In this basic relay circuit, the output socket is not connected to any load, and there is no contact sparking or interference observed. To begin with, the system uses a 5V total power supply, powered by an AC-DC power module, while the microcontroller runs on 3.3V, supplied by an LDO. The microcontroller’s IO pins directly drive a transistor, set in push-pull mode. There are two boards: one is the main control board, which includes the microcontroller and an LCD screen, and the other is the power board, which contains the AC-DC module, relays, and output sockets. These two boards are connected via an FPC cable.
The issue arises when the 220V AC power is turned on. The relay toggles instantly, and there is a 50% chance that the system crashes. During these crashes, an oscilloscope detects pulse interference on the 5V power line. Additionally, the power indicator on the power module flickers, indicating a significant voltage drop. Various tests have been conducted: adding a 22uH inductor in series with the relay coil still resulted in crashes. Adding a Schottky diode in series with the relay coil and a 1N4007 in parallel, then connecting it to the 5V supply also failed to prevent the crash. Even after multiple attempts, the problem persisted.
However, when the relay contacts were disconnected from the 220V AC (specifically, the terminal labeled as "L"), the pulse interference disappeared, and the system no longer crashed. Replacing the AC-DC power supply module reduced the pulse interference but did not eliminate the crashes entirely. This led to suspicion that the relay coil or the board's EMC design was at fault. The question remains: why does replacing the power supply prevent the crash?
Master Ant suggested that instead of just eliminating the interference, the root cause should be identified to ensure the system can withstand such disturbances—like passing EFT tests. Daily electrical devices experience similar pulses, such as when turning on a light or using a hairdryer. Master Uncle pointed out that the interference could originate from either the back EMF of the relay coil or inductive coupling from the contacts.
He recommended adding a small resistor in series with the 1N4007 to help dissipate the back EMF energy, as simply using the diode may lead to oscillation. He also advised checking if the relay is properly insulated, suggesting replacement or cleaning. If possible, increasing the relay's capacity could also help.
Master Chunyang believed that poor insulation between high-voltage and low-voltage circuits was the most likely cause. A less probable explanation was that the system’s EMC characteristics were too weak, making it vulnerable to electromagnetic interference. To test this, he suggested using another relay and connecting its coil to the drive circuit with short wires, ensuring the contacts were not connected to the PCB. After powering up, if the system still crashed, it might indicate an issue with the PCB itself, such as leakage.
If the power supply had a Y capacitor, proper grounding was essential to avoid floating voltages. Otherwise, the grid could introduce interference through the Y capacitor, causing instability. In such cases, grounding might temporarily fix the issue, but it would still point to a flaw in the EMC design.
User Yanruiqi raised an important question: Why did removing the L terminal stop the interference? It was speculated that even without a load, the relay’s contact movement might generate a magnetic field or capacitance, leading to interference. It was recommended to replace the relay or isolate the L terminal from the other contact to prevent any potential leakage.
The power supply issue was considered more likely. Could the transient response of the power supply be insufficient? When the relay activates, it requires a certain amount of current, and if the power supply cannot handle it, it could lead to instability. For example, a 5V relay typically draws about 89.3mA, while a 24V relay draws around 18.7mA. It was suggested that testing with a DC24V relay might help identify if the power supply was the root cause.
In general, using a single-chip to directly drive a relay is common but not always reliable in industrial settings. Many prefer optocouplers like the TLP127, which include internal flyback diodes and provide better isolation. This helps prevent interference from reaching the microcontroller. Without isolation, the relay’s switching could directly affect the power supply and microcontroller, potentially causing a crash.
Another possibility is that the sudden change in current during relay activation causes a transient voltage spike, which the power supply struggles to manage. Testing by manually controlling the relay contacts or isolating them from the PCB could help confirm the source of the issue.
Overall, the key takeaway is that while replacing the power supply might reduce the symptoms, it doesn’t address the underlying design flaws. Improving the circuit’s EMC performance, adding isolation, and ensuring proper power handling are essential for long-term reliability.
Solar Panel,,High Efficiency Mono Solar Panels,Bifacial Jinko Solar Panels
PLIER(Suzhou) Photovoltaic Technology Co., Ltd. , https://www.pliersolar.com