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Triac Testing

Discussion in 'General Electronics Discussion' started by polyal, Jun 25, 2017.

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  1. polyal

    polyal

    12
    0
    Sep 8, 2011
    I thought the resistances between terminals of a triac should read o/c
    with a ohmeter...i have a batch of BTA 20A 600B ( 10 ) that all read
    appx 100R or 0.4v between T1 and GATE ?
    is there an explanation for this apart from ' bin them they're faulty '
    regards
    reg
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,482
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    Jan 21, 2010
    It's a bit tricky to see in the TRIAC, but in the SCR it is obvious there is a diode junction between the gate and the cathode.

    The same sort of thing applies for a TRIAC.

    You should expect a high resistance between the main terminals, not the gate and MT1.
     
  3. polyal

    polyal

    12
    0
    Sep 8, 2011
    thanks for that
    are you saying that these devices are OK?
    although the 0.4 volts drop ( 100ohms ) is both ways
    so its seems its not the effect of a diode junction
    how do snubberless triacs measure?
    reg
     
  4. Minder

    Minder

    3,066
    657
    Apr 24, 2015
    Simple way to test a triac under power is use a 6v - 12v transformer and hook a suitable lamp and the triac up in series across the secondary, use a 33ohm resistor from G to A2 and it should switch on/off with when resistor connected.
    M.
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,482
    2,830
    Jan 21, 2010
    Best thing is to test them.

    An SCR has a diode junction between the gate and cathode.

    A TRIAC is sort of like a pair of SCRs in antiparallel, but that would mess up the triggering. If you look at their construction they have a pretty weird configuration that effectively places two antiparallel dudes shunted with some bulk silicon between the gate and MT1. That explains the lowish resistance in both directions. Understanding this demented licorice all-sorts is damn near impossible which is why looking at an SCR is more useful.
     
    Last edited: Jun 26, 2017
  6. Minder

    Minder

    3,066
    657
    Apr 24, 2015
    @polyal BTW, the method I showed for testing comes from the GE SCR/TRIAC Experimenters Manual.
    M.
     
  7. poor mystic

    poor mystic

    1,068
    31
    Apr 8, 2011
    :) Usually you'd expect to find a diode, testing across the main terminal of a triac.
     
  8. gsmhack

    gsmhack

    9
    0
    Sep 13, 2017
    Hello there, just registered, in hope to find expert advice. I just crafted a circuit, and would like your opinion about it. I built its DC version, it works OK, but I have no experience with triacs, so this might be totally off. Let me describe the problem, what the circuit does (at least what I believe it would do), and provide a link to az easyeda schematics. Thanks for your help in advance!
    Now the problem: The house is provided with two electricity meters. One is working 24/7, but is more expensive, the other one is working nighttime, *sometimes* during daytime too, and is cheaper. I want to use the cheaper meter when it is available, and switch back to normal, when the switched turns off. I normally do this with a relay. NC goes to "constant on" supply, NO goes to switched. The common goes in the house, powering everything with the 230V AC. The coil of the relay is powered from the switched supply. The problem is that sensitive electronics can not deal with the gap, the momentary lack of power during the switching. I would like to solve this, and came up with this circuit...
    EasyEDA link: https://easyeda.com/editor#id=a004ef0d79274fd2854d149cdbf04689
    What this is supposed to do:
    - normally "constant ON" powers the output
    - now "nighttime" gets energized
    - this should trigger the TRIAC, and power the coil
    - now, as the contact lever of the relay moved from the NC contact, the current goes through the D1 and D2 bridge rectifiers (DC pins shorted), that drops ~0.7V×4=2.8V.
    - Now we have 2.8V AC, D3 rectifies it, and through R1 starts to charge C1
    - By the time the LED in the MOC would be powered, the relay hopefully switched all the way to the NO contact. When it does, no more voltage drop through D1 and D2, the MOC will not trigger, the TRIAC continues to trigger.
    - Now the nighttime turns off.
    - The current starts to flow through D1 and D2 again
    - The current also flows back from the output (relay pin 3) to the NO contact (pin 2), from there to the triac, so the triac will not turn off. (Now we need the MOC)
    - the rectified 2.8V across D1 and D2 now starts to charge C1, and it should turn on the LED in the MOC.
    - the MOC should short between Gate and MT1, and this is supposed to turn off the TRIAC
    - with the triac turned off, the coil of the relay releases, and until it arrives back to the NC contact, current flows through D1 and D2.

    I am absolutely not sure if this could work at all, or how to select the capacitor and resistor values. Could you please help me? (Maybe should I post this in another thread?)
     
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