Switching circuit

Discussion in 'The Projects Forum' started by ryanspice, Mar 5, 2009.

  1. ryanspice

    Thread Starter New Member

    Mar 5, 2009
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    Hi guys. This may be a stupid question, but I am working on my EE freshman project which involves triggering strands of EL wire from an audio signal. I have everything figured out except the actual switching part. I'll be triggering the wires with about 10mA/12V from an LM3916 VU meter circuit.

    I need a relay/switching circuit that can respond quickly and handle the 120VAC/2kHz EL wire supply (current is miniscule, don't know how much wire we'll be using yet so I can't give a definite answer, but probably under 50mA). Anything that runs off 12VDC and is relatively cheap will do.

    Thanks
     
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I have never used these, but it looks like MOC3020-3023 might work. They appear to work like a small triac. The are designed, BTW, to trigger regular triacs.
     
  3. ryanspice

    Thread Starter New Member

    Mar 5, 2009
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    Thanks, I didn't even know about triacs until now. They sound like the right way to go unless someone else has any suggestions.
     
  4. Ron H

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    I doubt that a regular triac would work at the low current levels you are expecting. That's why I was looking at the MOCs.
     
  5. ryanspice

    Thread Starter New Member

    Mar 5, 2009
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    I got everything hooked up the other day, and the triac is staying on all the time when I use the 2kHz AC and EL wire. This happens even if the input side of the triac is disconnected. I tried using an LED as a load, and it turned on when the input side was triggered, but of course then stayed on since it was a DC load.

    Is my problem that my AC is at such a high frequency that the triac doesn't have enough time to turn off? I also vaguely recall reading something about triacs having a max dv/dt rating, and exceeding this rating makes the triac stay on.
     
  6. Ron H

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    Is your 2kHz a square wave? If so, the dv/dt will turn the device on.:(
     
  7. ryanspice

    Thread Starter New Member

    Mar 5, 2009
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    It's SUPPOSED to be a sine wave, but the power source I'm using right now is cheap, and I can't be sure unless I bring it in to the lab and check it with a scope. Even if it is a pure sine wave, is it possible that the frequency is high enough for the dv/dt to be turning the device on?

    FYI, the load I am testing it with is drawing about 36mA RMS, no sign of the MOC3021 getting hot or coming close to failure.
     
  8. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    How many current is going through the LED of the MOC ?
    The different types have different sensitivity.

    [​IMG]

    Greetings,
    Bertus
     
  9. ryanspice

    Thread Starter New Member

    Mar 5, 2009
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    It's about 15.5 mA. It doesn't seem to have any problem being triggered, it just won't stop the AC from getting through no matter what state it's in on the other side.
     
  10. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    What is the input to the EL power supply? If it is an automotive application, where 12VDC is boosted to 120 for EL, switching the supply input, or enabling/disabling the power supply oscillator with a MOSFET may be easier.
     
  11. ryanspice

    Thread Starter New Member

    Mar 5, 2009
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    It runs off of 9VDC. It would be nice to just be able to switch that, but unfortunately this project involves switching 10 different loads off that same power supply, so I can't do the switching that way.
     
  12. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Can you not "isolate" the EL cinverter from the rest to switch it ?
    Do you have a set-up diagram of what is connected to the battery ?

    Greetings,
    Bertus
     
  13. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    A solid state relay may be the only solution for an AC signal that a triac can't work with, and you have no other control of. Does the supply have a minimum load or can it tolerate the load connecting/disconnecting completely?
     
  14. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    As thatoneguy said a solid state relays may work.
    Take a look at the AQ21from panasonic.
    See datasheet.

    Greetings,
    Bertus
     
  15. ryanspice

    Thread Starter New Member

    Mar 5, 2009
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    Thanks, I'll try the AQ21 and let you know how it goes.
     
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