Thyristor Semiconductor Products Page 66 Thyristor

2014 Littelfuse, Inc. Specifications are subject to change without notice. Revised: 12/14/14 Teccor brand Thyristors AN1006 Introduction Since the SCR and the Triac are bistable devices, one of their broad areas of application is in the realm of signal and power switching. This application note describes circuits in which these Thyristors are used to perform simple switching functions of a general type that might also be performed non-statically by various mechanical and electromechanical switches. In these applications, the Thyristors are used to open or close a circuit completely, as opposed to applications in which they are used to control the magnitude of average voltage or energy being delivered to a load. These latter types of applications are described in Static AC Switches Normally Open Circuit The circuit shown in Figure AN1007.1 provides random loads and is ideal for applications with a high-duty cycle. It eliminates completely the contact sticking, bounce, and wear associated with conventional electromechanical relays, contactors, and so on. As a substitute for control relays, Thyristors can overcome the differential problem; that is, the spread in current or voltage between pickup and dropout because Thyristors effectively drop out every half cycle. Also, providing resistor R 1 is chosen correctly, the circuits are operable over a much wider voltage range than is a comparable relay. Resistor R 1 is provided to limit gate current (I GTM ) peaks. Its resistance plus any contact resistance (R C ) of the control device and load resistance (R L ) should be just greater than the peak supply voltage divided by the peak gate current rating of the Triac. If R 1 is set too high, the Triacs may not trigger at the beginning of each cycle, and phase control of the load will result with consequent loss of load voltage and waveform distortion. For inductive loads, an RC snubber circuit, as shown in Figure AN1007.1, is required. However, a snubber circuit is not required when an alternistor Triac is used. Figure AN1007.2 illustrates an analysis to better understand a typical static switch circuit. The circuit operation occurs when switch S 1 is closed, since the Triac Q 1 will initially be in the blocking condition. Current flow will be through load R L , S 1 , R 1 , and gate to MT1 junction of the Thyristor. When this current reaches the required value of I GT , the MT2 to MT1 junctions will switch to the conduction state and the voltage from MT2 to MT1 will be V T . As the current approaches the zero crossing, the load current will fall below holding current turning the Triac Q 1 device off until it is refired in the next half cycle. Figure AN1007.3 illustrates the voltage waveform appearing across the MT2 to MT1 terminals of Q 1 . Note that the maximum peak value of current which S 1 will carry would be 25 mA since Q 1 has a 25 mA maximum I GT rating. Additionally, no arcing of a Thyristors Used as AC Static Switches and Relays current value greater than 25 mA when opening S 1 will occur when controlling an inductive load. It is important also to note that the Triac Q 1 is operating in Quadrants I and III, the more sensitive and most suitable gating modes for Triacs. The voltage rating of S 1 (mechanical switch or reed switch) must be equivalent to or greater than line voltage applied. Load R L R 1 100 R 2 100 S 1 Control Device Reed Switch For Inductive Loads C 1 0.1 F Triac R 1 2 V (R L + R C ) Where I GTM is Peak Gate Current Rating of Triac I GTM V RMS Figure AN1007.1 Basic Triac Static Switch MT1 I GT MT2 AC Voltage Input 120 V rms, 60 Hz V IN R L S 1 I GT V GT Q 1 + - Load R 1 G Q4008L4

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