40 SCR Half wave Rectifier Figurea shows the circuit of a SCR half wave

40 scr half wave rectifier figurea shows the circuit

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which the SCR turns ON, may be controlled either by amplitude triggering or pulse triggering.
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40 SCR Half wave Rectifier Figure(a) shows the circuit of a SCR half-wave rectifier. In this circuit, the ac supply is connected to the SCR through a load resistance RL. The gate current is obtained from the ac supply through the resistances R and R1 and diode D connected in series. The diode D is connected in such a way, so that it blocks the reverse voltage on the gate during the negative half-cycle. Figure (b), (c) and (d) shows the waveforms of the voltage across SCR (V SCR ), the load current (I L ) and voltage across the load (V L ) respectively. The various curves in the waveforms may be expained as follows: During the positive half- cycle, the SCR remains ‘OFF’ till the input voltage reaches the gate trigger voltage i.e. point A in Fig. (b). As a result, there is no current through RL. Hence load currrent (IL) and load voltage (V L ) are zero. At point A, SCR is fired into conduction. It acts like a short and voltage across it drops to zero i.e., curve AB in Fig. (b). Under this condition, whole of the applied voltage drops across the load resistance. During the negative input half cycle, the SCR is reverse biased and hence does not conduct. As a result, all the applied voltage appears across SCR and none across the load resistance (RL). The horizontal distance between the points O and A in Fig. (b) represents the time in the positive half-cycle, when SCR is not conducting. This distance in degrees is called the firing angle, phase angle or delay angle ( α ). This angle gives us an idea about the position at which the SCR starts conduction with respect to the origin. It may may be noted from the figure that the SCR is coducting between the points B and C. This angle is called conduction angle ( ɸ ) and its value is equal to ( π α ).
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41 Average Values of Load Voltage and Current Average load voltage The equation of a.c. supply voltage is V in = V m .sin ω t = V m .sin θ We also know that for a half wave rectifier, the average value of a load voltage is determined by the relation, Vdc = = = -----(1) Now the load voltage is developed only during the period SCR conducts. This period lies between the angles α and π . Therefore, we must take the average over the limits α to π instead of 0 to 2 π in equation (1) Vdc = = = α π = = (1+cos α ) -------(2) Average load current We know that load current, iL = vL/RL, therefore, the average value of the load current, Idc = = (1 + cos α ) = (1 + cos α ) ------(3) Where Im is equal to Vm/RL and is the maximum value of the load current. It may be noted that if the firing angle ( α ) is equal to zero, then the average values of load voltage and current is obtained from equations (2) and (3) Therefore Vdc = and
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42 Idc = = = It may be noted that the above results are the same as obtained for an ordinary diode half-wave rectifier. RMS load voltage The rms voltage across the load is Vrms = [ 2 d( ω t)] 0.5 = Vm[ 2 ω td( ω t)] 0.5 = Vm[ d( ω t)] 0.5 = Vm[ {( ω t )}] 0.5 Or Vrms = Vm[ + ] 0.5 RMS load current And the rms load current is Irms = 90° Variable Half-wave Rectifier (Converter) The SCR half-wave rectifier circuit shown in the half-wave rectifier is known as 90° variable half-wave rectifier (or 90° variable phase control circuit).
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