Remote Sensing is a feature designed to automatically compensate for the "voltage drop" that occurs in the wiring between a DC power supply and the load (device). It ensures that the exact set voltage is applied directly to the input terminals of the load.
Normally, a power supply operates to keep the voltage at its own output terminals constant; this is known as "Local Sensing." However, in actual operation, the power supply and the load are often separated by a distance. The wiring, connectors, and terminal blocks between them inherently possess resistance. When current flows through this resistance, a portion of the voltage is lost a phenomenon known as "Voltage Drop" resulting in a lower voltage reaching the load than intended.
The Remote Sensing feature uses dedicated wires (Sensing Lines) connected directly to the load's input terminals. This allows the power supply to detect the actual voltage at the load in real-time, compensate for the loss by increasing its output accordingly, and maintain the correct voltage at the load at all times.
Calculating Voltage Drop
When designing or building a power system, the first step is to calculate how much voltage will be lost across the wiring. Ignoring this can lead to unstable operation of precision equipment, failure to meet device specifications, or performance degradation.
Basic Calculation Formula
In a DC circuit, the voltage drop E considering the "round-trip" (both positive and negative wires) is calculated based on Ohm's Law:
E = 2 x I x R x L
- E: Voltage Drop (V)
- I: Maximum Load Current (A)
- R: Resistance per unit length of the wire (Ω/m)
- L: One-way wiring distance (m)
Calculation Example for 5m Wiring
For example, let's calculate the voltage drop when using AWG12 wire (approx. 3.3 mm2) over a one-way distance of 5m (10m round-trip) with a current of 8A.
- Wire Resistance: The standard resistance for AWG12 wire is approximately 5.2Ω/km (0.0052 Ω/m).
- One-way Resistance: 0.0052 Ω x 5m = 0.026 Ω
- Round-trip Voltage Drop: E = 2 x 8A x 0.026 Ω = 0.416V
This result shows that even over a relatively short distance of 5m, a current of 8A causes a voltage drop of approx. 0.42V. If you are supplying a 5V digital circuit, this 0.4V loss (8% deficiency) likely exceeds allowable tolerances and could be a direct cause of system resets or malfunctions.
Ensuring Ampacity and Safety
When selecting wires, adhering to the "Ampacity" (current-carrying capacity) is a higher priority than even suppressing voltage drop.
Every wire size has a regulated maximum current for safe use. For AWG12, the rated ampacity is 9.3A (based on our corporate standards).
While the 8A in our example is within this safe range, you must never exceed the 9.3A limit just because the Remote Sensing feature can compensate for the resulting voltage drop. Exceeding the ampacity causes the wire to overheat abnormally, which can lead to melting insulation or fire. Always verify the rated ampacity for your specific wire gauge and ensure operation remains strictly within safety limits.
Output Voltage Compensation via Remote Sensing
If the calculated voltage drop is unacceptable for your application, use the Remote Sensing feature to compensate.
How Compensation Works
In a standard connection (Local Sensing), the power supply's control circuit monitors the voltage at the "Output Terminals." With Remote Sensing, the monitoring point shifts to the "Sensing Terminals" at the load.
- Direct Connection to Load: The sensing terminals (+S, -S) of the power supply are connected directly to the input terminals of the load.
- Accurate Detection: Since almost no current flows through the sensing lines (due to high impedance), they are unaffected by wire resistance and provide a precise feedback of the "raw voltage" the load is receiving.
- Automatic Adjustment: The power supply automatically increases its output voltage so that the voltage detected at the sensing terminals matches the set value.
Example: If the setting is 5.00V and the wiring loss is 0.42V, the power supply outputs 5.42V at its terminals to maintain exactly 5.00V at the load.
This ensures that any fluctuations caused by connector contact resistance or temperature-related changes in wire resistance are automatically cancelled out.
How to Use Remote Sensing
Remote Sensing is a highly precise feedback control. If not wired correctly, it can pick up noise or cause the output to become unstable ("oscillation"). Follow these guidelines for proper operation:
1. Selecting Appropriate Wiring
- Sensing Lines: Since very little current flows, thin wires (24AWG to 18AWG) are sufficient.
- Noise Countermeasures: Sensing lines are high-impedance and susceptible to external magnetic and electric fields. Keep them away from high-current cables, motors, or switching power supplies. Always use twisted-pair cables (or shielded twisted-pair).
- Power Leads: Keep the main power leads as short as possible and twist them together to reduce parasitic inductance. This improves the power supply's response to sudden load changes (dynamic loads) and prevents voltage spikes.
2. Stability and Oscillation Prevention
- Adding Capacitors: Connecting an electrolytic capacitor at the load end (sensing point) can stabilize the operation. A capacitance of 0.1μF to several hundred μF is recommended. Ensure the voltage rating is at least 1.5 to 2 times the rated output voltage.
3. Setup and Verification
- Remove Jumpers: Most power supplies are shipped with the output and sensing terminals shorted (jumpered). You must remove these jumpers or short bars before connecting the external sensing lines.
- Understand Compensation Limits: Power supplies have a "Maximum Compensation Voltage" (e.g., up to 0.5V). If the voltage drop exceeds this limit, the supply cannot maintain the target voltage at the load. In such cases, use thicker wires to reduce the initial voltage drop.
- Use High-Precision Measurement: For critical testing, connect a separate digital multimeter to the load end. If there is a slight discrepancy, fine-tune the power supply's set voltage to match the multimeter reading.
Remote Sensing is an essential technology for extracting "design-level performance" in environments with long wiring or high current requirements. By combining accurate calculations, proper noise measures, and safety-first wiring, you can significantly enhance the reliability of your power system.