34
9
oh
m
35
1 o
hm
34
9
oh
m
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.
