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I have been working on a project with a TRIAC and encountered some problems; I have been looking around on how TRIACs operate, I did find lots of recourses talking about them on the internet but non of them answer my question.

Does a TRIAC care about voltage/current phase shift? If so, how?

For example If I had a load attached to it with 30 or 60, 90 degrees and so on phase shift between voltage and current how would it affect the TRIAC? from my understanding it only cares about the current passing through its T1 and T2 pins and it resets when the current through it falls bellow latching current.

Related questions on my project (for extra clarification): Question 1 Question 2

winny
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Oli
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  • There is no such load that could have 180 degrees shift. Try to use some logic: Higher the positive voltage, higher the negative current. Higher the negative voltage, higher the positive current. It makes no sense. You did a wrong current measurement, instead of 180 degrees shift, you have 0 degrees i.e. : in phase. – Marko Buršič Jun 20 '21 at 13:43
  • @MarkoBuršič, I have measured some resistive loads, other AC devices and all of them were correct. I know it doesn't make sense which is exactly why I asked it here. have look at this [question](https://electronics.stackexchange.com/questions/571895/current-with-150-degree-phase-shift-and-wiggle-on-single-phase-motor) I have explained the measurement method. – Oli Jun 20 '21 at 13:49
  • @MarkoBuršič, The main point of this question is to help understand how a TRIAC would react on circumstances where the is a phase shift between current and voltage. The amount of it is arbitrary. – Oli Jun 20 '21 at 13:52
  • @Transistor, Thanks for letting me know, I know they don't make sense but the problem is I somehow observed them on my scope and it totally confused me. I have changed them though – Oli Jun 20 '21 at 14:43

2 Answers2

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The usual cause of a lagging load current with respect to the voltage would be an inductive load.

An inductive load (lagging current) can cause the triac to not commutate (switch off) properly when the current drops below the triac's holding current value at the end of each half cycle. What actually happens is that when the current drops low enough the triac attempts to switches off, but the voltage across the load at this point in time has started to rise again (leading voltage). When the triac tries to switch off, the voltage across the load tries to instantaneously reduce to zero volts. This rapid reduction in the voltage across the load (which is in effect a rapid increase in the voltage across the triac) can exceed the triac's maximum dv/dt specification which switches the triac back on again. The triac doesn't commutate (switch off).

For a resistive load, with its in phase voltage and current, when the triac attempts to switch off there is only a small increase in the voltage across it (voltage across the load is very small because it's in phase with the current) and so the dv/dt rating is not exceeded and the triac commutates (switches off).

The way to circumvent this problem is to include a RC snubber network across the triac when the load is inductive or use a snubberless triac. The snubber limits the rate of the rise of the voltage across the triac.

  • James, the OP is saying that the load + triac makes a 180 degrees phase shift. Do you have a vague idea what does this mean? An ideal inductor or ideal capacitor would have +/-90 degrees of shift, but very far away from 180. This is impossible in our solar system, maybe in a black hole? – Marko Buršič Jun 20 '21 at 21:41
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    @MarkoBursic OP has edited phase shift values. –  Jun 20 '21 at 21:58
  • @MarkoBuršič, I did not say "load + TRIAC makes a 180 degrees phase shift" I only mentioned above 90 degrees phase shifts because I some how measured them on my scope. and I know they don't make sense. Care to explain why am I getting this reading on this [question](https://electronics.stackexchange.com/questions/571895/current-with-150-degree-phase-shift-and-wiggle-on-single-phase-motor) ? – Oli Jun 21 '21 at 07:07
  • @James, Thanks a lot for the answer. – Oli Jun 21 '21 at 07:08
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With respect to TRIACs, you are correct: the devices are current dependent.

What happened to your measurement might be related to your oscilloscope. Maybe there is a delay on your current probe, not present on the voltage probe, or maybe one of these signals is inverted, which could give you an impression of a 180° offset. As others stated, a pure capacitive or inductive load (ideal, not real), can only result in a 90° offset.

ocrdu
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Marcelo
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