January 2020

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Triac vs. Relay

 When to choose a triac over a relay?

Monika Musielak

Monika Musielak
Product Manager,
Global Control 5

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Triacs are semiconductor devices designed to control the distribution level of AC current because, in 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 those features, we 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 de-energize 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 an instantaneous drop in current. To prevent this situation, you will need to add snubbing.

Another thing is that relay contacts are not invincible; if you open a contact under load, you can break it and it 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 features 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, triacs can be used in dangerous environments, particularly in explosive sensitive environments where sparking relay contacts are absolutely out.

Triacs outputs have a much longer lifetime than relays. Because they’re built of semiconductors, they can last millions of cycles. To put this in perspective, let's imagine that you need to control a PWM actuator to control heating; it will work with a frequency of 1Hz (once per second). The 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 best to use in hotel rooms or any place where you need to control light or heating, etc. silently. 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.

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If you need more information about iSMA I/O modules please contact:
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About the Author:

Monika Musielak is a Product Manager responsible for iSMA I/O modules and HMI panels in Global Control 5. She made her first steps in electronics world by working as Product Marketing Manager with displays. Now, she is expanding her knowledge in BMS world.


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