The diacs, because of their symmetrical bidirectional switching characteristics, are widely used as triggering devices in triac phase control circuits emÂployed 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 […]
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
Applications of Triac Next to SCR, the triac is the most widely used member of the thyristor family. In fact, in many of control applications, it has replaced SCR by virtue of its bidirectional conductivity. Motor speed regulation, temperature control, illumination control, liquid level control, phase control circuits, power switches etc. are some of its
Characteristics of  Triac Typical V-I characteristics of a triac are shown in figure. The triac has on and off state characteristics similar to SCR but now the char acteristic is applicable to both positive and negative voltages. This is expected because triac consists of two SCRs connected in parallel but opposite in direc tions. MT2
Introduction to Triac-Its construction and Operation The triac is another three-terminal ac switch that is triggered into conduction when a low-energy signal is applied to its gate terminal. Unlike the SCR, the triac conducts in either direction when turned on. The triac also differs from the SCR in that either a positive or negative gate
Description. Here is the circuit diagram of a very versatile emergency lamp that can be operated in two modes (flasher and dimmer). The circuit is based on the dual op-amp IC LM358.The dimmer and flasher mode can be selected by using the switch S1. In the dimmer mode the output of IC1b will be pulses
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
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
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
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
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.
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
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
Description. This is the circuit diagram of a very simple and cheap peak detector circuit. This circuit operates from 5V DC and can detect signals up to 150 KHz. Dual comparator IC LM393 is the heart of this circuit. First comparator IC1a is used for detecting the peak of the input signal. Second comparator IC1b
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)
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 maxiÂmum permitted value, although VFB0 is still a
SCR-Ratings and Specifications The performance of an SCR is affected by the junction temperature, because of change in carrier densities in the four layers and the junction temperature naturally depends upon the internal power losses of the device and the efficiency of heat transfer mechanism. The factors contributing toward rise in junction temperature are on-state
Turning-off Methods of an SCR As already mentioned in previous blog post, once the  SCR is fired, it remains on even when triggering pulse is removed. This ability of the SCR to remain on even when gate current is removed is referred to as latching. So SCR cannot be turned off by simply removing the
How to turn ON an SCR ? As mentioned earlier, the SCR can be switched on either by increasing the forward voltage beyond forward break over voltage VFB0 or by applying a positive gate signal when the device is forward biased. Of these two methods, the latter, called the gate-control method, is used as it
Turn-on and turn-off characteristics of an SCR are called the dynamic characteristics of the SCR. (a)   Turn-On CharacterÂistic. The turn-on time characÂteristic shows the variation of current and voltage during turn-on. Turn-on time is defined as the time from the initiation of triggering, when the SCR offers infinite impedance to the flow of anode current, to
As already mentioned, the SCR is a four-layer device with three terminals, namely, the anode, the cathode and the gate. When the anode is made positive with respect to the cathode, junctions J1 and J3 are forward biased and junction J2 is reverse-biased and only the leakage current will flow through the device. The SCR
How an SCR works?-Principle of Operation SCR Working Principle The SCR is a four-layer, three-junction and a three-terminal device and is shown in fig.a. The end P-region is the anode, the end N-region is the cathode and the inner P-region is the gate. The anode to cathode is connected in series with the load circuit.
Introduction to SCR-Silicon Controlled Rectifier As the terminology indicates, the SCR is a controlled rectifier constructed of a silicon semiconductor material with a third terminal for control purposes. Silicon was chosen because of its high temperature and power capabilities. The basic operation of the SCR is different from that of an ordinary two-layer semiconductor diode
Comparison of  Thyristor and Transistor As already mentioned, transistors and thyristors are both semiconductor devices. They are now widely employed in switching operations because of their numerous advantages such as noiseless operation owing to absence of moving parts, very high switching speed (say 109 operations per second), high efficiency, low maintenance, small size, little weight
Comparison between Thyristors and Thyratrons Before the discovery of thyristors, thyratrons were used for industrial control, but now they have been replaced by the thyristors because of the following drawbacks of thyratrons. 1. Thyratron needs a large anode-to-cathode voltage and a separate filament supply whereas the thyristor needs only one main supply and a control
