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Thyristors & Triacs
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About Thyristors & Triacs
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What was thyristor and triacs?
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What types and types of thyristors and triacs are available?
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Purchase criteria for thyristors & triacs - what's important?
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Our practical tip: Observe specifications
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FAQ - Frequently Asked Questions about Thyristors and Triacs
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Conclusion: How to buy the right thyristor or triac
What was thyristor and triacs?
Thyristors and triacs are electronic components of power electronics. The structure of a thyristor consists internally of several PN transitions, similar to a transistor.
Like a conventional diode, the thyristor has a blocking and a forward direction as well as an anode and a cathode connection. In addition, there is a third connection terminal, the gate.
The gate can be used to switch the thyristor to the conductive state by means of a positive current pulse. In addition to the control electrode (the gate), the transition from the blocking state to the forward state can also be achieved by exceeding the so-called ignition voltage in the forward direction.
A triac (short-wave from the English triode for alternating current) basically represents an anti-parallel circuit of two thyristors. This makes it possible to switch alternating currents, while a single thyristor always only allows current to pass in one direction.
Like a thyristor, a triac is equipped with three connections in the form of anode, cathode and gate and is ignited by the control electrode or by exceeding the ignition voltage, i.e. switched to conductive.
Both components have the common requirement that the current flow is maintained after the anode-cathode line has been ignition, until the so-called holding current is undershot. This is usually done in AC voltage applications in voltage zero crossing, so that an automatic deletion of the conducting line takes place. Thyristors and triacs are used, for example, for current rectification, for phase control ("dimmer"), as power controllers, electronic switches and in frequency converters.
What types and types of thyristors and triacs are available?
Both components are available in many different housing designs. While types for smaller currents usually have a plastic housing, more powerful versions either have a housing with cooling fin for screwing on or are integrated in a solid compact housing with metallic base plate.
There are also versions with a molded threaded bolt for screwing into metric threaded holes. This allows a good thermal coupling for safe removal of the electrical power loss.
Power regulators are triacs with an electronic circuit integrated in the housing, which permit a direct power control of connected consumers via an additional potentiometer or a control voltage.
Purchase criteria for thyristors and triacs - what's important?
When replacing defective components, it is not only necessary to ensure that the maximum permissible values for current and voltage are sufficient; the specifications for ignition voltage, ignition current and holding current must also match the previously used component. Also important are the mechanical compatibility, housing dimensions, grid dimension and, if necessary, thread type. Consideration also deserves the insulation voltage. This should be well above the maximum expected values to ensure high reliability and electrical safety.
For new designs in surface mounting technology (SMT), both component versions have been available as SMD components for some time. If higher operating frequencies play a role, the values for the so-called activation time ("tq") are interesting. If this time is not reached, the electrical conductivity is not deleted in the barrier layer, so that a shorter frequency period provides a permanent conductivity and no shutdown is possible.
Our practical tip: Observe specifications
Thyristors and triacs are relatively robust electronic components. However, the common thing is that they must be operated within their specifications with regard to the maximum permissible slew rate of the load current in order to avoid damage. The same applies to the ignition current: If it is too low, the switching of the entire semiconductor crystal from the blocking state to the conductivity is too slow and the entire load current flows – just as with too short rise times – initially only in partial areas of the silicon surface. This can easily cause local overheating, which can irreparably damage the component. Therefore, a sufficient control voltage must always be ensured.
FAQ - Frequently Asked Questions about Thyristors and Triacs
What was the difference between a triac and a diac?
While the triac has a control port - the gate - a diac resembles a two-direction diode and is not separately controllable. The line between the two connections becomes electrically conductive only when the applied voltage exceeds the specified breakdown voltage. Due to the anti-parallel design, AC voltage can also be switched.
Why do you need thyristors at all when triacs seem to be more universal?
Technically speaking, thyristors can be produced for significantly higher load currents than triacs. They can be used to switch currents up to more than 10,000 ampere, while triacs are available for currents from the one- to three-digit ampere range.
Conclusion: How to buy the right thyristors or triac
Thyristors are particularly suitable as rectifiers for high currents. If it is necessary, for example, for switching very high currents, two antiparallel thyristors fulfill the functionality of a triac. Fully assembled power regulators with a minimum of external wiring enable a simple power or speed control of electrical consumers such as lamps, heaters or motors. However, triacs and thyristors with 400 V dielectric strength, greater safety and reliability for short voltage peaks are also suitable for mains voltage applications in the 230 V AC power supply.