SCS-Silicon Controlled Switch

Silicon controlled switch (SCS), like the SCR, is a unilateral, four layer three junction P-N-P-N silicon device with four electrodes namely cathode C, cathode gate Gx, anode gate G2 and the anode A, as shown in figure. Infact, the SCS is a low power device compared with the SCR. It handles currents in milli amperes rather than amperes. SCS differs from an SCR in the following aspects. It has an additional gate—the anode gate.It is physically smaller than SCR.It has smaller leakage and holding currents than SCR.It needs small triggering signals. It gives more uniform triggering characteristics from sample to…

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GCS-Gate Controlled Switch

As mentioned earlier, low-current drop out is the normal way in which the SCR is turned off. Gate-controlled switch is designed for easy opening with a reverse-biased trigger. A gate controlled switch (GCS) is closed by a positive trigger and opened by a negative trigger (or by low-current drop out). Gate-controlled switch circuit is shown in figure. Each positive trigger closes the GCS, and each negative trigger opens it. Thus a square-wave output is obtained, as shown in the figure. The most obvious advantage of GCS over the SCR or SCS is the fact that it can be turned on…

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Diac Applications

The diacs, because of their symmetrical bidirectional switching characteristics, are widely used as triggering devices in triac phase control circuits em­ployed for lamp dimmer, heat control, universal motor speed control etc. Although a triac may be fired into the conducting state by a simple resistive triggering circuit, but triggering devices are typically placed in series with the gates of SCRs and triacs as they give reliable and fast triggering. Diac is the most popular triggering device for the triac. This is illustrated in the following applications. 1. Triac Lamp Dimmer Circuit. The circuit for a triac controlled by an R-C…

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Diac

Introduction to Diac-Operation and Construction A diac is an important member of the thyristor family and is usually employed for triggering triacs. A diac is a two-electrode bidirectional avalanche diode which can be switched from off-state to the on-state for either polarity of the applied voltage. This is just like a triac without gate terminal, as shown in figure. Its equivalent circuit is a pair of inverted four layer diodes. Two schematic symbols are shown in figure. Again the terminal designations are arbitrary since the diac, like triac, is also a bilateral device. The switching from off-state to on-state is achieved by simply exceeding…

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SCR Applications

The ability of an SCR to control large currents to a load by means of small gate current makes the device very useful in switching and control applications. A few of the possible applications for the SCR are listed in the introduction to SCR blog post. Here we will consider six applications of SCR like power control, switching, zero-voltage switching, over-voltage protection, pulse circuits and battery charging regulator. 1. Power Control. Because of the bistable characteristics of semiconductor devices, whereby they can be switched on and off, and the efficiency of gate control to trigger such devices, the SCRs are…

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Full wave rectifier using SCR

How to make a full wave rectifier using SCR ? For full-wave rectification two SCRs are connected across the centre taped secondary, as shown in figure-a. The gates of both SCRs are supplied from two gate control supply circuits. One SCR conducts during the positive half cycle and the other during the negative half cycle and thus unidirectional current flows in the load circuit. The main advantage of this circuit over ordinary full-wave rectifier circuit is that the output voltage can be controlled by adjusting the gate current. Now if the supply voltage v = VMAX sin θ and the…

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SCR as Half Wave Rectifier

How SCR functions as a Half Wave Rectifier ? SCRs are very useful in ac circuits where they may serve as rectifiers whose output current can be controlled by controlling the gate current. An example of this type of application is the use of SCRs to operate and control dc motors or dc load from an ac supply. The circuit using an SCR as an half-wave rectifier is shown in figure. The ac supply to be rectified is applied to the primary of the transformer ensuring that the negative voltage appearing at the secondary of the transformer is less than…

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SCR used as a Switch

How an SCR functions as a switch ? We have seen that SCR operates either in on-state or in off-state and no other state in between, that is SCR behaves like a mechanical switch. As such it is called electronic switch. An SCR has following advantages over a mechanical switch or electro-mechanical relay: Noiseless operation owing to absence of moving parts. Very high switching speed (say 109 operations per second). High efficiency. Low maintenance. Small size and trouble free service for long period. Large control current range (say from 30 A to 100 A) with small gate current of few…

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SCR Control circuits

90° Phase Control of SCR. In ac circuits, the SCR can be turned on by the gate at any angle a with respect to the applied voltage. This angle α is called the firing angle. Power control is obtained by varying the firing angle and this is known as phase control. In the phase-control circuit given in fig. 1, the gate triggering voltage is derived from the ac supply through resistors R1, R2 and R3. The variable resistance R2 limits the gate current during positive half cycles of the supply. If the moving contact is set to the top of resistor R2, resistance…

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SCR Protection Circuits

How to protect an SCR using protection circuits ? SCRs are sensitive to high voltage, over-current, and any form of transients. For satisfactory and reliable operation they are required to be protected against such abnormal operating conditions. Because of complex and expensive protection, usually some margin is provided in the equipment by selecting devices with ratings higher (3 or 4 times higher) than those required for normal operation. But it is always not economi­cal to use devices of higher ratings, hence their protection is imperative. Over-voltage Protection. High forward voltage protection is inherent in SCRs. The SCR will breakdown and…

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