Triac Not Triggering in Q3

Question

I tried simulating this circuit in LTSpice, but, if I'm interpreting this correctly, the triac isn't triggering in Q3.The gate current reaches -360mA when I would expect the triac to enter Q3.

I'm not sure what's going wrong. Am I not meeting a condition to turn on the triac? Could it be a problem with the triac manufacturer's LTSpice model?

I'm using the BTA24-600CW spice model from ST.

https://www.st.com/en/thyristors-scr-and-ac-switches/bta24.html#cad-resources

I'm also using an MOC3082 optocoupler model. The optocoupler has a constant current of 18mA as input, so it should always be switched ON. I suspect this is working because R5 has current.

Answer 1

If you look at the .zip file provided by ST, it includes a .lib (models) and a .olb (symbols). These are meant to be used with PSpice, and there is also a .pdf file provided which shows you how to import the models and symbols into PSpice.

Since you are using LTspice, you can use the models since LTspice has a bunch of undocumented PSpice model compatibility. However, you cannot use the symbols. Like you found, LTspice provides a generic TRIAC symbol to use with subcircuits (i.e. has a prefix of X) in the [Misc] folder. Before you can use this symbol, you need to understand the pin order for it. The easiest way to do this is to put an instance of the symbol onto your schematic and right-click on it. Then you can click on Open Symbol:.

This brings you to the symbol editor. Right-click on the top pin and you'll see this:

The important information here is the Netlist Order; a fancy way of saying "pin order". Jot this down and repeat this for the remaining pins. You'll notice that the GATE pin has a netlist order of 2. This is important. Since a triac is bi-directional, the other two pins don't matter too much (in this specific instance).

Now, if you open up the .lib file for the models and scroll through you can see the generic structure for ST's triac model library.

.subckt Triac_ST A K G PARAMS:
+ Vdrm=400v     
+ Igt=20ma
+ Ih=6ma        
+ Rt=0.01
+ Standard=1

They define a universal subcircuit called Triac_ST with some default parameters. Then if you scroll down further you'll see that for each specific triac device, it simply instantiates a Triac_ST with parameters that fit the device. For your BTA24-600CW:

.subckt BTA24-600CW A K G
X1 A K G Triac_ST params:
+ Vdrm=600v     
+ Igt=35ma
+ Ih=50ma        
+ Rt=0.016
+ Standard=0
* 1999 / ST / Rev 0
.ends

The main takeaway here is that the GATE pin (deduced from the G) is the 3rd pin in all these definitions. So, how to fix this? Well, you can do it two ways (actually three, but the third is so similar). You can swap K and G within the .subckt BTA24-600CW definition, such that it looks like this:

.subckt BTA24-600CW A G K

You'd have to do the above for each specific device, so this is the targeted approach. Since you're likely using this modified .lib exclusively in LTspice, it's probably better to modify the Triac_ST subcircuit definition instead so the swap will apply universally to all devices within the file, like such:

.subckt Triac_ST A G K PARAMS: