DIAC Hard

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 and Off SCS

• Turn on: Positive pulse on cathode gate or negative pulse on anode gate
• Turn off: Positive pulse on anode gate or negative pulse on cathode gate, or reducing anode current below holding current

DIAC

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

• A bi-directional thyristor used to control 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, and a gate lead used to control conduction
• Can be turned on by a pulse of gate current and does not require breakover voltage to initiate conduction

TRIAC Characteristic Curve

• Current direction depends on the polarity across the terminal
• Turns off when the current drops to a sufficiently 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, is the resistance of its n-type silicon bar
• Intrinsic standoff ratio, η, is the ratio of RB1 to RB1 + RB2

UJT Operations

• 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 negative resistance characteristic
• At valley point, VE = VV and IE = IV
• Beyond valley point, the device is in saturation, and VE increases very little with increasing IE

Programmable UJT (PUT or PUJT)

• A four-layer pnpn device with a gate connected directly to the sandwiched n-type layer
• RBB, η, and VP can be controlled through RB1 and RB2 (external to the device)