opto - isolation

[Neil Cowell]

I am currently doing an electronics assignment on PLCs and I have a question to complete as follows:

With regards to different types of INPUT and OUTPUT units specified by manufacturers, what is the purpose of providing OPTO -ISOLATION?

Can anybody assist me out there with a great answer?

 

[rob allan]

First off do you know what an opto isolator is and what it does?
If not go to the library read up and learn .that is the purpose of questions like this.
Secondly with regard to Plcs visit some of the manufacturs web sites find out
what type of input /outputs are available
look at the specification of them voltage/current rating in what applications they would be used and then try and think where an opto isolator would fit in.
Do some research and you should be able to answer your question and learn in the process.post again if you get stuck
Dont just expect someone to answer your assignment for you!

 

[Ken Roach]

I think this is a reasonable question; he's not asking "what is opto-isolation", but rather "why do PLC I/O modules use opto-isolation".

Consider, Neil, that the field wiring for a typical industrial logic controller is, electrically speaking, a NASTY environment. There are often mixed signals (0-10V analog, 24 VDC discrete, and 120 VAC discrete often share the same cabinet), induced voltages, spike voltages, and the occasional short circuit that can occur.

If the Input and Output circuits on a controller shared the same common point as the controller's internal power supplies, it would be easy for these disruptive signals to carry over into the delicate 5VDC digital circuits of the logic controller's CPU, memory, and bus systems.

Instead, opto-isolators are typically used to separate the field circuitry in an I/O module from the internal logic circuitry. Typically the withstand level of this isolation is 1700 to 2500 VDC; you could assert that level of voltage on an Input module and you'd fry the module itself, but you wouldn't affect the CPU. This doesn't make the I/O systems bulletproof, but it helps a lot !

Isolation between the individual channels of an I/O module is a different topic, so I'm not going to address it here.

Good luck with your class !

 

[rogerhollingsworth]

Optical Isolation

Just for comparison, prior to optics being very popular, we used to use relay isolation to achieve isolation between process and field.
As an example; Sometimes we would have a "Hatchway" limit switch with two sets of dry contacts, one contact would serve as a limit for a 250vdc device and the second contact would serve as a logic signal.
To prevent moisture or other environmental conditions form causing the 250vdc to bleed over to the logic signal wires, due to their close proximity in the "Hatchway" limit switch, we would have the second contacts in the "Hatchway" control a relay then the relay contacts would in turn switch the logic signal.
In this way the stray 250vdc would not be introduced to the logic portion of our system.
Optical coupling serves the same purpose. (ISOLATION)

Roger