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I have a simple question: what is the second gate lead on a power thyristor (see the figure below) ? What is it physically attached to ?

Power thyristor with two small gate leads

I understood that the thyristor turns on when you force a small current pulse into the gate. So I don't understand what is the purpose of the second one.

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The thin red lead is actually a cathode lead. It is used for triggering or squelching the device independent of the high-capacity cathode lead.

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    \$\begingroup\$ Which cathode is it? Is this thyristor a GTO then ? \$\endgroup\$
    – snickers
    Jan 18 '15 at 17:24
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    \$\begingroup\$ It's the same cathode as the thick red lead. \$\endgroup\$ Jan 18 '15 at 17:35
  • \$\begingroup\$ if you are extra sure please consider editing the second picture caption here then en.wikipedia.org/wiki/Thyristor honestly I trust you more than wikipedia but I really can't make the edit myself since I am not sure of the correct answer. \$\endgroup\$ Jan 18 '15 at 18:06
  • \$\begingroup\$ All I could find was this, a bit slim on info itself. \$\endgroup\$ Jan 18 '15 at 18:11
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One of the two leads goes to the gate. The other is connected as closely as possible to the cathode at the die.

schematic

simulate this circuit – Schematic created using CircuitLab

So even if a voltage appears across the cathode lead it won't affect the gate-to-cathode trigger voltage.

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