What Is a Load Cell Module? Types and Selection Criteria
A load cell module excites a load cell, digitizes its millivolt-level signal and passes it to the PLC as a weight value.
A load cell module is a weighing input module that supplies the bridge circuit of a load cell, digitizes the millivolt-level signal from the bridge and passes it to the PLC as a weight value. In filling, dosing and bagging machines and in tank and silo weighing, it lets the PLC program process the weight directly, without a separate weighing indicator.
Why a standard analog input module is not enough
A load cell does not output 0-10 V or 4-20 mA; its output is defined in mV/V relative to the excitation voltage. For example, a 2 mV/V cell gives 10 mV at full load with 5 V excitation. Measuring such a small signal requires bridge excitation and a high-resolution converter. If a weighing transmitter with a 4-20 mA output is placed between the cell and the PLC, a standard analog input module can be used.
Types
| PLC family | Connection | Models in the catalogue |
|---|---|---|
| Delta DVP slim series | Expansion module fitted on the left side of the PLC | DVP201LC-SL (1 channel), DVP202LC-SL (2 channels) |
| Delta AX-5 (CODESYS) | AX-5 expansion bus | AX-501LC10-0A (channel count in the datasheet) |
| GMT | GXM expansion series | GXM-10L, GXM-20L (channel count in the datasheet) |
The names of the Delta DVP201LC-SL, DVP202LC-SL and AX-501LC10-0A state 4- and 6-wire connection and 24 bit resolution; check the connection type of the GMT models in the datasheet.
Selection criteria
- PLC family: A module works only within its own family. DVP-SL modules connect to Delta DVP slim series PLCs with a left-side expansion bus, the AX-501LC10-0A to AX-5 systems, and GXM modules to GMT PLCs.
- Channel count: Each separate weighing point needs one channel. Several cells under one tank are connected in parallel in a junction box and enter a single channel.
- 4-wire or 6-wire connection: A 4-wire cell has excitation and signal lines. The two extra sense lines of a 6-wire cell measure the excitation voltage reaching the cell and compensate for cable voltage drop and temperature effects.
- Cell sensitivity (mV/V): The module's setting range must cover the value on the cell label. The Delta DVP201LC-SL and DVP202LC-SL offer range settings up to 80 mV/V.
- Real accuracy: 24 bit describes the module's converter; the real accuracy of the weighing is set by the accuracy class of the cell, the mechanical mounting and the calibration.
- Excitation current: The excitation current drawn rises with the number of parallel cells; check in the data sheet that the total stays within the module's excitation output.
Common mistakes
- Leaving the sense terminals open with a 4-wire cell: When a 4-wire cell is connected to a 6-wire input, the sense terminals must be bridged to the excitation terminals on the module side.
- Running the cable in the same duct as power cables: The millivolt signal is sensitive to interference; use a shielded load cell cable and earth the shield at one point.
- Skipping calibration: Until the module is calibrated at zero and with a known weight, the reading cannot be trusted.
- Fixing the vessel with rigid pipes: Rigid pipe and cable connections carry part of the load, so the cell does not see the full weight; use flexible connections.
Load cell modules in the Welibaba catalogue
The Load Cell Modules category has 5 active products: Delta Electronics 3 (DVP201LC-SL, DVP202LC-SL, AX-501LC10-0A) and GMT 2 (GXM-10L, GXM-20L). To connect the cell to the module see Load Cell Cables, and to read a 4-20 mA or 0-10 V signal from a weighing transmitter see Analog Input Modules.
Frequently asked questions
What is the difference between a 4-wire and a 6-wire load cell?
A 4-wire cell has two excitation and two signal lines. A 6-wire cell has two extra sense lines; through them the module measures the actual excitation voltage reaching the cell and compensates for the error caused by cable length.
Can more than one load cell be connected to a single channel?
Yes. Cells under the same vessel are connected in parallel in a junction box, and a single channel reads the total weight. The cells must have the same capacity and mV/V value, and the total excitation current must not exceed the module's limit.
Can an analog input module be used instead of a load cell module?
Not if the cell is connected directly; an analog input module does not provide bridge excitation and cannot resolve the millivolt signal. If a weighing transmitter with a 4-20 mA or 0-10 V output is placed in between, an analog input module can be used.
Can a PLC load cell module be used for trade weighing?
Weighing for commercial sale requires a weighing instrument with legal metrology approval. A PLC load cell module does not provide this approval on its own; check the applicable regulations before use.