Electronic Circuits and Diagram-Electronics Projects and Design

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UJT Characteristics The static emitter char­acteristic (a curve showing the relation between emitter voltage VE and emitter current IE) of a UJT at a given inter base voltage VBB is shown in figure.  From figure it is noted that for emitter potentials to the left of peak point, emitter current IE never exceeds IEo . The current IEo corresponds very closely to the reverse leakage current ICo of the conventional BJT. This region, as shown in the figure, is called the cut-off region. Once con­duction is established at VE = VP the emitter po­tential VE starts decreasing with the increase…

Unijunction transistor (abbreviated as UJT), also called the double-base diode is a 2-layer, 3-terminal solid-state (silicon) switching device. The device has-a unique characteristic that when it is triggered, its emitter current increases re generatively (due to negative resistance characteristic) until it is restricted by emitter power supply. The low cost per unit, combined with its unique characteristic, have warranted its use in a wide variety of applications. A few include oscillators, pulse generators, saw-tooth generators, triggering circuits, phase control, timing circuits, and voltage-or current-regulated supplies. The device is in general, a low-power-absorbing device under normal operating conditions and provides tremendous…

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…

Gate turn-off switch (GTO) is, like an SCR, is a four layer, three junction semiconductor device with three external terminals, namely, the anode, the cathode and the gate, as illustrated in figure. The basic construction, schematic symbol of a GTO are shown is figures  respectively.  Although the graphic symbol is different from either the SCR or the SCS, the transistor equivalent is exactly the same and the characteristics are similar. The main advantage of the GTO over the SCR or SCS is that it can be turned on or off by applying the proper pulse to the cathode gate (without…

The diac and the silicon bilateral switch are grouped as bilateral or bidirectional devices because they can breakover in either direction. There are also breakover devices which breakover in only one direction; they fall in the category of unilateral or unidirectional breakover devices. Although unilateral breakover devices are more frequently employed in SCR triggering, they can also be employed in triac triggering circuit if they have some extra supporting circuitry. Silicon unilateral switch (SUS) is one of the important unilat­eral breakover devices. V-I characteristic of SUS The schematic symbol and V-I characteristic of SUS are shown is figure a and…

Silicon bilateral switch (SBS) is another breakover device which is capable of triggering triacs, and it is popular in low-voltage trigger control circuits. They have breakover voltages lower than those for diacs, ± 8 V being the most popular rating. V-I characteristic curve of an SBS is similar to that of the diac, but it has more pronounced “negative resistance” region i.e., its decline in voltage is more drastic after it enters the conducting state. The schematic symbol and V-I characteristic of an SBS are shown in figure a and b respectively. When SBS switches into conducting state the voltage…

The four-layer diode, also called the Shockley diode after its inventor William Shockley, is essentially a low-current SCR without a gate. It is classified as a diode because it has only two external terminals through anode and cathode. Because of its four doped regions it is often called a P-N-P-N diode. The basic structure, two transistor version, equivalent tran­sistor circuit and schematic symbol of a Shockley diode are shown in figure. Shockley Diode Symbol The easiest way to understand how it operates is to visualize it to be formed of two transistors Qx and Q2 placed back to back, as…

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…

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…