When a switch turns on, it behaves according to the characteristics and properties of the switching element. Therefore, any element chosen for use in a switching system should approach the ideal switch model.
The figure below illustrates the schematic representations of the four types of electrically controllable switching elements. Because of the advancement of relay designs, the relay family of switching elements offers the best overall performance at a reasonable cost.
Switching Elements
Each of the four types shown above has advantages and disadvantages. For Automatic Testing purposes however, the Relay offers the best performance. A FET switch is the fastest switch of the group, although it has a high ON resistance and the switchable signal level is typically limited. The Triac is a good solution for high-current, high-voltage switching but is rarely being used in ATE. The same holds true for the Solenoid Switch, although some applications require its use. Since the relay is the most common switching type used in ATE system, the rest of this section will focus on types of relays and their advantages.