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I am trying to understand the parameter Vgt (max) for a triac (or any semiconductor with a gate threshold parameter), mainly the max. part of it. For example, on page 2 of the following datasheet:

http://www.littelfuse.com/~/media/electronics/datasheets/switching_thyristors/littelfuse_thyristor_lxx04xx_qxx04xx_datasheet.pdf.pdf

VGT (VD = 12V RL = 60 Ω) MAX. 1.3 V

Could anyone please help me understand how to interpret max. here? Does it mean gate threshold can be 1.3V at max. and that we need at least 1.3V to trigger it and allow the current flow?

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It's not an absolute max rating so, it can be assumed the supplier is saying that to guarantee switching you must exceed 1.3 volts. However, you must protect this part by limiting current to no more than 1.2 amps (see AMR table for \$I_{GTM}\$).

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  • \$\begingroup\$ Thanks for the confirmation. This is what I originally thought it to mean. I wonder why they chose to mention "Max." instead of "Min."! \$\endgroup\$ – LoveEnigma May 2 '18 at 12:18
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    \$\begingroup\$ If the term minimum was used, that would not guarantee a level that must be exceeded to 100% guarantee correct operation. A minimum level would be used to tell the reader that the device is guaranteed to be disabled if below the minimum. \$\endgroup\$ – Andy aka May 2 '18 at 12:30
  • \$\begingroup\$ Okay, thanks for the clarification. I understand your point. But the usage of minimum and maximum is different in this context and unlike the traditional usage in electrical specifications of most other devices. Am I correct? \$\endgroup\$ – LoveEnigma May 3 '18 at 10:54
  • \$\begingroup\$ No, the usage is consistant with other specifications I've seen. \$\endgroup\$ – Andy aka May 3 '18 at 11:13
  • \$\begingroup\$ You have to look at what they say in the DC from their point of view - the guaranteed trigger voltage is 1.3 volts (max) meaning that for the "user", the minimum voltage you should apply is 1.3 volts. \$\endgroup\$ – Andy aka May 3 '18 at 11:38
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It means that at the moment of trigger, when you are forcing just enough current into the gate to guarantee triggering of the thyristor , the gate voltage will be no higher than 1.3V (at 25°C).

It may be higher at lower temperature, and it may be higher if you force more current into the gate.

So, suppose you have a +5V supply that yields minimum 4.5V after a transistor switch drop and tolerance, you can calculate the required gate current-limiting resistor from Rgt = (4.5-1.3V)/Igt

schematic

simulate this circuit – Schematic created using CircuitLab

In the circuit above, I am showing the gate driven with negative current since the standard triac is not guaranteed to turn on in Quadrant-IV.

Note that at low temperatures, the triac will generally be less sensitive than at 25°C and the gate voltage will also be higher, so to guarantee triggering at low temperatures you need to either put more current in at 25°C than would otherwise be necessary (the usual method) or make the gate current temperature sensitive with a PTC or something like that (rare).

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  • \$\begingroup\$ Thanks for your answer. From your answer, my understanding is that the gate will trigger even at the voltage below 1.3V whereas per Andy's answer the gate will trigger only when the voltage is > 1.3V. Could you please clarify? \$\endgroup\$ – LoveEnigma May 2 '18 at 12:17
  • \$\begingroup\$ @LoveEnigma please don't misread me - that is NOT what I said. Think again. \$\endgroup\$ – Andy aka May 2 '18 at 12:31
  • \$\begingroup\$ @Andyaka Didn't you mean gate will trigger only when the voltage is above 1.3V? \$\endgroup\$ – LoveEnigma May 3 '18 at 10:55
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    \$\begingroup\$ No, to guarantee switching you must exceed 1.3 volts. It might switch at 1.2 volts but that isn't guaranteed. \$\endgroup\$ – Andy aka May 3 '18 at 11:28
  • \$\begingroup\$ Okay, I got it. Thanks. I think you can add this to your answer, please. :) \$\endgroup\$ – LoveEnigma May 3 '18 at 12:16

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