Electronics PLC

Relay VS Transistor (Output Voltage)


   If you are going to buy a PLC, types of output is one of the thing that you need to consider, I hope this article can help you to choose what kind of PLC output you need. There are two types of PLC output, the relay output and the transistor output. Watch: Types Of PLC Descrete Output (Relay and Transistor)

Relay VS Transistor (Polarity)

Relay output = Output Polarity can be Positive or negative depending on the wiring Configuration. Watch: PLC wiring configuration

Transistor output = Output polarity is fixed (sinking for negative output, sourcing for positive output).

Relay VS Transistor (Response)

Relay output = Relay output performs mechanical switching which response is the same as the response of traditional relay.

Transistor output = Transistor output performs electronic switching which is a lot faster than mechanical switching.

Note: For some applications like servo motor pulse control, transistor output is much better choice that relay output, because sometimes servo motor controller requires 100,000 pulse per seconds to control the servo motor and relay output is not capable of performing this requirement.

Relay VS Transistor (Output Voltage)

Relay output = Some of the relay output can be use to produce AC voltage (Check the PLC specifications).

Transistor output = Transistor output can only used to produce 24 Volts DC.

Relay VS Transistor (Stored Energy)

Relay output = Relay output is not affected by stored energy from inductive load.

Transistor output = Transistor output can be damage by stored energy from inductive load.

Note: In case that you need to use PLC transistor Output together with inductive load you can connect diode in parallel with the inductive load in reverse bias to protect the PLC transistor output from stored energy.

watch: Use Diode for Protection Reverse polarity protection and Stored Energy Protection






Arduino Problem (Response Delay)


Hi guys I don’t know if you encounter this problem with arduino, but I want to share this problem and the things I’ve done to solve this it.


I upload one of the example program(02.Digital / Button) to “arduino UNO”. Base on the program when  port 2 receives 5v port 13 must release 5v and when port 2 receives 0v, port 13 must release 0v. When I try to put 5v to port 2, port 13 releases 5v but when I remove the 5v supply to port 2, port 13 is still “ON” and it turns “OFF” after a few seconds. The question here is “ the program doesn’t contain any timer to delay port 13 to turn “OFF”, then  why port 13 is delayed?”, I also try to change the ports on the program, but the response of the program is still the same.

The program below is one of the example program of arduino, and base on this program if port 2 is “ON” port 13 must turn “ON”, and if Port 2 is “OFF” port 13 must turn “OFF”.

The Picture below shows the circuit diagram that I created for the program above. When I press the button at port 2, LED will turn “ON” and when I release the button at Port 2 LED must turn “OFF” but instead of turning “OFF” immediately it will turn “OFF” after a few seconds.


I try to research on the forums but I didn’t find any answer, my theory about this circuit is “there might be some voltage on port 2 that remains after releasing the button”, it sounds crazy because there is no component connected in port 2 that can store voltage like capacitor, but I still try my theory, I put a resistor and LED in parallel with port 2 (see the picture bellow), the purpose of these resistor and LED is to discharge the excess voltage on port 2 after releasing the button. I never expect it but I works, there is no delay on response anymore.

I hope my article is helpful thank you.

Note: the picture of arduino program above is not mine, it is one of the example program of arduino software.





Electronics PLC

PLC output Protection


“Prevention is Better than cure”
In automation industry there are some reasons why our equipment get damage or malfunction, but if we know these reasons we can come up with the solution to prevent the damage that may occur, engineers are one of the intelligent people here in the world, they can fix the damage but if we can put some protective equipment and preventive method, We can reduce the production downtime.

Causes of PLC Output Damage

  • Wrong wire termination
  • Over Current
  • Stored Energy

Wrong Termination


  • Human Error
  • Wrong Circuit Diagram


  • Supervisor / Engineer must check and approved the diagram before project execution
  • Loop checking
  • Yellow Tagging

Over Current


  • Defective Equipment / Field devices
  • Short Circuit


  • Preventive Maintenance
  • Install fuse for short circuit protection

Stored Energy


  • Inductive Loads


  • Install Diode (reverse bias) in parallel with the load

Related Article

Reference Video





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: