Thyristor Semiconductor Products Page 31 Thyristor

2014 Littelfuse, Inc. Specifications are subject to change without notice. Revised: 12/14/14 Teccor brand Thyristors AN1003 A relaxation oscillator is the simplest and most common control circuit for phase control. Figure AN1003.7 illustrates this circuit as it would be used with a Thyristor. Turn-on of the Thyristor occurs when the capacitor is charged through the resistor from a voltage or current source until the breakover voltage of the switching device is reached. Then, the switching device changes to its on state, and the capacitor is discharged through the Thyristor gate. Trigger devices used are neon bulbs, unijunction transistors, and three-, four-, or five-layer semiconductor trigger devices. Phase control of the output waveform is obtained by varying the RC time constant of the charging circuit so the trigger device breakdown occurs at different phase angles within the controlled half or full cycle. Switching Device Voltage or Current Source Triac R C SCR Figure AN1003.7 Relaxation Oscillator Thyristor Trigger Circuit Figure AN1003.8 shows the capacitor voltage-time characteristic if the relaxation oscillator is to be operated from a pure DC source. 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 1 2 3 4 5 6 Time Constants Ratio of ( Capacitor Voltage Supply Source Voltage ) Figure AN1003.8 Capacitor Charging from DC Source Usually, the design starting point is the selection of a capacitance value which will reliably trigger the Thyristor when the capacitance is discharged. Trigger devices and Thyristor gate triggering characteristics play a part in the selection. All the device characteristics are not always completely specified in applications, so experimental determination is sometimes needed. Upon final selection of the capacitor, the curve shown in Figure AN1003.8 can be used in determining the charging resistance needed to obtain the desired control characteristics. Many circuits begin each half-cycle with the capacitor voltage at or near zero. However, most circuits leave a relatively large residual voltage on the capacitor after discharge. Therefore, the charging resistor must be determined on the basis of additional charge necessary to raise the capacitor to trigger potential. For example, assume that we want to trigger an S2010L the necessary SCR gate current with the trigger DIAC. Assume a 50 V dc power supply, 30 minimum conduction angle, and 150 maximum conduction angle with a 60 Hz input power source. At approximately 32 V, the DIAC triggers leaving 0.66 V BO of DIAC voltage on the capacitor. In order for DIAC to trigger, 22 V must be added to the capacitor potential, and 40 V additional (50-10) are available. The capacitor must be charged to 22/40 or 0.55 of the available charging voltage in the desired time. Looking at Figure AN1003.8, 0.55 of charging voltage represents 0.8 cycle at 60 Hz is 8.33 ms.) To obtain this time delay: 6.92 ms = 0.8 RC RC = 8.68 ms 8.68 x 10 -3 then, R = = 86,000 0.1 x 10 -6 To obtain the minimum R (150 conduction angle), the delay is 30 or (30/180) x 8.33 = 1.39 ms 1.39 ms = 0.8 RC RC = 1.74 ms 1.74 x 10 -3 R = = 17,400 0.1 x 10 -6 Using practical values, a 100 k potentiometer with up to 17 k minimum (residual) resistance should be used. Similar calculations using conduction angles between the maximum and minimum values will give control resistance versus power characteristic of this circuit. Triac Phase Control The basic full-wave Triac phase control circuit shown in Figure AN1003.9 requires only four components. Adjustable resistor R 1 and C 1 are a single-element phase- shift network. When the voltage across C 1 reaches breakover voltage (V BO ) of the DIAC, C 1 is partially discharged by the DIAC into the Triac gate. The Triac is then Control Characteristics Phase Control Using Thyristors (continued)

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