Monika Musielak Product Manager, Global Control 5: Triac vs. Relay “When to choose a triac over a relay?”

Triacs are semiconductors devices designed to control the distribution level of AC current, because it is a solid-state its work is silent. Whereas relays are electromechanical devices and can work with AC and DC current but when they work you can hear a “click”. Because of that features we can use triacs or relays for different purposes.

Relays work in a simple way – when you energize the coil, the contacts engage. The contacts will open when you deenergize the coil. You will need a simple transistor to drive it. One thing that can happen is inductive kickback, if the switch in the circuit is initially closed and then immediately opened there will be instantaneous drop in current. To prevent this situation you will need to add snubbering.

Another thing is that relay contacts are not invincible , if you open the contact under load you can break them and they will not open up again. On the other hand if a relay rated for power is used and you try to switch small signals, the contacts can eventually get dirty and won’t get a good connection between the contacts.

One of the biggest feature of triacs is being silent, thanks to their solid-state. Triacs can be used as PWM (Pulse Width Modulation) for regulation control valve actuators in radiator heating. Because there is no inductive coupling, triacks can be used in dangerous environment, particularly in explosive sensitive environments where sparking relay contacts are absolutely out.

Triacs outputs have much longer lifetime than relays, because  they’re build of semiconductor, it can last millions of cycles. To put things in perspective, lets imagine that you need to control PWM actuator to control heating, it will work with frequency 1Hz (once per second). Relay can usually last 100, 000 cycles, which will give around 1 day and 4 hours of work and constant clicking noise. Triac can last/work millions of cycles and it is silent.

So: when you need a silent and frequently working output you should choose triac. It is the best to use in hotel rooms or any place that where you need to silently control light or heating, etc. In other cases you need to calculate what will be best for your application.

Global Control 5 offers two iSMA I/O modules with Triac outputs: iSMA-B-4TO-H and iSMA-B-4TO-H-IP.