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I have always been told that you should never untangle the wires of a thyristor or subset of thyristor (such as TRIAC and SCR). Why is this? Is this just for convenience or is there a use for the wires being twisted?

Thyristor with wires tangled up. Source: wikipedia commons

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Why are the wires of thyristors always twisted in pairs?

For the same reason that any twisted-pair cable is used: to reduce interference.

Thyristors such as that you have photographed are used in high-current circuits. The thyristor gates are relatively sensitive and an untwisted pair of wires running off to the control circuit will create a loop which will be susceptible to picking up induced currents which may trigger the thyristor.

By twisting the pairs a series of small loops are made and each one is in opposite orientation to the one before it. The result is that the induced interference is, to a very significant extent, cancelled out.

It appears that the device in your photo has four terminals: the stud and the heavy red lead for the high current circuit and the twisted pair for the control circuit. The reds are connected internally to eliminate problems trying to attach the control wire to the bolt on the large ring terminal.

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  • \$\begingroup\$ Ah nice, that explains the two control terminals! \$\endgroup\$ Jun 5, 2016 at 13:45
  • \$\begingroup\$ Besides convenience, a separate wire allows you to have a single point reference at the Cathode. The gate driver circuit is often an otherwise floating circuit so you want only one connection and you want that connection to be electrically as close to the Cathode as possible. If you connect the reference point to the heatsink where the big stud is attached, you could have noise spikes that could fire the SCR spuriously. \$\endgroup\$
    – Mattman944
    Feb 19, 2023 at 21:35
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Given these wires aren't twisted for about half their length, the effect can't be that important. So, I think this is mostly a convenience / superstition thing.

However, there is good reason to twist the wires that control a high power semiconductor. It's the same as why network cables are twisted pair: Any power coupled in to one wire will couple (approximately) as much in the other, leading to common-mode noise, rather than differential noise. Common mode noise can be ignored by things that couple in capacitively. However, that is not the case for TRIACs.

Also, the picture shown is not a TRIAC, but an SCR (which is a special type of thyristor used for rectification), and I really don't know why there are two control lines here.

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