When transistors are used as switches, a diode can be mounted across the transistor to prevent damage from high-voltage spikes (transients).įigure 1. Transistors are used to switch low-level DC only. Transistors have a very high resistance when open and very low resistance when closed. Transistors can be switched on and off quickly. Transistors may be NPN or PNP transistors. Transistors as a SwitchĪ transistor is a three-terminal device that controls current through the device depending on the amount of voltage applied to the base. Solid-state switches have a switch life in the billions of cycles while mechanical switches have a switch life of about 200,000 cycles, making solid-state switches the standard for most high-volume switching applications. Triacs, diacs, UJTs, and SCRs operate only as switches and may be used in a variety of switching applications. Triacs and SCRs are control devices.ĭiacs and UJTs form the triggering circuits for triacs and SCRs. Triacs, diacs, and unijunction transistors (UJTs), along with SCRs, are often found in the same circuitry. Solid-state switches include transistors, silicon-controlled rectifiers (SCRs), triacs, diacs, and unijunction transistors (UJTs). However, solid-state switches must be properly selected and applied to prevent potential problems. The advantages of solid-state switches include fast switching, no moving parts, long life, and the ability to be interfaced with electronic circuits (PLCs and PCs). Solid-state switches can be used in most motor control applications. Solid-state switches are electronic devices that have no moving parts (contacts). It is important to understand solid-state switches since they are being used more often to replace mechanical switches in circuits and switching applications. Solid-state switches have the disadvantages of being able to switch only AC or DC, being harder to understand, and requiring more knowledge of the circuit and components when a technician troubleshoots them. Solid-state switches have a much longer operating life, can control the amount of voltage/current between being totally open or closed, and do not produce arcing since there are no contacts. Solid-state switches such as silicon-controlled rectifiers (SCRs) and triacs can replace mechanical switches. Mechanical switches have the disadvantages of having a much shorter operating life than solid-state switches and producing arcing at the contacts, which can be dangerous in some applications. Mechanical switches have the advantages of being able to switch either AC or DC, having only two operating conditions (open or closed), and being easy to understand and troubleshoot.
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