Thyristor Semiconductor Products Page 69 Thyristor

2014 Littelfuse, Inc. Specifications are subject to change without notice. Revised: 12/14/14 Teccor brand Thyristors AN1007 against di/dt produced by the capacitor. Of course, it is most important that the capacitor be connected between proper terminals of Triac. For example, if the capacitor and series resistor are accidentally connected between the gate and MT2, the Triac will turn on from current produced by the capacitor, resulting in loss of control. For low current (mA) and/or highly inductive loads, it may be necessary to have a latching network (3.3 k + circuit shown in Figure AN1007.8 illustrates the additional latching network. 6 R in V cc 1 180 G 240 V ac MT2 MT1 2 180 0.1 F 3 4 5 0.047 F 3.3 k Load Figure AN1007.8 Optocoupler Circuit for Lower Current Inductive Loads (Triac or Alternistor Triac) In this circuit, the series gate resistors are increased to 180 each, since a 240 V line is applied. Note that the load is placed on the MT1 side of the power Triac to illustrate that load placement is not important for the circuit to function properly. Also note that with standard U.S. residential 240 V home wiring, both sides of the line are hot with respect to ground (no neutral). Therefore, for some 240 V line applications, it will be necessary to have a Triac switch circuit in both sides of the 240 V line input. If an application requires back-to-back SCRs instead of a Triac or alternistor, the circuit shown in Figure AN1007.9 may be used. R in V cc 1 G 120 V ac 2 3 0.1F Load 6 4 5 100 K A 100 G A K NS- SCR NS- SCR Figure AN1007.9 Optocoupled Circuit for Heavy-duty Inductive Loads All application comments and recommendations for optocoupled switches apply to this circuit. However, the snubber network can be applied only across the SCRs as shown in the illustration. The optocoupler should be chosen for best noise immunity. Also, the voltage rating of the optocoupler output Triac must be equal to or greater than the voltage rating of SCRs. Summary of Random Turn-on Relays As shown in Figure AN1007.10, if the voltage across the load is to be phase controlled, the input control circuitry must be synchronized to the line frequency and the trigger pulses delayed from zero crossing every half cycle. If the series gate resistor is chosen to limit the peak current through the opto-driver to less than 1 A, then on a 120 V ac line the peak voltage is 170 V; therefore, the resistor is 180 . On a 240 V ac line the peak voltage is 340 V; therefore, the resistor should be 360 . These gate pulses are only as long as the device takes to turn on (typically, 5 Triac or Alternistor MT2 0.1f 100 Load MT1 Hot Neutral 120/240 V ac G 180 for 120 V ac 360 for 240 V ac Input R in 1 6 5 4 3 2 Load could be here instead of lower location Figure AN1007.10 Random Turn-on Triac Driver Select the Triac for the voltage of the line being used, the current through the load, and the type of load. Since the Gpeak voltage of a 120 V ac line is 170 V, you would choose a 200 V (MIN) device. If the application is used in an electrically noisy industrial environment, a 400 V device should be used. If the line voltage to be controlled is 240 V ac with a peak voltage of 340 V, then use at least a 400 V rated part or 600 V for more design margin. Selection of the voltage rating of the opto-driver must be the same or higher than the rating of the power Triac. In electrically noisy industrial locations, the dv/dt rating of the opto-driver and the Triac must be considered. The RMS current through the load and main terminals of the Triac should be approximately 70% of the maximum be chosen to control a 1 A load due to low latching and holding current requirements. Remember that the case temperature of the Triac must be maintained at or below the current versus temperature curve specified on its data sheet. As with all semiconductors the lower the case temperature the better the reliability. Opto-driven gates normally do not use a sensitive gate Triac. The opto-driver can supply up to 1 A gate pulses and less sensitive gate Triacs have better dv/dt capability. If the load is resistive, it is acceptable to use a standard Triac. However, if the load is a heavy inductive type, then an alternistor Triac, or back-to-back SCRs as shown in Figure AN1007.9, is recommended. A series RC snubber network may or may not be necessary when using an alternistor Triac. Normally a snubber network is not needed when using an alternistor latching network as described in Figure AN1007.8 may be needed for low current load variations. Thyristors Used as AC Static Switches and Relays (continued)

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