Thyristor Semiconductor Products Page 68 Thyristor

2014 Littelfuse, Inc. Specifications are subject to change without notice. Revised: 12/14/14 Teccor brand Thyristors AN1007 With the thermostat open, the SCR will trigger each half cycle and deliver power to the heater load. When the thermostat closes, the SCR can no longer trigger and the heater shuts off. Maximum current through the thermostat in the closed position is less than 250 A rms. Figure AN1007.5 shows an all solid state, optocoupled, normally closed switch circuit. By using a low voltage SBS triggering device, this circuit can turn on with only a small delay in each half cycle and also keep gating power low. When the optocoupled transistor is turned on, the gate drive is removed with only a few milliamps of bypass current around the Triac power device. Also, by use of the be used to control various types of high current loads. Load Triac 51 k 0.02 F (4) IN4004 PS2502 + 120 V ac Q4008L4 BS08D Figure AN1007.5 Normally Closed Switch Circuit Optocoupled Driver Circuits Random Turn-on, Normally Open Many applications use optocouplers to drive Thyristors. The combination of a good optocoupler and a Triac or alternistor makes an excellent, inexpensive solid state relay. Application information provided by the optocoupler manufacturers is not always best for application of the power Thyristor. Figure AN1007.6 shows a standard circuit for a resistive load. R in V CC 1 6 4 180 G R L 120 V 60 Hz MT2 MT1 Hot Neutral Load Could Be in Either Leg 2 or Alternistor Triac) A common mistake in this circuit is to make the series gate resistor too large in value. A value of 180 is shown in a typical application circuit by optocoupler manufacturers. The 180 is based on limiting the current to 1 A peak at the peak of a 120 V line input for Fairchild and Toshiba optocoupler I TSM rating. This is good for protection of the optocoupler output Triac, as well as the gate of the power Triac on a 120 V line; however, it must be lowered if a 24 V line is being controlled, or if the R L (resistive load) is 200 W or less. This resistor limits current for worst case turn-on at the peak line voltage, but it also sets turn-on point (conduction angle) in the sine wave, since Triac gate current is determined by this resistor and produced from the sine wave voltage as illustrated in Figure AN1007.2. The load resistance is also important, since it can also limit the amount of available Triac gate current. A 100 gate resistor would be a better choice in most 120 V applications with loads greater than 200 W and optocouplers from Quality Technologies or Vishay with optocoupler output Triacs that can handle 1.7 A PK (I TSM rating) for a few microseconds at the peak of the line. For loads less than 200 W, the resistor can be dropped to 22 . Remember that if the gate resistor is too large in value, the Triac will not turn on at all or not turn on fully, which can cause excessive power dissipation in the gate resistor, causing it to burn out. Also, the voltage and dv/dt rating of the optocoupler's output device must be equal to or greater than the voltage and dv/ dt rating of the Triac or alternistor it is driving. Figure AN1007.7 illustrates a circuit with a dv/dt snubber network included. This is a typical circuit presented by optocoupler manufacturers. R in V CC 1 6 4 100 G Neutral 2 100 Z L 120 V 60 Hz MT2 MT1 Hot 0.1 F C 1 or Alternistor Triac) dt capability to either the optocoupler output Triac or the power Triac. The sum of the two resistors then forms the Triac gate resistor. Both resistors should then be standardized and lowered to 100 . Again, this sum resistance needs to be low, allowing as much gate current as possible without exceeding the instantaneous current rating of the opto output Triac or Triac gate junction. By having 100 for current limit in either direction from the capacitor, the optocoupler output Triac and power Triac can be protected Thyristors Used as AC Static Switches and Relays (continued)

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