THYRISTORS

THYRISTORSTHYRISTOR is a family of devices constructed of four semiconductor layers which exhibits latching. - They basically act as switch and is used mainly in controlling the amount of power to a load.

FOUR-LAYER DIODEalso known as SHOCKLEY DIODE or SILICON UNILATERAL SWITCH (SUS).It is forward break-over device.

FOUR-LAYER DIODE

FOUR-LAYER DIODE

SILICON CONTROLLED RECTIFIER

It is like four layer diode with added terminal called the gateIt may be turned on like thethe four-layer diode or by applying a pulse in the gate

SILICON CONTROLLED RECTIFIER

SILICON CONTROLLED RECTIFIERVBRF (forward breakover voltage)VBRR (reverse breakover voltage)VROM peak reverse voltage sameas VBRRReverse blocking currentForward blocking currentGate currentHolding current

SILICON CONTROLLED RECTIFIERLike the 4-layer diode, an SCR can also be turned on without gate triggering by increasingthe anode-to-cathode voltage to a value exceeding the forward-breakover voltage.The gate current controls the value of forward breakover voltage, VBR(F), required for turn-on.

SILICON CONTROLLED RECTIFIERIt may be turned off using anode current interruption

SILICON CONTROLLED RECTIFIERIt may be turned off using forced commutation

Typically, turn-off times for SCRs range from a few microseconds up to about 30 s.SILICON CONTROLLED RECTIFIERSCR Characteristics and RatingsForward-breakover voltage - this is the voltage at which the SCR enters the forward-conduction region. Holding current - this is the value of anode current below which the SCR switches from the forward-conduction region to the forward-blocking region.Gate trigger current - this is the value of gate current necessary to switch the SCR from the forward-blocking region to the forward-conduction region under specified conditions.Average forward current - this is the maximum continuous anode current (dc) that the device can withstand in the conduction state under specified conditions.Forward-conduction region - this region corresponds to the on condition of the SCR where there is forward current from anode to cathode through the very low resistance (approximate short) of the SCR.Forward-blocking and reverse-blocking regions - these regions correspond to the off condition of the SCR where the forward current from anode to cathode is blocked by the effective open circuit of the SCR.Reverse-breakdown voltage - this parameter specifies the value of reverse voltage from cathode to anode at which the device breaks into the avalanche region and begins to conduct heavily (the same as in a pn junction diode).SILICON CONTROLLED RECTIFIERThe Light-Activated SCR (LASCR)conducts current in one direction when activated by a sufficient amount of light and continues to conduct until the current falls below a specified value.The LASCR is most sensitive to light when the gate terminal is open. If necessary, a resistor from the gate to the cathode can be used to reduce the sensitivity.

SILICON CONTROLLED RECTIFIERHalf-Wave Variable-Resistance Phase Control Circuit

SILICON CONTROLLED RECTIFIERHalf-Wave Variable-Resistance Phase Control Circuit

SILICON CONTROLLED RECTIFIERHalf-Wave Variable-Resistance Phase Control Circuit

SILICON CONTROLLED RECTIFIERAn Over-Voltage Protection Circuit (Crowbar Circuit)

SILICON CONTROLLED RECTIFIERSawtooth Generator

DIACa two-terminal four-layer semiconductor device that can conduct current in either direction when properly activated.

DIACConduction occurs in a diac when the breakover voltage is reached with either polarity across the two terminals.Once breakover occurs, current is in a direction depending on the polarity of the voltage across the terminals

DIAC

TRIACIs like a diac with the gate terminalLike two SCRs connected in parallel and in opposite directions with a common gate terminalCan also conduct in either direction

TRIACThe breakover potential decreases as the gate current increases

TRIACBilateral operation of a triac.

TRIACLike the SCR, triacs are used to control average power to a load by the method of phase controlDuring each positive half-cycle of the ac, the triac is off for a certain interval, called delay angleThen it is triggered on and conducts current through the load for the remaining portion of the positive half-cycle, called the conduction angle

TRIACPhase Control

SILICON CONTROLLED SWITCH (SCS)Has two gate terminals that are used to trigger the device on and off (anode gate and cathode gate)It typically has a faster turn-off time than the SCR but limited in terms of maximum current and voltage ratings

UJT (UNIJUNCTION TRANSISTOR)Has three terminals emitter, base 1 and base 2It has only one pn junction therefore it is not a thyristor

UJT (UNIJUNCTION TRANSISTOR)It has internal resistance between base 1 and emitter rb1 and internal resistance between base 2 and emitter rb2The value of rb1 varies inversely with emitter current and therefore, it is shown as a variable resistor.The total resistance from base 1 to base 2 is called the interbase resistance rbbThe internal resistances rb1 and rb2 form a voltage divider when the device is biased and the voltage across the resistance can be expressed as

The ratio of rb1 to rb2 is called the intrinsic standoff ratio ()

UJT (UNIJUNCTION TRANSISTOR)To turn on the device, applied voltage should be greater than or equal to peak-point voltage VpVp = VBB + Vpnwhere: VBB- the supply voltage between base 1 and base 2Vpn the barrier potential of the pn junction

UJT (UNIJUNCTION TRANSISTOR)After turn-on, the UJT operates in a negative resistance region i.e. after the peak point, VE decreases as IE continues to increase until the valley pointBeyond the valley point, the device is in saturation, and VE increases very little with an increasing IE

UJT APPLICATIONThe UJT can be used as a trigger device for SCRs and triacs.Other applications include nonsinusoidal oscillators, sawtooth generators, phase control, and timing circuits.

PUT(PROGRAMMBLE UNIJUNCTION TRANSISTOR)The programmable unijunction transistor (PUT) is actually a type of thyristor and not like the UJT at all in terms of structure. The only similarity to a UJT is that the PUT can be used in some oscillator applications to replace the UJT.Its structure is more similar to SCR than UJT, the difference is that the gate is connected to the n region near the anode while in SCR the gate is connected to the p region near the cathode

PUT(PROGRAMMBLE UNIJUNCTION TRANSISTOR)The gate is always biased positive with respect to the cathode. When the anode voltage exceeds the gate voltage by approximately 0.7V, the pn junction is forward-biased and the PUT turns onThe PUT stays on until the anode voltage falls back below this level, then the PUT turns off

PUT(PROGRAMMBLE UNIJUNCTION TRANSISTOR)The gate is usually connected to voltage divider circuit, where the ratio of the voltage across the two resistors determines the turn on voltage.Programmable means that the turn-on voltage can be adjusted.

PUT(PROGRAMMBLE UNIJUNCTION TRANSISTOR)