Electronics PLC Protection

Protect your Instrument from stored energy cause by inductive load


In some cases in automation industry sometimes the instruments get damage because of direct mechanical contact like limit switch, but we know the reason behind this,  limits switches and other instrument that makes mechanical contact with actuators like pneumatic cylinders and it get damage because it receives force repeatedly that might  cause broken sensor mounting and broken spring, but some instruments might also receive damage from stored energy cause by inductive load.

What is inductive load?

Inductive loads are electrical instruments that compose of coils that generates magnetic field such as:

  • Relay
  • Magnetic Contactor
  • Motors
  • Incandescent lamp 

How does inductive load causes damage to instruments like PLC?

 The stored energy cause by inductive load will try to pull current at port Y0 even if Y0 is not energize anymore (refer to the picture below). let’s say that Y0 will turn “ON” for 1 second then it will turn “OFF” for 1 second, the stored energy at the relay will try to pull the current at Y0 every time that the relay turns “OFF”.

How to Protect the instrument like PLC from Stored energy cause by inductive load?

The only way to protect the PLC from stored energy is by consuming it, you must have a component that will absorb the energy so that it will not cause damage to the other instruments. You can terminate a diode in parallel with the inductive load to absorb the stored energy.(refer to the picture below)


  • Diode must be connected in reverse bias, if diode is connected in forward bias the load will be shorted and that will cause damage to your instrument as well.
  • To get the proper diode, the maximum current and voltage must be consider.


  • You can replace the Incandescent lamp to LED lamp if you don’t like to terminate a diode. (LED lamp is not an inductive load, but it can be use as an indicator like incandescent lamp)

Reference Video: