Understanding the key performance parameters of thyristor overvoltage protection devices will help in selecting the right SIDACtor device for a particular application.  Most of the parameters that will be discussed appear on what is known as a VI curve. This is the image generated by a specialized oscilloscope called a curve tracer. The horizontal axis shows the voltage applied to the device with zero volts being where the two axis cross. The vertical axis shows the resulting current through the SIDACtor device with zero current also being where the two axis cross. So it can be seen that negative voltages create negative currents in the device in the lower left quadrant while positive voltages force positive currents as shown in the upper right quadrant. Most SIDACtor devices are symmetric meaning that the upper right and lower left quadrants are mirror images of each other. This discussion will focus on the upper right quadrant of this graph.