Using a TRIAC in a simple AC timed control circuit. Is it suitable?

Discussion in 'The Projects Forum' started by Marcus2012, Feb 23, 2015.

  1. Marcus2012

    Thread Starter Member

    Feb 22, 2015
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    Hi everyone.

    I am designing a simple control circuit for an AC switch with a delayed ON duration. I have all the timing down and everything seems ok but I have no experience with thyristors or TRIACS so I was wondering if anyone here did. Below is my diagram so far and for the TRIAC I have adapted the thyristor symbol as unfortunately it isn't available. Basically I was wondering if a TRIAC is basically synonymous with a transistor but for AC and is it relatively simple to apply to my diagram with a DC driven gate on the TRIAC? Is this suitable for my purposes or would I be better of with some kind of transistor setup (I have researched this originally but couldn't find anything that worked well)? The supply it will be controlling will be high frequency @ max 12V 30A, the function generator in the diagram is just arbitrary at the moment.

    All and any help would be appreciated

    Thanks :)

    [​IMG]
     
  2. wayneh

    Expert

    Sep 9, 2010
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    Any reason you're not just using the 12V supply for all this? In other words, why the regulator? And the two forward-biased 1N4002 diodes are redundant. I could see the first one for reverse hookup protection (if there is any real risk of that happening) but the second one isn't necessary.
     
  3. Marcus2012

    Thread Starter Member

    Feb 22, 2015
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    I'm running multiple circuits from the same regulated supply its just there because of that as I copied and pasted that part. I just need a setup that can switch a AC current on command from the monostable output from the 555. Thanks for the diode suggestion I may have gone a bit far there lol.
     
  4. DickCappels

    Moderator

    Aug 21, 2008
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    A few changes are indicated base on what little we know about your application.

    If the Triac circuit does not have to be isolated from the control circuit, you might want to ground MT1 (main terminal 1) so that you can get current to flow through the gate to MT1.
    Rules for driving triacs:
    www.nxp.com/documents/application_note/AN_GOLDEN_RULES.pdf

    If your load is inductive and/or the frequency is very high, the triac might not turn off before the next cycle starts. These things are not very fast, especially when turning off.

    If your load is inductive, then you should consider a snubber to protect the triac from high rates of voltage change across MT1 and MT2, or use a "snubberless" triac. I know ST Microelectronics and Fairchild make them, probably several other semiconductor manufacturers also by now.
    Snubber design:
    www.st.com/web/en/resource/technical/document/.../CD00004096.pdf

    For a summary, have a look at the circuit in Figure 6 in this datasheet:
    www.vishay.com/docs/83627/il410.pdf
     
  5. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    Last edited: Feb 24, 2015
  6. Marcus2012

    Thread Starter Member

    Feb 22, 2015
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    Thanks for the replies guys. I'm gonna try and digest all of this now :)

    A few more points though.
    1) This is to control a high frequency (10-200KHz, undecided yet) supply of 12v/30-40A.
    2) The load across the controlled circuit will be from an inductor.
    3) It will need to be active for a period between 100-500ms.

    I mention this as you said

    so am I barking up the wrong tree here and should I be looking at something else?

    Would a mechanical relay suffice for this purpose?

    http://www.digikey.co.uk/Web Export...ER_Reedrelay_vs._Solid-State.pdf?redirected=1
     
    Last edited: Feb 24, 2015
  7. Brownout

    Well-Known Member

    Jan 10, 2012
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    Triacs aren't normally used for those frequencies. What not just use a relay?
     
  8. Marcus2012

    Thread Starter Member

    Feb 22, 2015
    349
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    Thanks I was just think the same :)

    Do you think something like this would do for the on time I'm looking for then?

    http://www.autoelectricalspares.co.uk/lucas-srb539-28ra-relay-12-volt-70a-2387-p.asp

    I would have to contact lucas for an accurate response time though.

    Are there solid state relays that can handle such variable high frequencies at all or if there is am I looking at a lot of money?
     
  9. Brownout

    Well-Known Member

    Jan 10, 2012
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    How accurate do you need? Also, mechanical relays exhibit contact bounce. That might be a show stopper. I think the 100 ms minimum time is a problem too. I don't know of a solution off-hand.
     
  10. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    i would consider an H bridge. or half of one for this app. 100 kHz is to much for triacs and relays.
     
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