LASCR-Light Activated SCR

Light activated SCR (LASCR) or a Photo SCR  is just an ordinary SCR except that it can also be light triggered. Most LASCRs also have a gate terminal for being triggered by an elec­trical pulse just as a conventional SCR. The basic construction of an LASCR is shown in figure. The schematic symbols most commonly used for the LASCR are shown in figure.  Some LASCRs have clear windows in their cases so that light sources from other devices can be cou­pled to them. Many have the light source encapsu­lated in the same package so that a relay is formed. When…

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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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SCR-Series and Parallel connections

How to make series and parallel connections of an SCR ? In many power control applications the required voltage and current ratings exceed the voltage and current that can be provided by a single SCR. Under such situations the SCRs are required to be connected in series or in parallel to meet the requirements. Sometimes even if the required rating is available, multiple connections are employed for reasons of economy and easy availability of SCRs of lower ratings. Like any other electrical equipment, characteristics/properties of two SCRs of same make and ratings are never same and this leads to certain…

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SCR Current Ratings

Current ratings of an SCR The current carrying capability of an SCR is solely determined by the junction temperature. Except in case of surge currents, in no other case the junction temperature is permitted to exceed the permissible value. Some of the current ratings used in industry to specify the device are given below. (i) Forward Current Rating. The maximum value of anode current, that an SCR can handle safely (without any damage), is called the forward current rating. The usual current rating of SCRs is from about 30 A to 100 A. In case the current exceeds the forward…

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Voltage ratings of an SCR

SCR Voltage Ratings (i) Breakover Voltage. The minimum forward voltage, when the gate is open, at which SCR starts conducting heavily (that is turned-on) is called the breakover voltage. To specify this value, the gate is in the open-circuit condition and the junction temperature is at its maxi­mum permitted value, although VFB0 is still a function of dv/dt. Though the SCR can be turned-on, with gate open, by making the supply voltage equal to breakover voltage, but in practice, the SCR is operated with supply voltage less than breakover voltage and it is turned-on by applying a small voltage (typically…

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