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What is a Thyristor?

Paul Scott
Paul Scott

A thyristor is a solid-state component used to switch and control electric current flow. Also known as a silicon controlled rectifier (SCR), a thyristor is a robust electronic component used in high current flow applications. They consist of four layers of alternating n and p type semiconductor materials equipped with anode, cathode, and gate terminals. Thyristors start to conduct when they receive a preset voltage on their gate terminal and will, subject to several variables, continue to conduct even if the gate voltage is removed. These operational variables and a wide power rating range make thyristors extremely useful current controllers.

Although thyristors may be broadly classified as simple current switching devices, the range of operational variables they posses makes them very useful in a number of control applications. Essentially thyristors are high current switching devices made up of four alternating p and n layers. An anode is located on the first p layer, a gate terminal on the second p layer, and a cathode on the last n layer. When idle, there is no current transfer across the anode/cathode path. The component requires a voltage of set value applied to the gate layer to switch it on and cause it to conduct current.

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The fact that the component will not become active if the gate voltage falls short of its rated threshold value is one of the useful variables a thyristor possesses. This allows for precise control over the switching conditions of the component. Once the thyristor has been turned on, it will remain active even if the gate voltage is removed and current it passes does not drop below the holding value of the component. This known holding voltage is another handy characteristic of thyristors. If the anode voltage value is below the holding level, the thyristor will not switch on even if it receives a gate pulse.

Thyristors can comfortably handle extremely high voltage and current ratings. They are commonly used in zero cross alternating current (AC) controllers, power supplies, phase fired controllers, and long distance power transmission facilities. This last application features huge thyristor banks arranged in Graetz bridge configurations which are capable of reliably switching power values of several megawatts (1,000,000 watts). On the other end of the scale, small AC/DC power supplies may use thyristors rated at 20 watts or less. This flexibility and range of operating power ratings makes the thyristor one of the most useful current flow controllers in the circuit designer's arsenal.

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Discussion Comments


I am at a complete loss here. When they say four layers, does this mean four separate I/O? I'm with David09 on this. How would you initially give the "gate layer" current and continue to have have current flow when the gate layer has ceased with just one input. There must be more than one inputs to do this correctly?


@David09 - Yeah, it’s important to note that the thyristor has four layers, each of which performs a function. So the gate is only one of those layers and it’s what receives the flip of the switch. I guess the other layers are used to conduct the flow of normal operating voltage. That’s what it looks like anyway.


@David09 - The article is describing gate voltage, not the voltage needed to keep the current going. Gate voltage is what turns the thyristor “on” so to speak. It’s what flips the switch.

I would guess that it’s an initial burst of current, probably minor, that turns the thyristor on. However, once it’s on the gate voltage is removed, but the regular voltage continues to flow through during the thyristor operation.

In other words, the thyristor continues to receive voltage during its operation but just not the gate voltage, which is something different altogether. I hope that makes sense.


@hamje32 - I don’t get the part about current continuing to flow after the thyristor circuits no longer receive the gate voltage. If there is no longer any voltage in the thyristor circuits, how you can you still have current flow?

Perhaps I am missing something; I’ve read that line several times and still don’t get what that means. I, too, am not an expert, but I think I’m a bit woefully ignorant in this regard.


I’m not an electronics expert, but I do work in the utilities industry. Just from my cursory reading of this article it would appear that a thyristor power controller would be used extensively in substations and perhaps with devices such as relays.

What relays do is to test for spikes or drops in voltage. You could have an overcurrent relay, for example, to test for spikes in voltage, and an undercurrent relay which would test for drops.

Since the thyristor has a holding voltage which does not change unless it drops below a threshold voltage, I could see that this would be a useful switching mechanism in an undercurrent relay.

When you get too little current, in essence, the thyristor would go off and the relay would trip. I am not a technician but that’s how I guess that it would work.

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