DIAC Medium

Silicon-Controlled Switch (SCS)

• Similar to SCR, but with two gate terminals (cathode and anode gate)
• Four-terminal thyristor used to trigger the device ON and OFF
• Faster turn-off time than SCR
• Used in counters, registers, and timing circuits

Turning On SCS

• Apply a positive pulse on the cathode gate or a negative pulse on the anode gate

Turning Off SCS

• Apply a positive pulse on the anode gate or a negative pulse on the cathode gate
• Reduce the anode current below the holding current by using BJT as a switch to interrupt anode current

DIAC (Diode for Alternating Current)

• Two-terminal four-layer thyristor that conducts in either direction when properly activated
• Requires breakover voltage to initiate conduction with either polarity across the 2 terminals
• Neither terminal is referred to as cathode
• Contains 2 anodes (anode 1 and anode 2)

DIAC Equivalent Circuit and Basic Operation

• From A1 to A2: Q1 & Q2 forward-biased, Q3 & Q4 reversed-biased
• From A2 to A1: Q3 & Q4 forward-biased, Q1 & Q2 reversed-biased

DIAC Applications

• Trigger circuit for the Triac
• Proximity Sensor circuit

TRIAC (Triode for Alternating Current)

• A bi-directional thyristor used to control the power in AC circuits
• A Diac with a gate control or two SCRs in parallel and in opposite directions with a common gate terminal
• Has two leads designated MT1 and MT2 or A1 and A2
• Has a gate lead which is used to control its conduction, which can be turned on by a pulse of gate current and does not require the breakover voltage to initiate conduction

TRIAC Characteristic Curve

• Current in direction depending on the polarity across the terminal
• Turns OFF when the current drops to a sufficient low level
• Breakover potential decreases as the gate current increases

UJT (Unijunction Transistor)

• A three-terminal semiconductor device with only one pn junction
• A breakover type switching device useful in timers, oscillators, waveform generators, and gate control circuits for SCRs and TRIACs
• Two base leads (B1 and B2) and an emitter (E) lead
• Interbase resistance (RBB) of a UJT is the resistance of its n-type silicon bar

UJT Characteristics

• The ratio of RB1RB1+RB2\frac{R_{B1}}{R_{B1} + R_{B2}}RB1​+RB2​RB1​​ is called the intrinsic standoff ratio, designated as η (eta)
• Used with SCRs and Triacs to control their conduction angle
• Vpn is the barrier potential of the pn junction
• Vp = ηVBB + Vpn, where Vp is the peak-point voltage
• At peak-point, VE = Vp and IE = Ip
• Then, VE decreases as IE continues to increase, producing the negative resistance characteristic
• At valley point, VE = Vv and IE = Iv
• Beyond the valley point, the device is in saturation, and VE increases very little with an increasing IE

Programmable UJT (PUT or PUJT)

• A four-layer pnpn device with a gate connected directly to the sandwiched n-type layer
• Unlike in UJT, RBB, η, and Vp can be controlled through R1 and R2 (external to the device)