Introduction

In control applications for motors the choice of the solid-state switch to be used is always very important for the designer: cost, reliability, ruggedness, and ease of drive must always be kept in mind. This is especially important in motor control circuits where the designer has to optimize the circuitry for controlling the motors.

In the large family of electronic switches the thyristor must be considered as a low cost and powerful device for motor applications. Thyristors can take many forms, but they have certain features in common. All of them are solid-state switches which act as open circuits capable of withstanding the rated voltage until triggered. When triggered thyristors become low-impedance current paths and remain in that condition (i.e. conduction) until the current either stops or drops below a minimum value called the holding level. Once a thyristor has been triggered, the trigger current can be removed without the device turning off.

Because Thyristors are reliable solid-state switches they have many applications, especially as controls. A useful application of triac is as a direct replacement for an ac mechanical relay. In this application the triac furnishes on-off control and the power regulating ability of the triac is used. The control circuitry for this application is usually very simple, consisting of a source for the gate signal and some type of small current switch, either mechanical or electrical. The gate signal can be obtained from a separate source or directly from the line voltage at terminal MT2 of the triac.

One of the most common uses for thyristors is to control ac loads such as electric motors. This can be done either by controlling the part of each ac cycle when the circuit will conduct current (phase control) or by controlling the number, or cycles, per time period that the current conducts (cycle control). In addition, thyristors can serve as the basis of relaxation oscillators for timers and other applications.

Click here for complete Tech Note in .pdf format

Analog Main | Product of the Week | Columns | Editorial | Tech Notes

		Click here to get your listing up.