AC - DC Converter / Doubler Circuit

To: EE 222 Students and Tracy Huber and Doug Waage

Date: November 6, 1995

RE: Lab 5, Circuit 3

Read the handout for Lab 5.

The circuit in Figure 1 is designed using two standard 4007 Si diodes. The capacitors are both 0.6 uF. A four volt sinusoidal voltage is placed at Vin using a function generator. The output voltage is measured using the oscilloscope. Once the circuit is constructed, the output voltage, Vout, can be measured on the oscilloscope.

Figure 1: AC- DC Converter / Doubler Circuit

When 4 volts AC is applied to the circuit, the peak voltage approximately doubles in DC voltage. The output voltage is 7.187 V DC, as shown in Figure 2 from the oscilloscope. The voltage is shown as a straight line, which represents positive DC voltage. A graph of Vin and Vout is shown in Figure 3.

Figure 2: Voltage at Vout

Figure 3: Voltage Out and Voltage In

The circuit is investigated in order to understand why the circuit doubles and only outputs DC voltage. Vin, C1, and D1 were observed as a single loop. The voltage through the capacitor is observed on the oscilloscope as 7.187 V. The input voltage is s inusoidal, producing an AC voltage. The voltage could only move accross the diode during reverse bias. There are no other places, such as a resistor, for the capacitors to discharge. Because of this, the voltage during forward bias remains across the c apacitor. Although the increase to eight volts does not happen instantaneously, the change is too quick to see on the oscilloscope.

The capacitor charges up to a peak of 7.187 V while the diode is forward biased, and a minimum of 0 V during reverse bias. For this reason, the output voltage is approximately 8 V. There is no other path, such as a resistor, for the capacitors to discha rge to. The voltage can only flow through D2 during forward bias. The eight volts from C1 flows through the diode into C2. Vout is measured over C2, and this is why the output is a constant 7.178 V.

The transfer character was also observed. Vout was plotted verses Vin, shown in Figure 3. Due to the fact that there are no resistive elements in circuit, once the capcitors charge up, they will keep their charge. We observed the transfer function is st ill a straight line at 7.187 V.

Figure 3: Vin verses Vout. Transfer Characteristics

The frequency of the input was varied, and the output voltage did not change. Vout was still 7.187 V. The sine was then changed to a square wave. Vin varied between -3.85 V and 3.85 V, and Vout still remained 7.187 V. A triangle wave was then applied. Vin varied between -3.950 V and 3.950 V. The output voltage remained 7.187 V.

The polarity of the two diodes were switched. Vout was then measured as -7.187 V. The transfer characteristics were plotted as a straight line at -7.187 V. One would expect this to be the output. When the polarity of D1 is switched, current only flows through it during forward bias. This would in turn cause the voltage to remain over the capacitor during reverse bias. The voltage across C1 varied between approximately 0V and -8 V.

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