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    How to replace a relay with triac?

    Hello, I want to use TRIAC instead of relay in my circuit "just in order to occupy less space". Relay is intended to switch 220VAC up to 8A. What is the minimum configuration for using TRIAC in my circuit?
    Last edited by electronicsIUST; 1st January 2019 at 07:32.

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    Re: How to replace a relay with triac?

    what type load you are planning to use with traic? if it inductive you need to consider surge current also. 12A min continuous current traic can use for 8A resistive load.



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    Re: How to replace a relay with triac?

    Also consider the other consequences:
    A relay provides isolation between the control (coil) and switch contacts, a triac does not,
    A relay gives an almost perfect on/off control with almost zero 'on' resistance, a triac will drop at least 0.6V across itself.
    That ~0.6V drop multiplied by the current will give you the power it dissipates which might be higher than a relay.

    For many applications, a triac and opto-coupler combination will work better than a triac alone, it buys back the isolation and with a suitable opto can also help to reduce inrush current and EMC problems.

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.



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    Re: How to replace a relay with triac?

    Quote Originally Posted by betwixt View Post
    Also consider the other consequences:
    A relay provides isolation between the control (coil) and switch contacts, a triac does not,
    A relay gives an almost perfect on/off control with almost zero 'on' resistance, a triac will drop at least 0.6V across itself.
    That ~0.6V drop multiplied by the current will give you the power it dissipates which might be higher than a relay.

    For many applications, a triac and opto-coupler combination will work better than a triac alone, it buys back the isolation and with a suitable opto can also help to reduce inrush current and EMC problems.

    Brian.
    Thanks for your reply.

    Actually isolation doesn't matter to me and also I think voltage drop is not so much that become a problem. But what about power dissipation? Should I consider using heatsink or other thermal dissipation considerations?



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    Re: How to replace a relay with triac?

    With the heatsink required for 8A, a triac will hardly require less space than a compact relays.



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    Re: How to replace a relay with triac?

    yes you have lots of current there.
    For you, i think its either relay, or back to back fets or back to back igbts.
    And of course you need some isolated way of driving the b2b fets/igbts...so that could mean a little murata isolated power module (eg NXJ series , say) and an opto coupler say....or a digital isolator and an isolated power module.
    IGBTS are better with surge currents

    - - - Updated - - -

    you say isolation doesnt matter but if you want to drive an AC switch from some DC level somewhere, you generally are going to need to consider the isolated drive that will be needed.



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    Re: How to replace a relay with triac?

    The actual voltage across the triac will be much higher than 0.6V. At this range of current (8A) the voltage will be about 1.5V or higher. The triac will need large heatsink to deal with it. You can easily calculate the power loss which will be approx. 8 * 1.5 = 12W. The heatsink is usually selected so that its temperature should not be higher than 60..70C. The ambient temperature should be taken into account as well. If the heatsink is placed inside the apparatus enclosure one usually assumes that the ambient temperature may achieve 40..50C. Taking all these factors into consideration you can calculate needed thermal resistance of the heatsink: Rth = (70-40)/12 = 2.5 C/Watt. It's a quite big piece of metal...



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