Triac or relay cards as output


Thread Starter

Jeffereson Shield

I want to know about the difference in the Triac and relay-based cards used in PLCs. Also, a brief about their application and effectiveness.


Relay Output: Relay output is a potential free contact. That means you can connect any voltage such as AC or DC.

Triac Output: Triac output is a constant output voltage. It's not a potential free contact. If potential free contact is required you must use the Relay Board.
To add to previous poster, the Triac is only suitable for AC, it would not switch Off on DC.

Dear Mr. Roy,

Thanks for your valuable information. We can say Transistor Output module as a DC channel output module.

Gerald Beaudoin

They also only work properly with loads greater than a minimum value. If the current draw is too small, you can run into problems where they won't want to turn on.
I have only found two advantages to triac outputs. On the design and construction side, they are easier to wire in that they only require one power source which usually supplies all outputs as opposed to two power sources or a jumper wire in the wiring arm. This does not reduce the number of wiring points because they require a neutral as well, but it does put a neutral for voltage checks right there for you. The main advantage is reaction time that can be critical in high speed applications. On the down side, triacs don't handle spikes or hard grounds very well and have a tendency to take out the entire module rather than one set of dry contacts. For higher current draw devices, an isolation relay has to be used anyway. Most important in my experiences is dealing with a lost or lifted neutral in the triac circuit as it can cause the output indicator on the card to turn on even though it is not on in your ladder logic as well as cause false voltage readings while trouble shooting. If at all possible, avoid using them in non grounded systems that are common in the automotive and foundry industries.