at delivers a dc voltage twice or rnore times of the peak value (amplitude) of the input ac voltage. Such power supplies are used for high-voltage and low-current devices such as cathode-ray tubes (the picture tubes in TV receivers, oscilloscopes and computer display). Here we will consider half-wave voltage doubler, full-wave voltage doubler and voltage tripler and quadrupler.
Half-Wave Voltage Doubler
The circuit of a half-wave voltage doubler is given in figure shown below. During the positive half cycle of the ac input, voltage, diode D1 being forward biased conducts (diode D2 does not conduct because it is reverse-biased) and charges capacitor C1 upto peak values of secondary voltage Vsmax with the polarity, as marked in figure shown below.
During the negative half-cycle of the input voltage diode D2 gets forward biased and conducts charging capacitor C2. For the negative half cycle, the lower end of the transformer secondary is positive while upper end is negative. The polarity of the capacitor C2 has also been marked in the figure. Now starting from the bottom of the transformer secondary and moving clockwise and applying Kirchhoffs voltage law to the outer loop we have
-Vsmax – Vc1 + Vc2 = 0
Or
Vc2 = Vsmax + Vc1= Vsmax + Vsmax = 2Vsmax = Twice the peak value of the transformer secondary voltage. (Since Vc1 = Vsmax)
During the next positive half-.cycle diode D2 is reverse-biased and so acts as an open and capacitor C2 discharges through the load If there is no load across the capacitor, C2 both capacitors stay charged – C1 to Vsmax and C2 to 2Vsmax. If, as expected there is a load connected to the output terminals of the voltage doubler, the capacitor C2 discharges a little bit and consequently the voltage across capacitor C2 drops slightly. The capacitor C2 gets recharged again in the next half-cycle. The ripple frequency in this case will be the signal frequency (that is, 50 Hz for supply mains.
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