Diac Triac.docx

**SILICON-CONTROLLED SWITCH** - Similar to SCR - Has 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. **How to turn on SCS** - A positive...

**SILICON-CONTROLLED SWITCH** - Similar to SCR - Has 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. **How to turn on SCS** - A positive pulse applied on the cathode gate or a negative pulse applied on the anode gate. **How to turn off SCS** - A positive pulse applied on the anode gate or a negative pulse applied on the cathode gate. - Reducing the anode current below the holding current by using BJT as a switch to interrupt anode current. **DIAC** - Two-terminal four-layer thyristor which conduct in EITHER direction when properly activated. - RIGHT SIDE: PNPN - LEFT SIDE: NPNP - Require breakover voltage to initiate conduction with either polarity is across the 2 terminals. - Neither terminal is referred to as CATHODE. - Contains 2 anodes, anode 1 (electrode 1) and anode 2 (electrode 2) - When anode 1 is positive, the applicable layers are p~1~ n~2~ p~2~ and n~3~. - When anode 2 is positive, the applicable layers are p~2~ n~2~ p~1~ and n~1~. **DIAC Equivalent Circuit and Basic Operation** From A1 to A2: - Q1 & Q2 forward-biased - Q3 & Q4 reversed-biased - Operate on the upper right portion of the characteristic curve. From A2 to A1: - Q3 & Q4 forward-biased - Q1 & Q2 reversed-biased - Operate on the lower right portion of the characteristic curve. **DIAC Applications** - Trigger circuit for the Triac - Proximity Sensor circuit **TRIAC** - 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. - It turn OFF when the current drop sufficient low level. - Breakover potential decrease as the gate current increase. **UJT (UNIJUNCTION TRANSISTOR)** - A three-terminal semiconductor device that has only one pn junction. - A breakover type switching device whose characteristics make it useful in timers, oscillators, waveform generators, and gate control circuits for SCRs and TRIACs. - Two base lead B~1~ and B~2~ and an emitter E lead. - Interbase resistance, R~BB~ of a UJT is the resistance of its n-type silicon bar. - The ratio [\$\\frac{R\_{B1}}{R\_{B1} + R\_{B2}}\$]{.math.inline} is called the INTRINSIC STANDOFF RATIO, designated as η (eta). - Used with SCRs and Triacs to control their conduction angle. - V~pn~ --barrier potential of the pn junction - V~P~ = ηVBB + V~pn~ where V~P~ is the peak-point voltage - After turn-on, the UJT operates in a negative resistance region up to a certain value of I~E~. - At peak-point, **V~E~ = V~P~ and I~E~ = I~P~**~.~ - Then, V~E~ decreases as I~E~ continues to increase, thus producing the **negative resistance** characteristic. - At valley point, **V~E~ = V~V~ and I~E~ = I~V~**. - Beyond the valley point , the device is in saturation, and V~E~ increases very little with an increasing I~E~. - All UJT circuits, the burst of current from E to B~1~ is short-lived, and the UJT quickly reverts back to the OFF condition. **Programmable UJT (PUT or PUJT)** - A four-layer pnpn device with a gate connected directly to the sandwiched n type layer. - Unlike in UJT, R~BB~, η, and V~P~ can be controlled through R~B1~ and R~B2~ (external to the device). - type of three-terminal thyristor that is triggered into conduction when the voltage at the anode exceeds the voltage at the gate. - The gate is connected to the *n* region adjacent to the anode. - This *pn* junction controls the *on* and *off* states of the device. - The gate is always biased positive with respect to the cathode. - When the anode voltage exceeds the gate voltage by approximately 0.7 V, the *pn* junction is forward biased and the PUT turns on. - The PUT stays on until the anode voltage falls back below this level, then the PUT turns off.