1. Problem Background: Anomalies Caused by Universal Power Supplies
During the use of KUS series ultrasonic level meters with 220V to 24V/12V universal power supplies, customers encountered two key issues:
- Serial communication failure (RS485 communication interruption);
- Measurement deviation (3m actual distance showed fluctuations around 1m).
Tested power modules:
- 220V AC to 24V DC module
- 220V AC to 12V DC module
2. 12V DC Power Module Test Record
Table 1: 12V Power Module Test Steps and Results
| Test Step | Operation Details | Test Results |
|---|---|---|
| 1. Ripple Measurement | Oscilloscope probes connected to V+ and V-, AC coupling, 20MHz bandwidth limit | Ripple: 20~30mV triangular wave, ruling out ripple as the cause of measurement errors |
| 2. Non-grounded Test | Power module ungrounded, sensor measures 3m actual distance | Measurement fluctuated around 1m, threshold adjustment invalid; normal after switching to bench power supply |
| 3. Grounded Test | Power module grounded, sensor cables kept away from 220V wires | Measurement accurate (3m); errors reappeared when cables near 220V wires |
3. 24V DC Power Module Test and Communication Failure Analysis
Table 2: 24V Power Module Test vs. Communication Issues
| Test Scenario | Grounding Status | RS485 Communication Performance | Measurement Results | Interference Wave Characteristics |
|---|---|---|---|---|
| Standard Test | Ungrounded | Frequent communication failure, occasional connection | Abnormal measurements, threshold adjustment invalid | Peak-peak interference 13V, 50Hz (mains coupling) |
| Optimized Test | Grounded + GND connected to V- | Stable communication, no interruptions | Accurate measurements matching actual distance | No obvious interference (Figure 3) |
Root cause of communication failure:
- Floating V+ and V- create potential difference between RS485 signals and ground, causing interference.
- Solution: Short GND to V- to eliminate potential difference (Figure 4).
4. Four Key Points for Universal Power Adaptation (Summary)
- Grounding Priority: Power module must be well grounded to align V- with earth potential;
- Signal Common Ground: Connect GND to V- to resolve RS485 potential difference;
- Cable Isolation: Maintain ≥30cm distance between sensor cables and 220V wires;
- Safety Warning: Prohibit sensor cables from contacting 220V high voltage to avoid electric shock.
5. Waveform Comparison and Interference Suppression
(1)V+ to ground interference (ungrounded): 42.8V peak-peak, 50Hz mains interference ;
(2)RS485 signal (grounded): Interference eliminated, communication stability 100% .
