Choosing TRIAC based on current rating

Discussion in 'General Electronics Chat' started by DanTheBrahman, Aug 2, 2011.

  1. DanTheBrahman

    Thread Starter New Member

    Aug 2, 2011
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    I am driving a heater at 4.3A rms and 120V rms (AC). TRIACs in the digikey product index have the parameter On State Current (RMS)(Max). Would I choose one that has that parameter >= 4.3A or would I choose 3 x 4.3 = 12.9A to be safe. Relatedly there is also a Non-Repetitive Peak Current parameter which is around 50A for a 6A (On State Current (RMS)) TRIAC.

    :confused:
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Voltage and current recommendations from a triac application note:

    "Select the triac for the voltage of the line being used, the current through the load, and the type of load. Since the peak voltage of a 120 V ac line is 170 V, you would choose a 200 V (MIN) device. If the application is used in an electrically noisy industrial environment, a 400 V device should be used. If the line voltage to be controlled is 240 V ac with a peak voltage of 340 V, then use at least a 400 V rated part or 600 V for more design margin.

    The RMS current through the load and main terminals of the triac should be approximately 70% of the maximum rating of the device."

    It's important to have a good heatsink since without it they can get quite hot and current rating decreases with higher temperatures.
     
  3. DanTheBrahman

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    Aug 2, 2011
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    Thank you, praondevou. Could you post a link to that AN? The device I am building is a consumer electronics vaporizer. The expected use of it will be at home out of the wall AC. Is that considered noisy?
     
  4. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    http://www.littelfuse.com/data/en/Application_Notes/an1007.pdf

    No, I don't think that's a noisy environment. If it's an residential area it's less likely to have big voltage transients because the switched loads are generally relatively "small".

    However, I would use a device with a bigger max peak voltage, so I'm on the safe side. I just had a look at Digikey and I probably would choose a 8A to 10A type in a TO-220 package. But this depends on your layout.
     
  5. DanTheBrahman

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    Aug 2, 2011
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  6. DanTheBrahman

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    Aug 2, 2011
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    One more question. What is the STANDARD reference designator used for TRIACs, ie Q? U?, or something else?

    Thanks.
     
  7. DanTheBrahman

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    Aug 2, 2011
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    I still haven't found a way to calculate what Vgt and Igt to use. Any ideas?
     
  8. praondevou

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    please read THIS application note.

    Since they only mention Igt, I think that's the maximum current that can be required to trigger the triac, i.e. if the current is less than this, it might not trigger.

    Don't worry destroying the device. Max peak trigger current is MUCH higher.

    For the designator I've seen Q, TR and T.
     
  9. DanTheBrahman

    Thread Starter New Member

    Aug 2, 2011
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    This is from another thread of mine, but will close up this thread nicely.

    Thanks, praondevou.

    That part will replace the optotriac / triac combo so knowing the answer to the Igt question is not necessary any more.
     
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