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As far as I understand when the thyristor is triggered it should turn on and remain on unless negative voltage applied to its gate. I found the model from littelfuse website.

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But I tried the following circuit with the model where I plot gate trigger voltage supply voltage and load current as follows:

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And when the gate voltage is off the load current also goes off instead of reaming on aka latching.

What could be the reason the thyristor is not latching as expected here?

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As far as I understand when the thyristor is triggered it should turn on and remain on unless negative voltage applied to its gate.

No, it remains on after the trigger has been removed until the anode current falls below the hold-on current which should be listed in the datasheet. Note that this value will depend on the temperature of the device.

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From the datasheet.

Links

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It is latching, don't you see? It latches only until the current falls below holding current, then you have to re-trigger each cylcle. It would latch constantly if you have DC supply voltage.

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