Adam CBC-M Series Service Manual page 9

gages 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 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.
10 VOLT POWER
SUPPLY
+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.
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
+E
A
B
-S
C
D
Vout = O VOLTS,
BETWEEN +S AND -s
-E
BALANCED BRIDGE
+E
A
10 VOLT POWER
SUPPLY
+S
C
-E
UNBALANCED BRIDGE
FULL CAPACITY LOAD
ON THE LOAD CELL
7
B
-S
D
Vout = 0.028 VOLTS (28mv),
BETWEEN +S AND -s
Typical load