Published on: | Author: Galoce Technical Applications Team
Excitation voltage is the electrical lifeblood of a strain gauge load cell—without it, there is no signal. But how does excitation voltage affect your measurements? What happens if you use the wrong voltage? Why do some load cells specify 5V while others require 10V or 20V? This guide answers these questions and more, covering the fundamentals of bridge excitation, ratiometric measurement, cable effects, and practical considerations for selecting and applying excitation voltage in real-world weighing systems.
1. What Is Excitation Voltage?
Excitation voltage (often denoted VEX or VIN) is the DC electrical power supplied to a load cell to energize its internal Wheatstone bridge circuit. The strain gauges within the bridge require this voltage to produce a measurable output signal when deformed by applied force .
Output Signal (mV) = Excitation Voltage (V) × Sensitivity (mV/V) × (Applied Load ÷ Rated Capacity)
Think of excitation voltage as the "carrier" that enables the strain gauges to translate mechanical strain into an electrical signal. Without excitation, the load cell is inert—it produces no output regardless of how much force is applied .
2. How Excitation Voltage Affects Output Signal
The relationship between excitation voltage and output signal is linear and proportional—within the load cell's specified range, doubling the excitation voltage doubles the output signal for the same applied load .
Example Calculation:
Load cell specs: 2.0 mV/V sensitivity, 1000 kg capacity
Applied load: 500 kg (50% of full scale)
| Excitation | Output Signal (mV) |
|---|---|
| 5 V | 2.0 × 5 × 0.5 = 5.0 mV |
| 10 V | 2.0 × 10 × 0.5 = 10.0 mV |
| 15 V | 2.0 × 15 × 0.5 = 15.0 mV |
Result: Higher excitation yields a larger signal, which can improve signal-to-noise ratio and allow finer resolution—but only within the load cell's design limits .
This proportionality is the foundation of ratiometric measurement (covered in Section 4).
3. Standard Excitation Voltages: 5V, 10V, and Beyond
Load cells are designed for specific excitation voltage ranges. The most common standards are :
| Excitation Voltage | Typical Applications | Characteristics |
|---|---|---|
| 5 VDC | Battery-powered devices, portable scales, low-power systems | Lower power consumption, reduced self-heating, compatible with many ADCs |
| 10 VDC | Industry standard for most industrial load cells | Balance between signal strength and power dissipation; widely supported by indicators |
| 12–15 VDC | Automotive systems, some heavy-capacity cells | Higher output; may require checking max voltage rating |
| 20–24 VDC | Specialized high-output sensors, long cable runs | Higher signal, but risk of overheating if not designed for it |
Most industrial load cells are rated for 10–15 VDC maximum, with 10 V being the most common recommended value .
4. Ratiometric Measurement: Why It Matters
Ratiometric measurement is a key concept in load cell systems. Because the output signal is proportional to excitation voltage, any change in excitation voltage causes a proportional change in output—if the measuring instrument also references the same excitation voltage .
In a ratiometric system, the analog-to-digital converter (ADC) uses the excitation voltage as its reference. If excitation drops by 1%, the signal drops by 1%, but the ADC's reference also drops by 1%—so the digital reading remains unchanged. This cancels out slow variations in excitation voltage .
Ratiometric vs. Absolute Measurement:
- Ratiometric: ADC reference = excitation voltage. Immune to excitation drift. Preferred for most industrial systems .
- Absolute: ADC uses fixed internal reference. Excitation variations directly affect readings. Requires highly stable excitation .
5. Regulated vs. Unregulated Power Supplies
The quality of the excitation voltage source directly impacts measurement stability .
| Supply Type | Characteristics | When to Use |
|---|---|---|
| Regulated Linear Supply | Very low noise, excellent stability, minimal ripple. Heavier and less efficient. | Precision laboratory systems, high-accuracy applications, sensitive measurements . |
| Regulated Switching Supply | Efficient, compact, but may have high-frequency ripple. Look for "low-noise" versions. | General industrial use—acceptable with proper filtering and ratiometric measurement . |
| Unregulated Supply | Output varies with load and line voltage. Not recommended for precision measurements. | Only acceptable in ratiometric systems with very stable loads and wide ADC reference range . |
6. Cable Length and Voltage Drop: 4-Wire vs. 6-Wire
Long cables introduce resistance, which causes a voltage drop between the power supply and the load cell. This reduces the actual excitation voltage at the load cell, affecting output .
4-Wire Configuration (Standard)
- Uses two wires for excitation (+EXC, –EXC) and two wires for signal (+SIG, –SIG).
- Voltage drop in the excitation wires is not compensated—the load cell receives less than the supply voltage, reducing output .
- Suitable for short cable runs (typically <10–15 meters) .
6-Wire Configuration (Remote Sensing)
- Adds two additional "sense" wires that measure the voltage at the load cell and feed it back to the power supply for regulation .
- The supply automatically adjusts to maintain the correct excitation voltage at the load cell, compensating for cable resistance .
- Essential for long cable runs (>15 meters) or when highest accuracy is required .
Vdrop = I × Rcable
Where I is excitation current (load cell input resistance determines current). Example: 350 Ω load cell at 10V draws ≈28.6 mA. With 10 Ω cable resistance (round trip), drop = 0.286 V—a 2.86% error uncompensated .
7. Excitation Voltage and Power Consumption
While load cells consume very little power, understanding the relationship helps in battery-powered or multi-cell applications .
- Bridge resistance: Typical values are 350 Ω or 700 Ω (some are 1000 Ω or higher).
- Current draw: I = VEX / Rbridge. For a 350 Ω cell at 10V: 10 / 350 = 28.6 mA.
- Power dissipation: P = VEX² / R. At 10V: 100 / 350 ≈ 0.286 W (286 mW).
For multi-cell systems (e.g., 4 cells in a scale), total current adds up—about 114 mA for four 350 Ω cells at 10V. This must be within the power supply's capacity .
8. Effects of Over-Voltage and Under-Voltage
Operating outside the specified excitation range has consequences .
Over-Voltage:
- Self-heating: Higher current causes the strain gauges to warm up. This can create thermal expansion mismatches, zero drift, and in extreme cases, damage the gauge bonding .
- Permanent damage: Exceeding the maximum rating (e.g., 20V on a 15V max cell) can burn out the strain gauges or damage compensation resistors .
- Non-linearity: Heating effects can cause the output to become non-linear .
Under-Voltage:
- Reduced signal: Lower output voltage means lower signal-to-noise ratio; the measurement becomes more susceptible to electrical noise .
- Potential under-performance: If excitation is too low, the load cell may not reach its specified accuracy .
- Some indicators have minimum excitation requirements—if too low, they may not function correctly .
9. Noise and Stability Considerations
Excitation voltage quality directly affects noise in the output signal .
Sources of Excitation-Related Noise:
- Ripple on DC supply: AC components on the excitation voltage appear in the output (modulated by the bridge).
- Power supply fluctuations: Even with ratiometric measurement, very fast transients may not be fully rejected .
- Ground loops: As covered in the previous guide, improper grounding introduces noise that affects both excitation and signal .
Mitigation Strategies:
- Use low-noise power supplies (linear supplies are best; if using switching supplies, add additional LC filtering) .
- Keep excitation and signal wires separate from power cables .
- Consider using a dedicated excitation supply just for load cells, separate from digital or relay power .
- In noisy environments, add ferrite beads or common-mode chokes on excitation lines .
10. Excitation for Digital vs. Analog Load Cells
The role of excitation differs between traditional analog load cells and modern digital types .
Analog Load Cells:
- Require continuous, stable excitation voltage to produce the mV signal .
- Signal is directly proportional to excitation .
- Vulnerable to excitation noise and voltage drop .
Digital Load Cells (with built-in electronics):
- Still require power—typically 5–24 VDC—but this powers the internal ADC, microprocessor, and communication circuits .
- The internal electronics regulate excitation for the strain gauge bridge, so external excitation quality is less critical .
- Digital output (RS-485, CAN, etc.) is immune to excitation variations after the ADC .
- Power consumption is higher due to electronics (often 0.5–2 W per cell) .
11. Practical Selection Guide
Choosing the right excitation voltage and power supply for your application involves several considerations .
| Application | Recommended Excitation | Power Supply Type | Special Considerations |
|---|---|---|---|
| General industrial weighing | 10 VDC | Regulated switching (low-noise) or linear | Standard for most platform scales, tanks, hoppers |
| Laboratory / high precision | 5–10 VDC | Ultra-low-noise linear supply | May use higher excitation if cell allows, for better SNR |
| Portable / battery-powered | 5 VDC | Battery with LDO regulator | Use high-resistance cells (700–1000 Ω) to minimize current |
| Long cable runs (>15 m) | 10–15 VDC | Regulated, with 6-wire sensing | Compensate for voltage drop; increase voltage if needed to maintain signal level |
| Hazardous areas (intrinsic safety) | As per barrier specs | Via Zener or galvanic isolator barriers | Must comply with entity parameters; excitation limited by barrier |
- ☐ Confirm load cell's specified excitation range (datasheet).
- ☐ Choose 10V for most industrial applications.
- ☐ Ensure power supply can deliver sufficient current (I = V/R × number of cells).
- ☐ Use 6-wire sensing for cables >15 m.
- ☐ Verify indicator uses ratiometric measurement.
- ☐ Add filtering if supply noise is suspected.
Conclusion: Getting Excitation Right
Excitation voltage is a fundamental but often overlooked aspect of load cell system design. Proper selection and application ensure that your load cell operates within its designed parameters, delivers the expected accuracy, and maintains long-term stability.
Key takeaways:
- Excitation voltage powers the Wheatstone bridge and determines output signal amplitude.
- Higher excitation improves signal-to-noise ratio but must stay within manufacturer limits.
- Ratiometric measurement cancels slow excitation variations—ensure your indicator supports it.
- Cable length affects actual excitation at the load cell; use 6-wire sensing for long runs.
- Power supply quality matters—clean, stable excitation reduces noise and drift.
- Digital load cells handle excitation internally, simplifying external requirements.
Whether you're designing a new weighing system or troubleshooting an existing one, paying attention to excitation voltage will pay dividends in measurement reliability and accuracy.
At Galoce, our load cells are engineered for robust performance across a range of excitation voltages, and our application engineers can help you select the optimal power configuration for your specific needs. Contact us for expert guidance on load cell excitation and complete system design.
