Here's the circuit:

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T1 and T2 are two thyristors where i provide pulse at their gates.

Here's the output:

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For some reason Vout follow Vin before the pulse at T1 happens when Vout before the pulse should be 0. Why does that happens?

Also when i put a capacitor in paraller to change the Vout voltage shape the plot doesn't even finish loading to the 0.2 seconds for some reason.

When i make some modifications in the circuit:

enter image description here

This is the output:

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So here the thyristor does what it's intended to do. But in the first case it doesn't and i dont know why.

Edit: So now i connected the load and the 2 diodes to ground and shorted the gate to cathode:

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This is the result:

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Everything is good except that the pulse generator doesn't seem to be periodic anymore, atleast it's not doing parameters i entered in LTSpice. In the first 0.1s everything is good but then for some reason it stops and starts making pulses out of order

  • 1
    \$\begingroup\$ Why would it work without a ground where the current would flow through the load? \$\endgroup\$
    – Justme
    Dec 14, 2021 at 12:41
  • 2
    \$\begingroup\$ Your trigger pulses can't be referenced to ground as you show it; instead, they need to be referenced to the cathodes of the SCRs. If you plot the gate-to-cathode voltage (V(pulse) - V(vout)) on one of the SCRs, you'll see the flaw in your design. \$\endgroup\$
    – Dave Tweed
    Dec 14, 2021 at 13:10
  • \$\begingroup\$ Yeah, i needed a ground. \$\endgroup\$
    – Scipio
    Dec 14, 2021 at 14:00
  • \$\begingroup\$ @DaveTweed Yeah, you're right. So i made the changes you guys suggest and things are looking better but for some reason the pulse generator is making pulses out of order. See my last edit \$\endgroup\$
    – Scipio
    Dec 14, 2021 at 15:33

1 Answer 1


Take care with grounding connections. In the last update, the voltage generators are shorcircuited with the diodes. The voltage generators should be floating if you ground the load (R1). In case the voltage generator is grounded, the load should be floating. enter image description here

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