The load cell transmitter converts the low-level mV/V signal of the load cell into a standard analog or digital output that can be read by a PLC, data acquisition system, or controller. Choosing the right transmitter depends not only on the type of output but also on the load cell excitation, input range, isolation, resolution, and environmental conditions.
Why is a transmitter used?
The load cell output is often only a few millivolts. This signal can be affected by interference when transmitted over long cables or in environments with powerful motors and inverters. The transmitter amplifies the signal, filters it, and converts it into an industrial form such as 4–20 mA, 0–10 V, RS485, or Modbus.
When is a 4–20 mA output chosen?
The current signal is less affected by voltage drop over long distances, and thanks to the 4 mA live zero, cable breakage can be distinguished from the true zero. It is a strong choice in process automation and remote PLC panels. The receiver input resistance, supply voltage, and loop load should be calculated.
Characteristics of 0–10 V output
Voltage output provides simple integration over short and controlled cable distances. Common reference, grounding, and voltage drop are important. In long lines with high electromagnetic noise, shielding and cable routing should be carefully planned.
Advantages of RS485 and Modbus
RS485 is a physical communication layer; Modbus RTU is one of the frequently used protocols over this line. Multiple devices can be addressed on a single line, weight can be transmitted digitally, and in some devices, status/setting information can be accessed. Termination, polarity, baud rate, parity, and device address must be compatible across all nodes.
Transmitter selection criteria
- Load cell nominal accuracy and input signal range
- Supported excitation voltage and total bridge resistance
- Analog or digital output requirement
- Galvanic isolation and EMC resistance
- Supply voltage, panel or field mounting
- Calibration method, filtering and sampling rate
- Required accuracy and operating temperature
Multiple load cell connections
When cells are connected in parallel in the junction box, the minimum load resistance that the transmitter can supply must be checked. Four 350-ohm cells do not create the same load as a single 350-ohm cell electronically. The number of cells allowed in the datasheet or the minimum bridge resistance is taken as a basis.
How is PLC scaling done?
First, the zero and span calibration of the mechanical system is completed. Then the minimum and maximum values of the transmitter output are scaled to the engineering unit in the PLC. For 4–20 mA, 4 mA can represent zero load, and 20 mA can represent the determined full scale. Limit checks should be added for overflow, negative values, and sensor breakage.
Why is isolation important?
When the load cell, transmitter, PLC, and power supply are at different ground potentials, circulating current and noise can occur. Galvanic isolation can reduce these effects; however, it does not replace proper panel grounding and shield connection.
Common wiring errors
- Guessing load cell colors without a datasheet
- Short-circuiting the sensor terminals incorrectly
- Grounding the screen uncontrollably from both ends
- Using star topology and incorrect termination on the RS485 line
- Connecting the analog output common terminal to the wrong reference
- Hiding the error with only PLC scaling without calibration
Frequently Asked Questions
Is the transmitter the same as the indicator?
No. Although some functions overlap, the indicator usually provides a user screen and weighing functions; the transmitter focuses more on signal conversion and automation integration.
Which is better, 4–20 mA or RS485?
There is no single universal answer. For a simple and durable process signal, 4–20 mA; for multiple devices, data integrity, and digital diagnostics, RS485/Modbus may be suitable.
Does the transmitter increase the accuracy of the load cell?
The correct device reliably processes the signal; however, it does not eliminate mechanical error or sensor fault.
You can examine Kobastar load cell transmitter options and contact our technical team for PLC integration ..