Thyristors
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About Thyristors
The term thyristor is composed of the two terms thyratron and transistor. A thyristor is used in electrical engineering as a controllable diode. It is controlled by a voltage that can be applied to the gate connection. You can find more information here in our guide.
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Structure and function of thyristors
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What was to be considered when buying thyristors?
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Conclusion: New possibilities
Structure and function of thyristors
Figure: Thyristors designed for 7.5 Ampere with feedthrough hole
Thyristors are similar to diodes in the design and are shown in similar diagrams. A diode is a simple semiconductor component that blocks the current in one direction of flow and allows it to flow in another direction from a low voltage.
At one end there is the positively charged anode at one diode, at the other end there is the negatively charged cathode. The current can only flow from the anode to the cathode, in the other direction the current is blocked up to a certain breakdown voltage. This is usually in the three-digit voltage range.
A thyristor adds controllability to this principle. While a diode has only two connections – anode and cathode – the component still has a third connection – visible on the three contacts. The third contact is the so-called gate connection. This is used to control the thyristor.
In the basic state, the thyristor is locked in both directions and no current flows. If a certain voltage is applied to the gate connection, it is opened in one direction. Now, like a diode, the current can flow in one direction, while the other flow direction remains blocked.
As long as a certain minimum current flows through the thyristor, the so-called holding current, the thyristor remains conductive, even if no voltage is present at the gate. Thus, a short firing pulse is sufficient for opening. As soon as the holding current is undershot or the polarity of the current is reversed, the thyristor blocks again in both directions.
A thyristor consists of a semiconductor material such as silicon. The exact material used depends on the manufacturer. It is divided into four layers doped with pnpn. This results in three pn transitions.
The gate connector is attached to the third layer, i.e. the last cathode-side p-layer. If a voltage is present between the anode and cathode and voltage is also applied to the gate connection, the thyristor switches to the low-resistance state and becomes conductive.
Special designs of the thyristor are, for example, triacs. In a triac, two thyristors are switched anti-parallel. This means that both components are used in parallel but with different polarity. Thus it is possible to control the current in both directions and thus enables the phase angle control for alternating current.
Another specification, the GTO thyristor, can be manually blocked with a negative current at the gate. With a normal thyristor, this would lead to the destruction of the component.
Figure: Thyristor with a starting voltage of 2 V and a voltage of 1200 V.
What was to be considered when buying thyristors?
Thyristors are used as controllable diodes. The type of construction you choose depends on your circuit and the application area in which it is to be used. In order to find the right component for your application, it is worth paying attention to some key data.
You can already find some important key data in the technical data that you can find on each article page at Conrad. Under "Documents & Downloads" you will find the data sheet of the component. It lists all the important data and properties of the thyristor so that you can select the correct one.
The ignition voltage indicates the voltage that must be applied at least at the gate in order to open the thyristor. For this purpose, the thyristor also requires a small current, which is the ignition current.
Figure: Thyristor with solder connection and screw fixing for 800 V voltage
If the thyristor is open, it is limited in terms of the maximum current that it can conduct. This is referred to as IT in the data sheet and is expressed in several values.
The peak value indicates the short-term permissible current, as it can occur at a current peak, the average value is the maximum value. The RMS value ("Root Mean Square") indicates the maximum possible permanent current of the square by means. For sine alternating voltage, this is about 70% of the maximum value.
The current is limited because the component heats up during operation and only remains operational up to a certain temperature. Therefore, the maximum operating and ambient temperature should not be exceeded.
The breakdown voltage, also specified as UDRM, specifies the value at which the current breaks in reverse direction, similar to a diode, despite the blocked thyristor.
You can buy or build a triac directly as a component from Conrad by switching two conventional thyristors anti-parallel. If you would like to buy one or more thyristors, you should be aware of their area of application. The circuit must provide a sufficient ignition voltage and at the same time must not exceed the maximum current, otherwise there is a risk of overheating of the component.
Conclusion: New possibilities
Thyristors are diodes that can be actively blocked and opened using the control current. This opens up new possibilities for current control, especially for applications with alternating current. You can also build a triac yourself that allows you to perform a phase angle on alternating current.