Applicable Systems
Unidrive SP
Commander SK
EV2000
Mentor MP
Symptoms
The controller power supply has failed. The OK LED is off and the controller does not respond to communication attempts. All outputs are de-energized.
Possible Causes
- Power supply input voltage outside the rated range (20.4-28.8V DC)
- Power supply output overloaded by too many I/O modules
- Power supply internal fuse blown after a short circuit on the output bus
- Power supply component failure from age or thermal stress
Troubleshooting Steps
- Step 1: Measure the input voltage at the power supply terminals
Voltage should be 24V DC +/-10%; if outside this range, investigate the source - Step 2: Disconnect all I/O modules and measure the power supply output voltage
If voltage returns to normal, an I/O module short is overloading the supply - Step 3: Reconnect modules one at a time, monitoring the output voltage
The module that causes the voltage to drop is the faulty one - Step 4: If the output remains low with no load, the power supply is defective
Replace the power supply with an OEM-equivalent unit
How to Reset
Reset: After the power supply is replaced or the overload is removed, the controller restarts automatically when input power is restored.
Common Replacement Parts
- Power supply: 24V DC, 5-10A output (Match to controller power requirement)
- Power supply fuse: Fast-acting, 5A, 5x20mm (Match to existing fuse rating)
- I/O module: Digital or analog (Test each module with a bench power supply)
Frequently Asked Questions
- Q: How do I size a controller power supply?
Sum the current draw of all modules and the controller base. Add 25% margin. For a typical system: controller 0.5A + 8 modules x 0.15A = 1.7A; use a 2.5A supply.
- Q: What causes power supply failure?
Overload, input voltage spikes, thermal stress from poor ventilation, and aging of electrolytic capacitors are the most common causes.
- Q: Can I use a switching power supply?
Yes, most industrial controllers accept switching supplies. Ensure the supply has low ripple (<50mV) and can handle inrush current from relay outputs.
Source: OEM Technical Manual
Category: PLC & VFD