6-Wire vs. 4-Wire Wiring Guide – Optimizing Voltage Compensation (Sense Wiring) for Load Cells
Introduction: The Necessity of Stable Excitation Voltage
The accuracy of any load cell measurement hinges on the stability of the excitation voltage supplied by the weighing indicator. This voltage (typically 5V or 10V DC) powers the Wheatstone bridge within the load cell. If the voltage reaching the load cell fluctuates—primarily due to changes in cable resistance caused by temperature variations over long distances—the load cell’s output signal will be inaccurate, leading to measurement drift. The decision between 4-wire and 6-wire load cell wiring is critical for mitigating this risk, particularly in large industrial installations.
The 4-Wire System: Simplicity and Its Limitations
The 4-wire configuration is the simplest and most common wiring method, used predominantly in short-run cable installations or where high precision is not the primary requirement.
Configuration
- Excitation Wires (2): Supply voltage from the indicator to the load cell (typically +Excitation and -Excitation).
- Signal Wires (2): Return the measured voltage signal from the load cell back to the indicator (typically +Signal and -Signal).
The Problem of Lead Resistance
In a 4-wire setup, the indicator measures the excitation voltage at the indicator itself, not at the load cell terminals. As cable length increases or ambient temperature changes, the resistance of the copper cable changes. This drop in voltage (voltage loss) occurs between the indicator and the load cell, meaning the actual voltage powering the load cell is lower than the indicator expects. The resulting error is a systematic, uncompensated drift in the weight reading.
The 6-Wire System: The Power of Sense Wiring
The 6-wire configuration solves the voltage drop problem by introducing two dedicated feedback wires, known as Sense or Reference wires. This system is the standard for high-accuracy applications, long cable runs, and demanding industrial environments.
Configuration
- Excitation Wires (2): Supply voltage to the load cell.
- Signal Wires (2): Return the measured signal to the indicator.
- Sense Wires (2): Dedicated feedback wires that run parallel to the excitation wires (typically +Sense and -Sense).
Voltage Compensation Mechanism
The indicator uses the two Sense wires to continuously monitor the excitation voltage directly at the load cell's terminals. If the indicator detects a voltage drop via the Sense wires, it automatically and instantaneously increases the output voltage on the Excitation wires until the correct, stable voltage reaches the load cell. This process is called remote sensing or voltage compensation.
Technical Trade-Offs and Selection Criteria
The choice between 4-wire and 6-wire systems is an engineering decision based on cost, distance, and required accuracy.
| Feature | 4-Wire System | 6-Wire System (Sense) | Recommendation |
|---|---|---|---|
| Accuracy | Susceptible to drift and temperature errors. | Highly stable; voltage compensation eliminates cable resistance error. | Use for Legal-for-Trade and high-precision. |
| Cable Length | Short runs (typically under 15m). | Long runs (up to 300m or more). | Required for remote weighbridges or silo scales. |
| Cost/Complexity | Lower cost, simpler wiring. | Higher cost (more conductor cores), more complex junction box. | Avoid if the application is simple or non-critical. |
| Application | Bench scales, internal non-critical use. | Tank weighing, truck scales, automated batching. | Required for systems demanding certified repeatability. |
Junction Boxes and Multi-Cell Systems
In systems using multiple load cells (e.g., a 4-cell platform scale), the Sense wires are connected in parallel at the junction box. This ensures that the indicator receives a single, accurate voltage feedback signal, compensating for the cumulative resistance of the cables from the indicator all the way out to the junction box, which is a major source of error in multi-cell applications.
The 6-wire configuration is not just an optional upgrade; it is a mandatory feature in professional, certified industrial weighing systems where long-term stability and immunity to environmental fluctuations are paramount to maintaining the integrity of the weight data.
