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An AC generator produces a sinusoidal voltage (with zero DC offset) because of its rotation. You can use diode circuits to prevent this voltage from reversing polarity; this process is called rectification and is a step towards AC to DC conversion. You can also use a diode to clip the voltage to a desired maximum value so that you can protect other connected electronic components from high voltages.

Figure 1

The fact that a diode allows current to flow in only one direction means that it can rectify a sinusoidal voltage waveform by blocking its negative part.

Figure 2

We analyze this circuit by assuming that the diode follows the offset ideal model and we treat the diode's OFF and ON states separately.

Figure 3

Figure 4

Combining the results from Fig. 3 and Fig. 4 gives

\begin{equation}

V_{\text{out}} = \begin{cases}

0, & \text{if } V_{\text{in}} < V_{\text{on}}, \\

V_{\text{in}}-V_{\text{on}}, & \text{if } V_{\text{in}}>V_{\text{on}}.

\end{cases} \label{DCS-HWE}

\end{equation}

Figure 5

Diode circuits can protect electronic equipment from high voltages. An example is the diode clipper, which prevents its output voltage from exceeding a set limit.

Figure 6

Just like the half-wave rectifier, we analyze this circuit by assuming that the diode follows the offset ideal model and we treat the diode's OFF and ON states separately.

Figure 7

Figure 8

Combining the results from Fig. 7 and Fig. 8 gives

\begin{equation}

V_{\text{out}} = \begin{cases}

V_{\text{in}}, & \text{if } V_{\text{in}} < V_1+V_{\text{on}}, \\

V_1+V_{\text{on}}, & \text{if } V_{\text{in}} > V_1+V_{\text{on}}.

\end{cases} \label{DCS-DCE}

\end{equation}

Figure 9

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A full wave-rectifier uses four diodes to reverse the polarity of the negative half of the input voltage waveform while preserving the polarity of the positive half.

Figure 10

We assume the diodes are identical and follow the offset ideal model. Then there are three possible combinations of diode ON/OFF states depending on the value of $V_{\text{in}}$.

Figure 11

Figure 12

Figure 13

Combining the results from Fig. 11, Fig. 12 and Fig. 13 gives

\begin{equation}

V_{\text{out}} = \begin{cases}

{-}V_{\text{in}}-2V_{\text{on}}, & \text{if } V_{\text{in}} < -2V_{\text{on}}, \\

0, & \text{if } -2V_{\text{on}} < V_{\text{in}} < 2V_{\text{on}}, \\

V_{\text{in}}-2V_{\text{on}}, & \text{if } V_{\text{in}}>2V_{\text{on}}.

\end{cases} \label{DCS-FWE}

\end{equation}

Figure 14

A basic AC to DC converter can be made with a rectifier and a capacitor. This kind of circuit is inside most AC adapters used to power household electronic devices.

Figure 15

The full-wave rectifier by itself would provide an output voltage that oscillates between zero and a maximum value, as shown in Fig. 14. This fluctuation is undesirable for most electronics. The role of the capacitor across the output terminals is to smooth the output voltage.

Figure 16

One way to understand this circuit is to see that the capacitor charges when the rectifier output voltage is large and discharges when the rectifier output voltage is small.

Figure 17

In ECE 210: Analog Signal Processing, you will learn how to analyze the AC to DC converter circuit in Fig. 16 mathematically. You will also see how the same circuit can demodulate a transmitted AM radio signal so that you can play the signal on a speaker.