Theory of Operation Load Cell

This section describes the theory of operation of load cells and the weight indicator.

A. Each load Cell consists of block of steel specially machined so that as force is applied to it a section in the center of cell (the strain web) deforms a very specific amount for each pound of force applied to it.

B. In the strain web four strain gauges are glued to the metal. They are arranged electrically in a
bridge. In addition a 5th resistor is typically used for calibration at the factory.

C. R1-R4 are the strain gauge resistors glued to the metal strain web. Each is approximately 350
ohms (certain load cells may be 750 ohm or higher, refer to your manufacturer’s spec sheet) , nd setup so that when there is no pressure on the load cell all four resistors are equal in value. As pressure is put to the load cell, thus deforming the metal slightly, the resistance of the strain gauge resistors changes slightly.

D. The strain gauges are configured so that as force is applied to the cell R1 and R3 slightly increase in value, and R2 and R4 decrease in value (by less than 1 ohm).

E. The excitation voltage applied to the bridge causes current to flow from +EXC thru the strain gauges to –EXC. As long as R1-R4 are all equal, then there will be no voltage difference between the +SIG and +SIG legs (the unloaded condition of the load cell).

F. As force is applied to the load cell, and the strain gauge bridge becomes slightly un-balanced, then a voltage will appear between the +SIG and –SIG legs.

G. The magnitude of this voltage is based on load cell design, and is rated in Mv/V (milliVolts of signal per Volt of Excitation) at full load. A lot of load cells are rated at 3mv/V, which means that for every volt of excitation we will get 3mv of signal if the load cell is loaded to capacity. Other load cells may be rated at 2mv/V.

H. The calibration resistor (R5) is approximately 50 ohms, and represents a combination of calibration resistors that are installed at the factory to adjust the mv/V specification, and to compensate for temperature changes.

I. Placing an Ohm Meter across the signal leads (with all other leads disconnected) you will measure approximately 350 ohms (+/- 5 ohms). Placing the meter across the Excitation leads you will measure 395 ohms (+/- 5 ohms). Placing the meter across a signal and excitation lead you will measure less than 300 ohms.

J. The load cell cable also has a shield wire that is attached to ground at the indicator end. You should never be able to measure any resistance between an Excitation or Signal wire and the shield.

K. The primary calibration specification for load cells is how much the mv output of the cell changes per pound of load applied. This is factory calibrated to be within .03% accuracy. The output with no load is roughly factory calibrated (since this gets zero’d out during calibration anyway), and is specified to be within 1% of full scale output with no load (around +/- 1mv in most applications).