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LTspice Triac .model doesnt work

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cupoftea

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Hi,
Do you know why this triac model doesnt work in LTspice.
I got it here.
https://www.powerelectronicsnews.com/power-supply-design-note-simulating-a-triac/

Triac.jpg
 

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  • triac.zip
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Solution
Hi,

no, it´s not.

Fig9 on page 7 does it correctly, but on your schematic diode D1 is the other way round.
* Your schematic triggers with a too low current, positive gate pulse. (Q1, Q4). It triggers on falling edge of microcontroller pin.
* In the datasheet they trigger the gate with a high peak current, negative voltage, gate pulse. (Q2, Q3). It triggers on rising edge of microcontroller pin.

Klaus
Maybe the trigger network is inadequate, datasheet indicates
20+ mA for a trigger current and your 24V/3.3K would fall short
of that (even if not capacitor-blocked, and the MOSFET turnoff
might be so leisurely that through-current to trigger is even less).

Might start with just simulating it as an SCR and see what your
(simulated) trigger current threshold is / must be, for a simpler
case.
 
Thanks, yes, my bad, i mis-sized the RC in the trig cct

Triac1.jpg
 

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  • triac1.zip
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Hi,

I don´t undersand your trigger idea at all.

You want to trigger at positive edge of gate.
But positive edge is determined by the weak resistor R2.
then you have the strong M1 .. but provides nothing to generate a trigger pulse.

why don´t you use the MOSFET to generate the trigger?

Klaus
 
Hi,

no, it´s not.

Fig9 on page 7 does it correctly, but on your schematic diode D1 is the other way round.
* Your schematic triggers with a too low current, positive gate pulse. (Q1, Q4). It triggers on falling edge of microcontroller pin.
* In the datasheet they trigger the gate with a high peak current, negative voltage, gate pulse. (Q2, Q3). It triggers on rising edge of microcontroller pin.

Klaus
 
Last edited:
Solution
Thanks very much indeed KlausST, now corrected.
The shown cct here, has the triac drive cct far from the triac, with much stray inuctance, but it appears that triac drive ccts need no snubbers, would you agree?
And the triac needs no snubber because it always turns itself off at the zero crossing anyway...would you agree?

LTspice and jpeg attached

triac drive _no snubber.jpg
 

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  • triac no snubber.zip
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