Difference between transistor and relay PLC?

I am trying to understand the difference between transistor and relay plcs. When do you use one over the other?. I have tried to research about this but have not found anything too specific that will give me a concrete answer. Is there any links you can refer me to?

I think the biggest difference is that transistor cards are solid-state where relay cards are not.

A couple of difference I have noticed are that transistor cards have a lower ampacity rating, they are almost always one single voltage per card, they cause less noise (AB relay cards are bad for this) and they don't burn out nearly as often as relay cards.


Relay cards do have advantages such as you can mix voltages on a card and they can switch higher levels of current.

THese are just my own observations working with AB SLC series plcs. Other manufacturers might be different.

Transistor switching offers greater cycling frequency than a relay, and a greater life as it is electrical rather than mechanical.

As Christoff mentioned they normally have a lower current switching capacity and can be damaged easier than a relay.

With transistors you also have to consider whether you need sink or source output as they are not reversible.

Jon.

XIO said:

I am trying to understand the difference between transistor and relay plcs. When do you use one over the other?. I have tried to research about this but have not found anything too specific that will give me a concrete answer. Is there any links you can refer me to?

Click to expand...

I am just going to give nuts n bolts answer.

The decision is made based on what you are switching and HOW OFTEN.

IF you are switching every second 24/7 then use transistor.
IF you are switching every couple hours or more then you can go with relay output.

The reason for this choice is that the relay switching is electromechanical and the life cycle of the switches is a million or so. The more you switch em the sooner they will wear out.

With transistor you must be careful of how the "signal operates"
ie NPN or PNP.
For reading look up NPN vs PNP on this site. There is a post within last week on this question. Ron Beaufort gave very good answer that goes into the theory a little deeper.

In either case you have to be careful of the collapsing fields of relays or solenoids. MOV across the ouptut will take care of this.

Dan Bentler

I use transistor outputs - end of line. Then place relays on outputs where I need them. Easier to replace a relay than a PLC relay card if one blows up.

The relays I use are either Omron G2R series or the new G2RV series with diodes to protect the output. I drive inidator lights direct from the transistor output. The ones I use are IDEC (IZUMI) and they come with bi-directional diodes fitted to protect the output. The bi-directional diode setup means that they are not polarity sensitive. Have never had a failure.