# SCR/Thyristor LTSpice simulation.

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• #16319
ville91
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Hello!

I have a problem, that should be easy to solve, but apparently isn’t. I’m trying to simulate the typical voltage-current characteristics of a thyristor/SCR in forward-blocking mode and in forward-conduction mode in the same graph.  See my circuit below. So, I’m measuring the anode-cathode current (V2, x-axis) and anode voltage (y-axis). What should happen is that when V2 exceeds a certain breakover voltage, the voltage then drops. After that, when the voltage is raised, the anode current should rise linearly. But that’s not what I get, see my graph below. V2 is a certain sine wave, and I used .step param -function to raise that gate voltage by steps. When the gate voltage is great enough, the thyristor should go into forward-conduction mode, but it doesn’t happen in my simulation. What should I do? I would really appreciate some help and clear instructions. Any method would be great. All I want to do, is to get the voltage-current characteristics in forward-blocking mode and in forward-conduction mode.

PS. Basically I did the same stuff as in this video, where tha same is done with Pspice:

#16464
Faman2012
Participant
Credits: 2

Hi,

I think that your SCR model with two BJT is completely wrong. You can see from the I(R4) vs V(anode) characteristic that I(R4) ranges from positive to negative (and high) values. This can’ t be, because a SCR works alternatively as an open circuit and a diode, depending on the gate voltage and on the voltage applied between the anode and the cathode. I’ve attached a basic SPICE cirduit model of a SCR with two ideal BJT.

Below the anodic current and the current vs anodic voltage characteristic.

It can still be improved by introducing the capacitances beetwen Q1 base and emitter, Q1-Q2 bases, and Q2 base and emitter.

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