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The Villard Circuit

Voltage multipliers are electric circuits which achieve gains of multiples of the input peak voltage generally in several stages. The term voltage multiplier is usually applied to circuits consisting of rectifying diodes and capacitors only; other means of achieving gains are not included in this type of circuit. There are several configurations with which the increase in dc output voltage levels can be achieved, either half or full-wave. However, here we will mainly concentrate on the Villard circuit, whose simplest version consists of a capacitor and a diode. While its benefit is highly simple design, its main disadvantage is very poor ripple characteristics, as you will find out during the experiments. Essentially, the circuit is a diode clamper which shifts the dc value of the waveform.

The following animation together with the explanation will help you better understand what this voltage multiplier is all about; but this is only a simplified explanation, the circuit's behaviour being a bit more complicated. The final steady state is actually only achieved after several complete cycles. In the following sentences, we describe the multiplier's behaviour in a simplified way.

During the negative input half-cycle: C1 is charged to approx. the peak voltage Vp through D1.

During the following positive input half-cycle: C2 charges to 2*Vp through D2. You can consider C1 as a battery in a diode clamper circuit, so that the voltage C2 "sees" is approximately the voltage of C1 plus the input voltage.

During the next negative input half-cycle: The positive terminal of C1 is connected to 0V at the beginning of the cycle through D1. In this way, C3 can get charged to 2*Vp through D3.

During the following positive input half-cycle: Now the 2*Vp can be transferred to C4 through D4, so that a 4*Vp voltage is measured at the output which corresponds to the sum of the voltages of C4 and C2.

villard-circuit.gif

Due to the internal resistance of the power supply and the diodes, the final steady state is only reached after several periods have elapsed. The following graph shows the first few cycles; the capacitors are not charged to their final values during these cycles, which means there is a transient response you should take into account. You will measure this transient response in the following experiments.

transient-response-villard-circuit.jpg

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