S400 – S401 – S402 – S420 Application Notes








General Description

The SMD S400, S401, S402, & S420 Load Cells are a family of stainless steel-bodied assemblies (depending on capacity) containing a monolithic sensing element with a strain gage applied, capable of producing a linear, analog voltage output in response to loadings of 0‐50 lbs up to 1000 lbs depending on the model is chosen.

Physical Consideration

Mounting: This sensor should be evenly supported around the entire perimeter of its cylindrical body on a flat and rigid “mechanical ground” plane. Lateral and longitudinal positioning should be controlled by placing it in a recess or other controlling geometry. It should not be press-fit or retained with any excessive force to the sides of the body.

Loading: For models S400, S401 and S420, load forces may only be applied in a compressive direction to the load button of the sensor, perpendicular to the sensor body and on the center axis of the load button. The S402 can be loaded in compression or tension. For accuracy and repeatability, a single point of contact is best. The configuration of the S400 series sensors provide for no inherent overload protection (loading in excess of 150% of rated capacity) therefore provisions should be designed into the application hardware to preclude excessive deflection and damage to the sensor.

Wiring: Color-coded lead wires are provided for electrical connections and the cable guide tube provides an adequate form of strain relief under normal conditions. On the S420, electrical connections are made to the
sensor via the 4 gold plated pins at its bottom. While designed for direct soldering through holes in a printed circuit board, individual wire “push‐on” terminals may be used as well.

Electrical Considerations

As seen in the schematic drawing, 4 resistive elements are configured in the strain gauge pattern in a Wheatstone bridge configuration. The application of a load to the sensor causes an electrical imbalance between the resistive elements and produces an output voltage (in millivolts) that is linear and proportionate to the load applied. Less than the maximum excitation voltage may be used however the resulting output will be reduced proportionately.

Environmental Concerns & Precautions

Care must be taken to avoid condensing moisture and direct water exposure. The S400 series sensors are very stable over time and temperature. The robust stainless steel housing provides adequate physical protection for the strain gauge elements however, during handling and assembly procedures, anti‐static discharge practices should be observed. The S420 should not be exposed to moisture in any circumstance.

Mounting Recommendation

Ideally, load interface should be via a single point of contact, such as the intersection of the load button to a plate.


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