without a load on the load cell all the resistances are the same. However
when the load cell has a weight on it 2 of the strain gages will be in
compression (A and D) and the resistance will decrease and 2 will be in
tension (B and C) and their resistance will increase.
350
oh
m
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 20-30mv of signal if
the excitation voltage is 10volts and the load cell is fully loaded.
Most load cells used in scales are not as simple as this example. They
have additional resistance elements added to compensate for temperature
variations and to set the outputs to correct voltages. The circuit of a typical
load cell is shown below.
@Adam Equipment Company 2007
15
