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General Electric Device That Revolutionized Electrical Machines Is Now an IEEE Milestone

The silicon-controlled rectifier, or thyristor, can be found in flash bulbs, motors and manufacturing equipment

3 min read

Demonstration of prototype SCR in 1957
Photo: Schenectady Museum & Suits-Bueche Planetarium

Two C501\u2019s, a type of silicon-controlled rectifier, in parallel mounted in a G-7 heat exchangerTwo C501’s, a type of silicon-controlled rectifier, in parallel mounted in a G-7 heat exchangerPhoto: GE

THE INSTITUTEMore than 60 years after General Electric introduced the silicon-controlled rectifier, it is still a dominant control device in the power industry because of its efficiency. The SCR, also known as the thyristor, is a three-terminal p-n-p-n device that has an anode, a cathode, and a gate. It was introduced in 1957 and developed at a GE facility in Clyde, N.Y.

The invention of the SCR led to improvements in the control of the rectification, or conversion, of line voltage from AC to DC and became the basis of modern speed control in both AC and DC motors. The device’s application to motor control made possible the displacement of DC motors by the more efficient and reliable AC motors, particularly in trains, according to the Engineering and Technology History Wiki. SCRs also allowed for DC electrical transmission at much higher voltages and power levels than previously obtainable, says IEEE Life Senior Member Sreeram Dhurjaty, chair of the IEEE Power Electronics Society’s Rochester Section chapter.

The SCR was dedicated as an IEEE Milestone on 14 June. Administered by the IEEE History Center and supported by donors, the Milestone program recognizes outstanding technical developments around the world. The Rochester Section’s Power Electronics Society chapter was the sponsor.

Representatives from the IEEE Power Electronics Society unveil the Milestone plaque for the silicon-controlled rectifier on 14 June 2019. From left: Sreeram Dhurjaty, John Kassakian, Ahmed Elasser, James Mazzarella, Jos\u00e9 Moura, Frede Blaabjerg, and Gerard HurleyRepresentatives from the IEEE Power Electronics Society unveil the Milestone plaque for the silicon-controlled rectifier on 14 June 2019. From left: Sreeram Dhurjaty, John Kassakian, Ahmed Elasser, James Mazzarella, José Moura, Frede Blaabjerg, and Gerard HurleyPhoto: IEEE


Prior to 1955, triode vacuum tubes, which control the flow of electric current between electrodes, were used for machine control. They were difficult to operate and frequently failed in large machines.

To fix those issues, IEEE Member John Bardeen and Walter H. Brattain of Bell Laboratories in 1947 developed the point-contact transistor, an interconnection between two transistors.

According to IEEE Life Fellow Edward Owen, who wrote the article “SCR Is 50 Years Old” about the technology in the IEEE Industry Applications Magazine, the complexity of the point-contact transistor’s circuits and the fragile nature of the technology spurred GE power engineers Frank "Bill" Gutzwiller and Gordon Hall to develop a new technology in 1956 that would improve upon Bardeen and Brattain’s device. But, as IEEE Life Fellow Gerard Hurley, history chair of the IEEE Power Electronics Society, explained during the Milestone ceremony, the two engineers encountered several issues.

Hurley said Gutzwiller and Hall did not realize until later in their research that silicon, not germanium, was the appropriate semiconductor material to use for the SCR. Germanium has a smaller band gap—which means less energy is required to pull electrons into conduction. That makes it easier for the material to heat up and degrade.

Gutzwiller and Hall also encountered problems with false triggering. Heat alone could cause the device to turn on. The device also could be triggered by induced current, when the anode to cathode voltages rose too fast. Both instances could cause leakage, which could increase power consumption or result in complete circuit failure.

The first SCRs that Gutzwiller and Hall built could tolerate only low voltages, but refinements to the manufacturing process ultimately produced devices capable of handling voltages exceeding 10 kilovolts. Gutzwiller and Hall also designed a silicon-wafer bonding process capable of better accommodating thermally induced stresses.

Modern SCRs are used for AC power control for lights and motors, AC power switching circuits, and photographic flashes.

The SCR also made an impact on manufacturing, according to IEEE Life Fellow John Kassakian, founding president of the IEEE Power Electronics Society.

“The steel, electrochemical, automotive, and welding industries, among many others, benefited greatly by the improved efficiency, more precise control, and reduced cost made possible by the application of SCR-based equipment to their processes,” Kassakian said at the Milestone ceremony.

A plaque honoring the SCR was mounted at the entrance of the Advanced Atomization Technologies headquarters, in Clyde. AAT is a joint venture between GE Aviation and Parker Aerospace.

The plaque reads:

General Electric introduced the silicon-controlled rectifier (SCR), a three-terminal p-n-p-n device, in 1957. The gas-filled tubes used previously were difficult to operate and unreliable. The symmetrical alternating-current switch (TRIAC), the gate turn-off thyristor (GTO), and the large integrated gate-commutated thyristor (IGCT) evolved from the SCR. Its development revolutionized efficient control of electric energy and electrical machines.

This article was written with assistance from the IEEE History Center, which is funded by donations to the IEEE Foundation’sRealize the Full Potential of IEEE campaign.

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