Troubleshooting: DC Power Supply Triggers Overload Protection Before Reaching Rated Current

1.  Inrush Current / Excessive Capacitive Load

When the load generates inrush current during startup or presents a large capacitive load, the power supply can easily trigger overload protection before reaching its rated current. If the output is connected to devices with large input capacitors—such as motors, drives, servos, VFDs, or switching power supplies—the capacitors will draw a surge current several times higher than the normal operating current at power-on. Even if the steady-state current is small, the power supply's sensitive current limiting or overload protection circuit may act immediately, interpreting the surge as an overload and preventing startup.

2.  Short Circuit or Insulation Degradation on the Load Side

A short circuit or localized insulation failure on the load side can also cause the power supply to trip immediately upon connection. For example, exposed wiring, poor contact causing a short, or internal issues within the load—such as a partially shorted circuit, MOSFET failure, or a semi-short in the drive bridge—may not blow a fuse outright, but can generate enough instantaneous current to exceed the protection threshold and trigger the power supply into protection mode.

3.  Excessive Voltage Drop Due to Long or Undersized Output Cables

If the output cables are too long or undersized, a significant voltage drop can occur during startup. The power supply's internal sensing circuit may interpret this voltage anomaly as an output overload, triggering protection and preventing normal load startup.

4.  Internal Power Supply Malfunction

A fault within the power supply itself can also cause false overcurrent protection. Components such as current sensing resistors, op-amps, or comparators may drift in value or become damaged, leading the control board to detect an overcurrent condition even when the supply is unloaded. This will prevent the power supply from operating correctly.

5.  Input Side (Mains Power) Issues

Problems on the AC input side can also lead to abnormal protection events. If the input voltage is too low, the power supply will draw higher current in an attempt to maintain a stable output, potentially hitting the overload threshold. Additionally, grid fluctuations or poor contact in the input wiring can cause unstable input power, ultimately triggering overload protection.

Simplified Quick Diagnostic Steps:

• Test with no load: Disconnect all loads and power on the supply with no load. If it starts up normally, the issue lies with the load or wiring. If it still trips, the power supply itself is likely faulty.
• Check output cables: Replace long or thin output cables with shorter, thicker gauge wires to eliminate voltage drop as a contributing factor.
• Test with a small load: Connect a single, small load to see if the power supply starts up. This can help determine if the issue is related to inrush current from a larger or more complex load.

Updated on: 28/02/2026