# SCR Applications.docx

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SCR Applications - On-Off Control of Current - Half-Wave Power Control - Full-Wave Power Control - Crowbar On-Off Control of Current The figure shows an SCR circuit that permits current to be switched to a load by the momentary closure of switch SW1 and removed from the load by the mom...

SCR Applications - On-Off Control of Current - Half-Wave Power Control - Full-Wave Power Control - Crowbar On-Off Control of Current The figure shows an SCR circuit that permits current to be switched to a load by the momentary closure of switch SW1 and removed from the load by the momentary closure of switch SW2. - Half-Wave Power Control A common application of SCRs is in the control of ac power for lamp dimmers, electric heaters, and electric motors. A half-wave, variable-resistance, phase-control circuit is shown in the figure. By adjusting *R*2, the SCR can be made to trigger at any point on the positive half-cycle of the ac waveform between 0° and 90°, as shown in the figure. When the SCR triggers near the beginning of the cycle (approximately 0°), as in the figure below, it conducts for approximately 180° and maximum power is delivered to the load. When it triggers near the peak of the positive half-cycle (90°), as in the figure below, the SCR conducts for approximately 90° and less power is delivered to the load. The figure shows triggering at the 45° point as an example. When the ac input goes negative, the SCR turns off and does not conduct again until the trigger point on the next positive half-cycle. - Firing and Conduction Angles The phase angle at which the SCR turns on is called the **firing angle,** [**θ**~**f**~]{.math.inline}. The angle between the start and end of conduction is called the **conduction angle,** [**θ**~**c**~]{.math.inline}. For example, as seen in the diagram, the SCR turns on at 90°. So the firing angle is, \ [*θ*~*f*~ = 90^**∘**^]{.math.display}\ The SCR remains to be in the on state (or conduction state) from [90^**∘**^]{.math.inline} to approximately [180^**∘**^]{.math.inline}. So its conduction angle is, \ [*θ*~*c*~ = 180^∘^ − 90^∘^ = 90^∘^]{.math.display}\ Average Voltage: \ [\$\$V\_{\\text{AVE}} = \\frac{1}{2\\pi}\\int\_{\\theta\_{f}}\^{\\theta\_{e}}V\_{m}\\sin\\theta\\text{dθ}\$\$]{.math.display}\ \ [\$\$V\_{\\text{AVE}} = \\frac{1}{2\\pi}\\int\_{\\theta\_{f}}\^{\\pi}V\_{m}\\sin\\theta\\text{dθ}\$\$]{.math.display}\ \ [\$\$V\_{\\text{AVE}} = \\frac{V\_{m}}{2\\pi}\\left( 1 + \\cos\\theta\_{f} \\right)\$\$]{.math.display}\ Average Current: \ [\$\$I\_{\\text{AVE}} = \\frac{V\_{\\text{AVE}}}{R\_{L}}\$\$]{.math.display}\ \ [\$\$I\_{\\text{AVE}} = \\frac{V\_{m}}{2\\pi R\_{L}}\\left( 1 + \\cos\\theta\_{f} \\right)\$\$]{.math.display}\ - Example A half-wave rectifier circuit employing an SCR is adjusted to have a gate current of 1mA. The forward breakdown voltage of SCR is 100 V for Ig = 1mA. If a sinusoidal voltage of 200 V peak is applied, find : \(i) firing angle (ii) conduction angle (iii) average current. Assume load resistance [*R*~*L*~ ]{.math.inline}= 100Ω and the holding current to be zero. - Solution Firing Angle: Conduction Angle: [*v* = *V*~*m*~sin *θ*]{.math.inline} [*θ*~*c*~ = *θ*~*e*~ − *θ*~*f*~]{.math.inline} [100 = 200sin *θ*~*f*~]{.math.inline} [*θ*~*c*~ = 180^∘^ − 30^∘^]{.math.inline} [\$\\theta\_{f} = \\sin\^{- 1}\\frac{100}{200}\$]{.math.inline} [*θ*~*c*~ = 150^∘^]{.math.inline} \ [*θ*~*f*~ = 30^∘^]{.math.display}\ - Solution Average Current: \ [\$\$V\_{\\text{AVE}} = \\frac{V\_{m}}{2\\pi}\\left( 1 + \\cos\\theta\_{f} \\right) = \\frac{200}{2\\pi}\\left( 1 + \\cos{30{\^\\circ}} \\right) = 59.4V\$\$]{.math.display}\ \ [\$\$V\_{\\text{AVE}} = \\frac{1}{2\\pi}\\int\_{\\pi/6}\^{\\pi}200\\sin\\theta d\\theta = 59.4V\$\$]{.math.display}\ \ [\$\$I\_{\\text{AVE}} = \\frac{V\_{\\text{AVE}}}{R\_{L}} = \\frac{59.4}{100} = 594mA\$\$]{.math.display}\ - Full-Wave Power Control The operation is similar to the half-wave power control except that in this configuration, either one of the SCRs are conducting in either positive or negative cycle of the input signal. Average Voltage: \ [\$\$V\_{\\text{AVE}} = \\frac{1}{\\pi}\\int\_{\\theta\_{f}}\^{\\theta\_{e}}V\_{m}\\sin\\theta\\text{dθ}\$\$]{.math.display}\ \ [\$\$V\_{\\text{AVE}} = \\frac{1}{\\pi}\\int\_{\\theta\_{f}}\^{\\pi}V\_{m}\\sin\\theta\\text{dθ}\$\$]{.math.display}\ \ [\$\$V\_{\\text{AVE}} = \\frac{V\_{m}}{\\pi}\\left( 1 + \\cos\\theta\_{f} \\right)\$\$]{.math.display}\ Average Current: \ [\$\$I\_{\\text{AVE}} = \\frac{V\_{\\text{AVE}}}{R\_{L}}\$\$]{.math.display}\ \ [\$\$I\_{\\text{AVE}} = \\frac{V\_{m}}{\\pi R\_{L}}\\left( 1 + \\cos\\theta\_{f} \\right)\$\$]{.math.display}\ - Example Power (brightness) of a 100 W, 110 V lamp is to be varied by controlling firing angle of SCR full-wave circuit; the r.m.s. value of a.c. voltage appearing across each SCR being 110 V. Find the r.m.s. voltage and current in the lamp at firing angle of 60°. - Solution RMS Voltage and Current of the conduction state of the SCR: \ [\$\$V\_{\\text{RMS}}\^{2} = \\frac{1}{\\pi}\\int\_{\\theta\_{f}}\^{\\theta\_{e}}V\_{m}\^{2}\\sin\^{2}\\theta d\\theta = \\frac{1}{\\pi}\\int\_{\\frac{\\pi}{3}}\^{\\pi}V\_{m}\^{2}\\sin\^{2}\\theta\\text{dθ}\$\$]{.math.display}\ We are not given [*V*~*m*~]{.math.inline}, but we know 110[*V*~RMS~]{.math.inline} appear across each SCR. \ [\$\$V\_{m} = V\_{\\text{RMS}} \\times \\sqrt{2} = 155.56V\$\$]{.math.display}\ \ [\$\$V\_{\\text{RMS}}\^{2} = \\frac{1}{\\pi}\\int\_{\\frac{\\pi}{3}}\^{\\pi}{155.56}\^{2}\\sin\^{2}\\theta d\\theta = 9734\$\$]{.math.display}\ \ [*V*~RMS~ = 98.7*V*]{.math.display}\ - Solution \ [\$\$I\_{\\text{RMS}} = \\frac{V\_{\\text{RMS}}}{R\_{L}}\$\$]{.math.display}\ We are not given the resistance value of the lamp, but we can solve it using, \ [*P* = *V*~RMS~^2^/*R*~*L*~]{.math.display}\ \ [100 = 110^2^/*R*~*L*~]{.math.display}\ \ [*R*~*L*~ = 121*Ω*]{.math.display}\ \ [\$\$I\_{\\text{RMS}} = \\frac{98.7}{121} = 816mA\$\$]{.math.display}\ - SCR Crowbar The figure shows a power supply of *VCC* applied to a protected load. Under normal conditions, *VCC* is less than the breakdown voltage of the zener diode. **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.