Testing a triac without connection to mains

Discussion in 'The Projects Forum' started by tracecom, Oct 24, 2013.

  1. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
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    In the attached circuit, I would like to test the operation of the triac without a connection to mains voltage. When the input to the base of Q1 is low, there is infinite resistance between MT1 and MT2; when the input is high, there is 6.43kΩ between MT1 and MT2 (regardless of polarity.) Is that sufficient to conclude that the triac is performing correctly? Thanks.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Use any small low voltage transformer with a suitable lamp load etc.
    Max.
     
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  3. tracecom

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    Thanks. Yes, I can do that and it works, but I am trying to keep it even simpler.
     
    Last edited: Oct 24, 2013
  4. #12

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    Nov 30, 2010
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    I don't think you can get any simpler. Every part in that circuit has a job that can not be ignored or deleted.
     
  5. tracecom

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    Thanks. What I am trying to simplify is the testing. I would like to test the operation of the triac without a connection to mains voltage. When the input to the base of Q1 is low, there is infinite resistance between MT1 and MT2; when the input is high, there is 6.43kΩ between MT1 and MT2 (regardless of polarity.) Is that sufficient to conclude that the triac is performing correctly?
     
  6. strantor

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    Oct 3, 2010
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    I think you need a higher voltage source than the ~1V your DMM uses to measure resistance. Try on the "Diode" test setting and listen for the beep.
     
  7. #12

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    Nov 30, 2010
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    Yes and no. You have demonstrated any change at all, but a Triac is not designed to support its current avalanche procedure with ohm meter quality current. Max already described the minimum way to test the Triac. Give it current in the range it needs to work.
     
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  8. strantor

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  9. tracecom

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    Thanks to you all. I tested the circuit with 15VAC and then with 127VAC, and it is working well.
     
  10. #12

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    Nov 30, 2010
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    Please excuse the nit-pickyness of this, but...

    Some triacs require 30 ma to 50 ma gate current. I've never seen one rated at less than 1 ma. An ohm meter delivers less than 1 ma. Strantors example shows 15 to 50 ohms A-K after the gate is triggered. Tracecoms measurements show 4.3K after an external current source triggers the gate. Both of these show, "any change at all" but they are as weak as using 1 ma from a 9V battery to test a 1N4007.

    The best test of a triac is under normal operating currents. If it is still 4.3K when an amp tries to get through at power line voltages, it is going to smoke. Just as a 1N4007 can survive a 9V battery in an ohm meter, that says nothing about surviving the 1000V it is rated for.

    Tracecom asked if the ohm meter test shows the triac is "performing correctly", and I say it shows any change at all. To meet the full qualifications of performing correctly, it has to perform the current avalanche which enables it to operate under real world conditions.

    I am timid about saying an ohm meter can validate a triac. This is from a real world experience where I bought a triac rated for 3 amps, assigned it to carry 1 amp, and the internal resistance was high enough to melt the insulation off its own legs. An ohm meter can show if it is working at all, but can not validate correct performance in the large scale.

    Thank you for your patience.
     
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