Thyristor Semiconductor Products Page 70 Thyristor

2014 Littelfuse, Inc. Specifications are subject to change without notice. Revised: 12/14/14 Teccor brand Thyristors AN1007 Zero Crossing Turn-on, Normally Open Relay Circuits When a power circuit is mechanically switched on and off mechanically, generated high-frequency components are generated that can cause interference problems such as RFI. When power is initially applied, a step function of voltage is applied to the circuit which causes a shock excitation. Random switch opening stops current off, again generating high frequencies. In addition, abrupt current interruption in an inductive circuit can lead to high induced- voltage transients. The latching characteristics of Thyristors are ideal for eliminating interference problems due to current interruption since these devices can only turn off when the on-state current approaches zero, regardless of load power factor. On the other hand, interference-free turn-on with Thyristors requires special trigger circuits. It has been proven experimentally that general purpose AC circuits will generate minimum electromagnetic interference (EMI) if energized at zero voltage. The ideal AC circuit switch, therefore, consists of a contact which closes at the instant when voltage across it is zero and opens at the instant when current through it is zero. For applications that require synchronized zero-crossing turn-on, the illustration in Figure AN1007.11 shows a circuit which incorporates an optocoupler with a built-in zero- crossing detector R in V cc 1 120 V ac MT2 MT1 2 3 Load 6 0.1 F 4 5 Hot Neutral Zero Crossing Circuit G 100 22 Figure AN1007.11 Optocoupled Circuit with Zero-crossing Turn- on (Triac or Alternistor Triac) Also, this circuit includes a dv/dt snubber network connected across the power Triac. This typical circuit illustrates switching the hot line; however, the load may be connected to either the hot or neutral line. Also, note that the series gate resistor is low in value (22 ), which is possible on a 120 V line and above, since zero-crossing turn-on is ensured in any initial half cycle. Zero Voltage Switch Power Controller The UAA2016 (at is designed to drive Triacs with the Zero Voltage technique which allows RFI- free power regulation of resistive loads. Operating directly on the AC power line, its main application is the precision regulation of electrical heating systems such as panel heaters or irons. It is available in eight-pin I.C. package variations. A built-in digital sawtooth waveform permits proportional temperature regulation action over a 1 C band around temperature reduction function, and for security a sensor failsafe inhibits output pulses when the sensor connection is broken. Preset temperature (in other words, defrost) application is also possible. In applications where high hysteresis is needed, its value can be adjusted up to 5 C around the set point. All these features are implemented with a very low external component count. Triac Choice and R out Determination The power switching Triac is chosen depending on power through load and adequate peak gate trigger current. The illustration in Figure AN1007.12 shows a typical heating control. Thyristors Used as AC Static Switches and Relays (continued)

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