Silicon Controlled Rectifier (SCR/Thyristor), 1957
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.
Street address(es) and GPS coordinates of the Milestone Plaque Sites
43.084319, -76.875856 124 Columbia St, Clyde, NY 14433, USA
Details of the physical location of the plaque
Location near the entrance, on the outside of the building, at eye level and fully visible to the public.
How the intended plaque site is protected/secured
Fully visible and accessible to the public.
Historical significance of the work
The invention of the SCR/Thyristor revolutionized the control of electric power conversion by replacing the gas-filled controlled rectifier tube (the thyratron) with a three-terminal solid-state device consistiing of an anode, a cathode and a gate. Unlike the two terminal diode in which current flows when a positive voltage is applied between the anode and the cathode, the SCR will not conduct with a positive anode-cathode voltage until a small voltage is applied between the gate and the cathode. Conduction is stopped by reducing the current to a low value known as the latching current.
The invention of the SCR/Thyristor led to dramatic efficiency and control improvements in the rectification of line voltages and is the basis of modern speed control of ac and dc motors. Its application to motor control had a substantial impact in electric traction, making possible the displacement of dc motors by the more efficient and reliaable ac motors, particlulaly in railroads. The SCR and its derivative, the GTO, have made possible HVDC transmission at much higher voltages and power levels than previously obtainable with mercury arc rectifiers and thyratrons.
The SCR has also had a dramatic impact on manufacturing.The steel, electrochemical, automotive and welding industries, among many others, benefited greaatly by the improved efficiency, more precise control, and reduced cost made possible by the application of SCR based equipment to their processes.
Features that set this work apart from similar achievements
Among similar achievements are the thyratron, the bipolar transistor (BJT) and the field effect transistor (FET), devices which were invented before the SCR. The SCR made the thyratron obsolete by manifesting it’s function in a smaller, more efficient, more reliable and more easily controlled solid state device. In addition, the SCR can exhibit power ratings far exceeding the thyratron’s. Besides its ability to control much higher power levels, the singular feature of the SCR that sets it apart from the BJT and FET is its bipolar voltage blocking capability. The BJT can support a voltage of only one polarity and thus cannot function in applications for which the SCR is appropriate. Due to materials issues the FET had a long gestation period before it became a practical device, and it wasn’t until the 1970’s that the vertical channel power MOSFET was developed. Like the BJT, the FET is incapable of bilateral voltage blocking and cannot be used in place of the SCR.
The Milestone was dedicated on 14 June 2019 by the IEEE Power Electronics Society and the IEEE Rochester Section.
 E. L. Owen, "SCR Is 50 Years Old", IEEE Industry Applications Magazine, Volume 13, Issue 6, Pages 6 - 10, 2007
 N. Holonyak, "The Silicon p-n-p-n Switch and Controlled Rectifier (Thyristor)",IEEE Transactions on Power Electronics, Volume 16, Issue 1, Pages 8 - 16, 2001
 Deepak Tiku,, "dc Power Transmission: Mercury-Arc to Thyristor HVdc Valves", IEEE Power and Energy Magazine, Volume 12, Issue: 2 Pages: 76 - 96, 2014
 M. Korytowski, "Uno Lamm the father of HVdc transmission", IEEE Power and Energy Magazine, Volume 15, Issue:3 Pages: 92-102, 2017
 GE SCR Manual, 5th Ed., GE Syracuse New York, 1972