An Introduction to Thyristors and Power Control

Often confused with transistors and sometimes referred to as silicon controlled rectifiers (SCRs), thyristors perform vital functions in AC circuits and are used extensively in industry due to their robustness and reliability.


Thyristors were invented in 1957 but are still used today to switch electric currents ranging from a few milliamps at tens of volts to thousands of amps at thousands of volts.

At CD Automation, we supply a full range of thyristors for all kinds of applications.




What is a Thyristor.

Thyristors are made up of four layers of semiconductors, rather than three like a transistor, and the most common variety have three connectors like a transistor. Current is controlled by a gate between the positive and negative terminals. A small current at the gate triggers the larger current flowing between the terminals. The gate can be switched on and off very quickly and there are no moving parts involved which gives the thyristor its robust qualities. They can’t be used as amplifiers like transistors can but can control much higher electrical currents. As they conduct current in one direction only, two need to be used back to back to work in AC circuits.


How can they be used?

In the industrial world, their uses include extruders, ovens, dryers, curing equipment, environmental chambers and electric furnaces. They are the preferred power control method for electrical heating applications because of their high tolerances. In many of these applications, the switching frequency is so high that although electromechanical contactors can do the job, they wear out so quickly that a thyristor is a far better choice. Some applications, such as transformer loads, require phase angle control or delayed triggering so at CD Automation, we offer thyristor power controllers with a wide range of firing modes to match almost every electrical switching application.

“As the temperature rises to about 500 degrees centigrade for optimal operation, the resistance of the element also increases. We are only talking about 250 milliseconds, but in that time you can do a lot of damage with high inrush current into complex heating elements such as silicon carbide or Kanthal.

What are the benefits of using them?

Accurate, reliable and long lasting power control is a must for many industries as it protects components, leads to the most efficient working and saves energy. Thyristors are particularly good for this as they come in such a variety of sizes, capacities and with different functions so are suitable for a wide range of applications. Their lack of moving parts and switching speed mean they are reliable as well as efficient with minimum power loss during switching. This also means less maintenance and a faster more dynamic response in the systems which use them. Size is also a bonus with thyristors as they are very small so don’t require much space, have low power ratings, fewer handling issues and lower transportation costs.

CD Automation have a wealth of experience in supplying thyristors so can advise on exactly the right one for your process as well as providing fast and reliable support to help you install them and use them in the best way possible. Call 01323 811100 to speak to one of our team about the most effective thyristor for your process.