Momentary Load ON at Power-Off (Transistor Output)
Mitsubishi Electric · MELSEC iQ-R I/O Module
What does MOMENTARY-ON-POWEROFF mean?
When an inductive load is connected to a transistor output module, the load momentarily turns on while in the OFF state during system power-off. This is caused by a sneak current generated from the back EMF of the inductive load at the moment of shutoff, which can briefly re-energize the load.
Common Causes
- Inductive load connected to the transistor output module generating back EMF during power-off.
- Sneak current path created by the back EMF at shutoff (1), causing current to flow to the load (2) which is in the OFF state.
Repair Steps & Checklist
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- 1
To suppress the back EMF, connect a diode in parallel with the inductive load (3).
- 2
If applicable, connect a diode (D1) into the path between the positive and negative terminals of the external power supply to provide a circulation path. (Note: For source output modules like RY40PT5P, RY41PT1P, RY42PT1P, only the diode in parallel with the load is effective.)
- 3
Ensure the diode's reverse voltage VR(VRM) is approximately 10 times the rated voltage (e.g., 200V for 24VDC) and its forward current IF(IFM) is more than twice the maximum load current (e.g., 4A or more for 2A/common).