Thyristors and Transistors Compared

Transistors are the tiny electronic components that changed the world: you'll find them in everything from calculators and computers to telephones, radios, and hearing aids. They're amazingly versatile, but that doesn't mean they can do everything. Although we can use them to switch tiny electrical currents on and off (that's the basic principle behind computer memory), and transform small currents into somewhat larger ones (that's how an amplifier works), they're not very useful when it comes to handling much bigger currents. Another drawback is that they turn off altogether as soon as the switching current is removed, which means they're not so useful in devices such as alarms where you want a circuit to trigger and stay on indefinitely. For those sorts of jobs, we can turn to a somewhat similar electronic component called athyristor, which has things in common with diodes, resistors, and transistors. Thryristors are reasonably easy to understand, though most of the explanations you'll find online are unnecessarily complex and often confusing beyond belief. So that's our starting point: let's see if we can take a clear and simple look at what thyristors are, how they work, and what kinds of things we can use them for!
What are thyristors?
First, let's nail some terminology. Some people use the term silicon-controlled rectifier (SCR) interchangeably with "thyristor." In fact, silicon-controlled rectifier is a brand name that General Electric introduced to describe one particular kind of thyristor that it made. There are various other kinds of thyristors too (including ones called diacs and triacs, which are designed to work with alternating current), so the terms aren't completely synonymous. Nevertheless, this article is about keeping things simple, so we'll just talk about thyristors in the most general terms and assume SCRs are exactly the same thing. We'll refer to them as thyristors throughout.
Three connections
So what is a thyristor? It's an electronic component with three leads called the anode (positive terminal), cathode (negative terminal), and gate. These are somewhat analogous to the three leads on a transistor, which you'll remember are called the emitter, collector, and base (for a conventional transistor) or the source, drain, and gate (in a field-effect transistor, or FET). In a conventional transistor, one of the three leads (the base) acts as a control that regulates how much current flows between the other two leads. The same is true of a thyristor: the gate controls the current that flows between the anode and the cathode. (It's worth noting that you can get thryistors with two or four leads, as well as three-lead ones. But we're keeping things simple
here, so we'll just talk about the most common variety.)
Transistors versus thyristors
If a transistor and a thyristor do the same job, what's the difference between them? With a transistor, when a small current flows into the base, it makes a larger current flow between the emitter and the collector. In other words, it acts as both a switch and an amplifier at the same time:

A similar thing happens inside a FET, except that we apply a small voltage to the gate to produce an electric field that helps a current flow from the source to the drain. If we remove the small current at the base (or gate), the large current immediately stops flowing from the emitter to the collector (or from the source
to the drain in a FET). 
Now often that's not what we want to happen. In something like an intruder alarm circuit (where maybe an
intruder steps on a pressure pad and the bells start ringing), we want the small current (activated by the pressure pad) to trip the larger current (the ringing bells) and for the larger current to keep on flowing even when the smaller current stops (so the bells still ring even if our hapless intruder realizes his mistake and steps back off the pad). In a thyristor, that's exactly what happens. A small current at the gate triggers a much larger current between the anode and the cathode. But even if we then remove the gate current, the larger current keeps on flowing from the anode to the cathode. In other words, the thyristor stays ("latches") on and remains in that state until the circuit is reset.
Where a transistor generally deals with tiny electronic currents (milliamps), a thyristor can handle real (electric) power currents (several hundred volts and 5–10 amps is typical). That's why we can use them in such things as factory power switches, speed controls for electric motors, household dimmer switches, car ignition switches, surge protectors, and thermostats. Switching time is practically instantaneous (measured in microseconds), and that useful feature, coupled with a lack of moving parts and high reliability, is why thyristors are often used as electronic (solid-state) versions of relays (electromagnetic switches).

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